[PATCH 0/33] utrace for 3.2 kernel

[Oleg Nesterov]

Hello,

utrace patches for 3.2 kernel.

Also available at git://github.com/utrace/linux utrace-3.2

Oleg.

 Documentation/DocBook/Makefile    |    2 +-
 Documentation/DocBook/utrace.tmpl |  589 +++++++++
 arch/s390/kernel/traps.c          |    6 +-
 arch/x86/kernel/ptrace.c          |    1 -
 fs/exec.c                         |    5 +-
 fs/proc/array.c                   |   14 +-
 include/linux/ptrace.h            |    7 +
 include/linux/sched.h             |   25 +-
 include/linux/signal.h            |    2 +
 include/linux/tracehook.h         |   59 +-
 include/linux/utrace.h            |  773 ++++++++++++
 init/Kconfig                      |    9 +
 kernel/Makefile                   |    1 +
 kernel/exit.c                     |    5 +
 kernel/fork.c                     |    9 +
 kernel/ptrace.c                   |   57 +-
 kernel/sched.c                    |    2 +-
 kernel/signal.c                   |   97 ++-
 kernel/utrace.c                   | 2462 +++++++++++++++++++++++++++++++++++++
 19 files changed, 4069 insertions(+), 56 deletions(-)

[PATCH 01/33] utrace core

[Oleg Nesterov]

From: Roland McGrath <roland(a)redhat.com&gt;

This adds the utrace facility, a new modular interface in the kernel for
implementing user thread tracing and debugging.  This fits on top of the
tracehook_* layer, so the new code is well-isolated.

The new interface is in <linux/utrace.h> and the DocBook utrace book
describes it.  It allows for multiple separate tracing engines to work in
parallel without interfering with each other.  Higher-level tracing
facilities can be implemented as loadable kernel modules using this layer.

The new facility is made optional under CONFIG_UTRACE.
When this is not enabled, no new code is added.
It can only be enabled on machines that have all the
prerequisites and select CONFIG_HAVE_ARCH_TRACEHOOK.

In this initial version, utrace and ptrace do not play together at all,
the next patches try to fix this.

This is is same/old utrace-core patch except:

	- use task->jobctl/JOBCTL_STOP_DEQUEUED instead of removed
	  signal->flags/SIGNAL_STOP_DEQUEUED in utrace_get_signal()

	- do not include the tracehook changes, this comes with the
	  next patches

Signed-off-by: Roland McGrath <roland(a)redhat.com&gt;
Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 Documentation/DocBook/Makefile    |    2 +-
 Documentation/DocBook/utrace.tmpl |  589 +++++++++
 fs/proc/array.c                   |    3 +
 include/linux/sched.h             |    5 +
 include/linux/utrace.h            |  692 +++++++++++
 init/Kconfig                      |    9 +
 kernel/Makefile                   |    1 +
 kernel/utrace.c                   | 2440 +++++++++++++++++++++++++++++++++++++
 8 files changed, 3740 insertions(+), 1 deletions(-)
 create mode 100644 Documentation/DocBook/utrace.tmpl
 create mode 100644 include/linux/utrace.h
 create mode 100644 kernel/utrace.c

diff --git a/Documentation/DocBook/Makefile b/Documentation/DocBook/Makefile
index 66725a3..08ed954 100644
--- a/Documentation/DocBook/Makefile
+++ b/Documentation/DocBook/Makefile
@@ -14,7 +14,7 @@ DOCBOOKS := z8530book.xml mcabook.xml device-drivers.xml \
 	    genericirq.xml s390-drivers.xml uio-howto.xml scsi.xml \
 	    80211.xml debugobjects.xml sh.xml regulator.xml \
 	    alsa-driver-api.xml writing-an-alsa-driver.xml \
-	    tracepoint.xml drm.xml media_api.xml
+	    tracepoint.xml utrace.xml drm.xml media_api.xml
 
 include $(srctree)/Documentation/DocBook/media/Makefile
 
diff --git a/Documentation/DocBook/utrace.tmpl b/Documentation/DocBook/utrace.tmpl
new file mode 100644
index 0000000..0c40add
--- /dev/null
+++ b/Documentation/DocBook/utrace.tmpl
@@ -0,0 +1,589 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
+"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
+
+<book id="utrace">
+  <bookinfo>
+    <title>The utrace User Debugging Infrastructure</title>
+  </bookinfo>
+
+  <toc></toc>
+
+  <chapter id="concepts"><title>utrace concepts</title>
+
+  <sect1 id="intro"><title>Introduction</title>
+
+  <para>
+    <application>utrace</application> is infrastructure code for tracing
+    and controlling user threads.  This is the foundation for writing
+    tracing engines, which can be loadable kernel modules.
+  </para>
+
+  <para>
+    The basic actors in <application>utrace</application> are the thread
+    and the tracing engine.  A tracing engine is some body of code that
+    calls into the <filename>&lt;linux/utrace.h&gt;</filename>
+    interfaces, represented by a <structname>struct
+    utrace_engine_ops</structname>.  (Usually it's a kernel module,
+    though the legacy <function>ptrace</function> support is a tracing
+    engine that is not in a kernel module.)  The interface operates on
+    individual threads (<structname>struct task_struct</structname>).
+    If an engine wants to treat several threads as a group, that is up
+    to its higher-level code.
+  </para>
+
+  <para>
+    Tracing begins by attaching an engine to a thread, using
+    <function>utrace_attach_task</function> or
+    <function>utrace_attach_pid</function>.  If successful, it returns a
+    pointer that is the handle used in all other calls.
+  </para>
+
+  </sect1>
+
+  <sect1 id="callbacks"><title>Events and Callbacks</title>
+
+  <para>
+    An attached engine does nothing by default.  An engine makes something
+    happen by requesting callbacks via
<function>utrace_set_events</function>
+    and poking the thread with <function>utrace_control</function>.
+    The synchronization issues related to these two calls
+    are discussed further below in <xref linkend="teardown"/>.
+  </para>
+
+  <para>
+    Events are specified using the macro
+   
<constant>UTRACE_EVENT(<replaceable>type</replaceable>)</constant>.
+    Each event type is associated with a callback in <structname>struct
+    utrace_engine_ops</structname>.  A tracing engine can leave unused
+    callbacks <constant>NULL</constant>.  The only callbacks required
+    are those used by the event flags it sets.
+  </para>
+
+  <para>
+    Many engines can be attached to each thread.  When a thread has an
+    event, each engine gets a callback if it has set the event flag for
+    that event type.  For most events, engines are called in the order they
+    attached.  Engines that attach after the event has occurred do not get
+    callbacks for that event.  This includes any new engines just attached
+    by an existing engine's callback function.  Once the sequence of
+    callbacks for that one event has completed, such new engines are then
+    eligible in the next sequence that starts when there is another event.
+  </para>
+
+  <para>
+    Event reporting callbacks have details particular to the event type,
+    but are all called in similar environments and have the same
+    constraints.  Callbacks are made from safe points, where no locks
+    are held, no special resources are pinned (usually), and the
+    user-mode state of the thread is accessible.  So, callback code has
+    a pretty free hand.  But to be a good citizen, callback code should
+    never block for long periods.  It is fine to block in
+    <function>kmalloc</function> and the like, but never wait for i/o or
+    for user mode to do something.  If you need the thread to wait, use
+    <constant>UTRACE_STOP</constant> and return from the callback
+    quickly.  When your i/o finishes or whatever, you can use
+    <function>utrace_control</function> to resume the thread.
+  </para>
+
+  <para>
+    The <constant>UTRACE_EVENT(SYSCALL_ENTRY)</constant> event is a special
+    case.  While other events happen in the kernel when it will return to
+    user mode soon, this event happens when entering the kernel before it
+    will proceed with the work requested from user mode.  Because of this
+    difference, the <function>report_syscall_entry</function> callback is
+    special in two ways.  For this event, engines are called in reverse of
+    the normal order (this includes the <function>report_quiesce</function>
+    call that precedes a <function>report_syscall_entry</function> call).
+    This preserves the semantics that the last engine to attach is called
+    "closest to user mode"--the engine that is first to see a thread's
user
+    state when it enters the kernel is also the last to see that state when
+    the thread returns to user mode.  For the same reason, if these
+    callbacks use <constant>UTRACE_STOP</constant> (see the next section),
+    the thread stops immediately after callbacks rather than only when it's
+    ready to return to user mode; when allowed to resume, it will actually
+    attempt the system call indicated by the register values at that time.
+  </para>
+
+  </sect1>
+
+  <sect1 id="safely"><title>Stopping Safely</title>
+
+  <sect2 id="well-behaved"><title>Writing well-behaved
callbacks</title>
+
+  <para>
+    Well-behaved callbacks are important to maintain two essential
+    properties of the interface.  The first of these is that unrelated
+    tracing engines should not interfere with each other.  If your engine's
+    event callback does not return quickly, then another engine won't get
+    the event notification in a timely manner.  The second important
+    property is that tracing should be as noninvasive as possible to the
+    normal operation of the system overall and of the traced thread in
+    particular.  That is, attached tracing engines should not perturb a
+    thread's behavior, except to the extent that changing its user-visible
+    state is explicitly what you want to do.  (Obviously some perturbation
+    is unavoidable, primarily timing changes, ranging from small delays due
+    to the overhead of tracing, to arbitrary pauses in user code execution
+    when a user stops a thread with a debugger for examination.)  Even when
+    you explicitly want the perturbation of making the traced thread block,
+    just blocking directly in your callback has more unwanted effects.  For
+    example, the <constant>CLONE</constant> event callbacks are called when
+    the new child thread has been created but not yet started running; the
+    child can never be scheduled until the <constant>CLONE</constant>
+    tracing callbacks return.  (This allows engines tracing the parent to
+    attach to the child.)  If a <constant>CLONE</constant> event callback
+    blocks the parent thread, it also prevents the child thread from
+    running (even to process a <constant>SIGKILL</constant>).  If what you
+    want is to make both the parent and child block, then use
+    <function>utrace_attach_task</function> on the child and then use
+    <constant>UTRACE_STOP</constant> on both threads.  A more crucial
+    problem with blocking in callbacks is that it can prevent
+    <constant>SIGKILL</constant> from working.  A thread that is blocking
+    due to <constant>UTRACE_STOP</constant> will still wake up and die
+    immediately when sent a <constant>SIGKILL</constant>, as all threads
+    should.  Relying on the <application>utrace</application>
+    infrastructure rather than on private synchronization calls in event
+    callbacks is an important way to help keep tracing robustly
+    noninvasive.
+  </para>
+
+  </sect2>
+
+  <sect2 id="UTRACE_STOP"><title>Using
<constant>UTRACE_STOP</constant></title>
+
+  <para>
+    To control another thread and access its state, it must be stopped
+    with <constant>UTRACE_STOP</constant>.  This means that it is
+    stopped and won't start running again while we access it.  When a
+    thread is not already stopped, <function>utrace_control</function>
+    returns <constant>-EINPROGRESS</constant> and an engine must wait
+    for an event callback when the thread is ready to stop.  The thread
+    may be running on another CPU or may be blocked.  When it is ready
+    to be examined, it will make callbacks to engines that set the
+    <constant>UTRACE_EVENT(QUIESCE)</constant> event bit.  To wake up an
+    interruptible wait, use <constant>UTRACE_INTERRUPT</constant>.
+  </para>
+
+  <para>
+    As long as some engine has used <constant>UTRACE_STOP</constant> and
+    not called <function>utrace_control</function> to resume the thread,
+    then the thread will remain stopped.  <constant>SIGKILL</constant>
+    will wake it up, but it will not run user code.  When the stop is
+    cleared with <function>utrace_control</function> or a callback
+    return value, the thread starts running again.
+    (See also <xref linkend="teardown"/>.)
+  </para>
+
+  </sect2>
+
+  </sect1>
+
+  <sect1 id="teardown"><title>Tear-down Races</title>
+
+  <sect2 id="SIGKILL"><title>Primacy of
<constant>SIGKILL</constant></title>
+  <para>
+    Ordinarily synchronization issues for tracing engines are kept fairly
+    straightforward by using <constant>UTRACE_STOP</constant>.  You ask a
+    thread to stop, and then once it makes the
+    <function>report_quiesce</function> callback it cannot do anything else
+    that would result in another callback, until you let it with a
+    <function>utrace_control</function> call.  This simple arrangement
+    avoids complex and error-prone code in each one of a tracing engine's
+    event callbacks to keep them serialized with the engine's other
+    operations done on that thread from another thread of control.
+    However, giving tracing engines complete power to keep a traced thread
+    stuck in place runs afoul of a more important kind of simplicity that
+    the kernel overall guarantees: nothing can prevent or delay
+    <constant>SIGKILL</constant> from making a thread die and release its
+    resources.  To preserve this important property of
+    <constant>SIGKILL</constant>, it as a special case can break
+    <constant>UTRACE_STOP</constant> like nothing else normally can.  This
+    includes both explicit <constant>SIGKILL</constant> signals and the
+    implicit <constant>SIGKILL</constant> sent to each other thread in the
+    same thread group by a thread doing an exec, or processing a fatal
+    signal, or making an <function>exit_group</function> system call.  A
+    tracing engine can prevent a thread from beginning the exit or exec or
+    dying by signal (other than <constant>SIGKILL</constant>) if it is
+    attached to that thread, but once the operation begins, no tracing
+    engine can prevent or delay all other threads in the same thread group
+    dying.
+  </para>
+  </sect2>
+
+  <sect2 id="reap"><title>Final callbacks</title>
+  <para>
+    The <function>report_reap</function> callback is always the final event
+    in the life cycle of a traced thread.  Tracing engines can use this as
+    the trigger to clean up their own data structures.  The
+    <function>report_death</function> callback is always the penultimate
+    event a tracing engine might see; it's seen unless the thread was
+    already in the midst of dying when the engine attached.  Many tracing
+    engines will have no interest in when a parent reaps a dead process,
+    and nothing they want to do with a zombie thread once it dies; for
+    them, the <function>report_death</function> callback is the natural
+    place to clean up data structures and detach.  To facilitate writing
+    such engines robustly, given the asynchrony of
+    <constant>SIGKILL</constant>, and without error-prone manual
+    implementation of synchronization schemes, the
+    <application>utrace</application> infrastructure provides some special
+    guarantees about the <function>report_death</function> and
+    <function>report_reap</function> callbacks.  It still takes some care
+    to be sure your tracing engine is robust to tear-down races, but these
+    rules make it reasonably straightforward and concise to handle a lot of
+    corner cases correctly.
+  </para>
+  </sect2>
+
+  <sect2 id="refcount"><title>Engine and task
pointers</title>
+  <para>
+    The first sort of guarantee concerns the core data structures
+    themselves.  <structname>struct utrace_engine</structname> is
+    a reference-counted data structure.  While you hold a reference, an
+    engine pointer will always stay valid so that you can safely pass it to
+    any <application>utrace</application> call.  Each call to
+    <function>utrace_attach_task</function> or
+    <function>utrace_attach_pid</function> returns an engine pointer with a
+    reference belonging to the caller.  You own that reference until you
+    drop it using <function>utrace_engine_put</function>.  There is an
+    implicit reference on the engine while it is attached.  So if you drop
+    your only reference, and then use
+    <function>utrace_attach_task</function> without
+    <constant>UTRACE_ATTACH_CREATE</constant> to look up that same engine,
+    you will get the same pointer with a new reference to replace the one
+    you dropped, just like calling <function>utrace_engine_get</function>.
+    When an engine has been detached, either explicitly with
+    <constant>UTRACE_DETACH</constant> or implicitly after
+    <function>report_reap</function>, then any references you hold are all
+    that keep the old engine pointer alive.
+  </para>
+
+  <para>
+    There is nothing a kernel module can do to keep a <structname>struct
+    task_struct</structname> alive outside of
+    <function>rcu_read_lock</function>.  When the task dies and is reaped
+    by its parent (or itself), that structure can be freed so that any
+    dangling pointers you have stored become invalid.
+    <application>utrace</application> will not prevent this, but it can
+    help you detect it safely.  By definition, a task that has been reaped
+    has had all its engines detached.  All
+    <application>utrace</application> calls can be safely called on a
+    detached engine if the caller holds a reference on that engine pointer,
+    even if the task pointer passed in the call is invalid.  All calls
+    return <constant>-ESRCH</constant> for a detached engine, which tells
+    you that the task pointer you passed could be invalid now.  Since
+    <function>utrace_control</function> and
+    <function>utrace_set_events</function> do not block, you can call those
+    inside a <function>rcu_read_lock</function> section and be sure after
+    they don't return <constant>-ESRCH</constant> that the task pointer
is
+    still valid until <function>rcu_read_unlock</function>.  The
+    infrastructure never holds task references of its own.  Though neither
+    <function>rcu_read_lock</function> nor any other lock is held while
+    making a callback, it's always guaranteed that the <structname>struct
+    task_struct</structname> and the <structname>struct
+    utrace_engine</structname> passed as arguments remain valid
+    until the callback function returns.
+  </para>
+
+  <para>
+    The common means for safely holding task pointers that is available to
+    kernel modules is to use <structname>struct pid</structname>, which
+    permits <function>put_pid</function> from kernel modules.  When using
+    that, the calls <function>utrace_attach_pid</function>,
+    <function>utrace_control_pid</function>,
+    <function>utrace_set_events_pid</function>, and
+    <function>utrace_barrier_pid</function> are available.
+  </para>
+  </sect2>
+
+  <sect2 id="reap-after-death">
+    <title>
+      Serialization of <constant>DEATH</constant> and
<constant>REAP</constant>
+    </title>
+    <para>
+      The second guarantee is the serialization of
+      <constant>DEATH</constant> and <constant>REAP</constant>
event
+      callbacks for a given thread.  The actual reaping by the parent
+      (<function>release_task</function> call) can occur simultaneously
+      while the thread is still doing the final steps of dying, including
+      the <function>report_death</function> callback.  If a tracing engine
+      has requested both <constant>DEATH</constant> and
+      <constant>REAP</constant> event reports, it's guaranteed that the
+      <function>report_reap</function> callback will not be made until
+      after the <function>report_death</function> callback has returned.
+      If the <function>report_death</function> callback itself detaches
+      from the thread, then the <function>report_reap</function> callback
+      will never be made.  Thus it is safe for a
+      <function>report_death</function> callback to clean up data
+      structures and detach.
+    </para>
+  </sect2>
+
+  <sect2 id="interlock"><title>Interlock with final
callbacks</title>
+  <para>
+    The final sort of guarantee is that a tracing engine will know for sure
+    whether or not the <function>report_death</function> and/or
+    <function>report_reap</function> callbacks will be made for a certain
+    thread.  These tear-down races are disambiguated by the error return
+    values of <function>utrace_set_events</function> and
+    <function>utrace_control</function>.  Normally
+    <function>utrace_control</function> called with
+    <constant>UTRACE_DETACH</constant> returns zero, and this means that no
+    more callbacks will be made.  If the thread is in the midst of dying,
+    it returns <constant>-EALREADY</constant> to indicate that the
+    <constant>report_death</constant> callback may already be in progress;
+    when you get this error, you know that any cleanup your
+    <function>report_death</function> callback does is about to happen or
+    has just happened--note that if the <function>report_death</function>
+    callback does not detach, the engine remains attached until the thread
+    gets reaped.  If the thread is in the midst of being reaped,
+    <function>utrace_control</function> returns
<constant>-ESRCH</constant>
+    to indicate that the <function>report_reap</function> callback may
+    already be in progress; this means the engine is implicitly detached
+    when the callback completes.  This makes it possible for a tracing
+    engine that has decided asynchronously to detach from a thread to
+    safely clean up its data structures, knowing that no
+    <function>report_death</function> or
<function>report_reap</function>
+    callback will try to do the same.  <constant>utrace_detach</constant>
+    returns <constant>-ESRCH</constant> when the <structname>struct
+    utrace_engine</structname> has already been detached, but is
+    still a valid pointer because of its reference count.  A tracing engine
+    can use this to safely synchronize its own independent multiple threads
+    of control with each other and with its event callbacks that detach.
+  </para>
+
+  <para>
+    In the same vein, <function>utrace_set_events</function> normally
+    returns zero; if the target thread was stopped before the call, then
+    after a successful call, no event callbacks not requested in the new
+    flags will be made.  It fails with <constant>-EALREADY</constant> if
+    you try to clear <constant>UTRACE_EVENT(DEATH)</constant> when the
+    <function>report_death</function> callback may already have begun, or if
+    you try to newly set <constant>UTRACE_EVENT(DEATH)</constant> or
+    <constant>UTRACE_EVENT(QUIESCE)</constant> when the target is already
+    dead or dying.  Like <function>utrace_control</function>, it returns
+    <constant>-ESRCH</constant> when the
<function>report_reap</function>
+    callback may already have begun, or the thread has already been detached
+    (including forcible detach on reaping).  This lets the tracing engine
+    know for sure which event callbacks it will or won't see after
+    <function>utrace_set_events</function> has returned.  By checking for
+    errors, it can know whether to clean up its data structures immediately
+    or to let its callbacks do the work.
+  </para>
+  </sect2>
+
+  <sect2 id="barrier"><title>Using
<function>utrace_barrier</function></title>
+  <para>
+    When a thread is safely stopped, calling
+    <function>utrace_control</function> with
<constant>UTRACE_DETACH</constant>
+    or calling <function>utrace_set_events</function> to disable some events
+    ensures synchronously that your engine won't get any more of the callbacks
+    that have been disabled (none at all when detaching).  But these can also
+    be used while the thread is not stopped, when it might be simultaneously
+    making a callback to your engine.  For this situation, these calls return
+    <constant>-EINPROGRESS</constant> when it's possible a callback is
in
+    progress.  If you are not prepared to have your old callbacks still run,
+    then you can synchronize to be sure all the old callbacks are finished,
+    using <function>utrace_barrier</function>.  This is necessary if the
+    kernel module containing your callback code is going to be unloaded.
+  </para>
+  <para>
+    After using <constant>UTRACE_DETACH</constant> once, further calls to
+    <function>utrace_control</function> with the same engine pointer will
+    return <constant>-ESRCH</constant>.  In contrast, after getting
+    <constant>-EINPROGRESS</constant> from
+    <function>utrace_set_events</function>, you can call
+    <function>utrace_set_events</function> again later and if it returns
zero
+    then know the old callbacks have finished.
+  </para>
+  <para>
+    Unlike all other calls, <function>utrace_barrier</function> (and
+    <function>utrace_barrier_pid</function>) will accept any engine pointer
you
+    hold a reference on, even if <constant>UTRACE_DETACH</constant> has
already
+    been used.  After any <function>utrace_control</function> or
+    <function>utrace_set_events</function> call (these do not block), you
can
+    call <function>utrace_barrier</function> to block until callbacks have
+    finished.  This returns <constant>-ESRCH</constant> only if the engine
is
+    completely detached (finished all callbacks).  Otherwise it waits
+    until the thread is definitely not in the midst of a callback to this
+    engine and then returns zero, but can return
+    <constant>-ERESTARTSYS</constant> if its wait is interrupted.
+  </para>
+  </sect2>
+
+</sect1>
+
+</chapter>
+
+<chapter id="core"><title>utrace core API</title>
+
+<para>
+  The utrace API is declared in
<filename>&lt;linux/utrace.h&gt;</filename>.
+</para>
+
+!Iinclude/linux/utrace.h
+!Ekernel/utrace.c
+
+</chapter>
+
+<chapter id="machine"><title>Machine State</title>
+
+<para>
+  The <function>task_current_syscall</function> function can be used on any
+  valid <structname>struct task_struct</structname> at any time, and does
+  not even require that <function>utrace_attach_task</function> was used at
all.
+</para>
+
+<para>
+  The other ways to access the registers and other machine-dependent state of
+  a task can only be used on a task that is at a known safe point.  The safe
+  points are all the places where <function>utrace_set_events</function> can
+  request callbacks (except for the <constant>DEATH</constant> and
+  <constant>REAP</constant> events).  So at any event callback, it is safe
to
+  examine <varname>current</varname>.
+</para>
+
+<para>
+  One task can examine another only after a callback in the target task that
+  returns <constant>UTRACE_STOP</constant> so that task will not return to
user
+  mode after the safe point.  This guarantees that the task will not resume
+  until the same engine uses <function>utrace_control</function>, unless the
+  task dies suddenly.  To examine safely, one must use a pair of calls to
+  <function>utrace_prepare_examine</function> and
+  <function>utrace_finish_examine</function> surrounding the calls to
+  <structname>struct user_regset</structname> functions or direct
examination
+  of task data structures.  <function>utrace_prepare_examine</function>
returns
+  an error if the task is not properly stopped, or is dead.  After a
+  successful examination, the paired
<function>utrace_finish_examine</function>
+  call returns an error if the task ever woke up during the examination.  If
+  so, any data gathered may be scrambled and should be discarded.  This means
+  there was a spurious wake-up (which should not happen), or a sudden death.
+</para>
+
+<sect1 id="regset"><title><structname>struct
user_regset</structname></title>
+
+<para>
+  The <structname>struct user_regset</structname> API
+  is declared in <filename>&lt;linux/regset.h&gt;</filename>.
+</para>
+
+!Finclude/linux/regset.h
+
+</sect1>
+
+<sect1 id="task_current_syscall">
+  <title><filename>System Call Information</filename></title>
+
+<para>
+  This function is declared in
<filename>&lt;linux/ptrace.h&gt;</filename>.
+</para>
+
+!Elib/syscall.c
+
+</sect1>
+
+<sect1 id="syscall"><title><filename>System Call
Tracing</filename></title>
+
+<para>
+  The arch API for system call information is declared in
+  <filename>&lt;asm/syscall.h&gt;</filename>.
+  Each of these calls can be used only at system call entry tracing,
+  or can be used only at system call exit and the subsequent safe points
+  before returning to user mode.
+  At system call entry tracing means either during a
+  <structfield>report_syscall_entry</structfield> callback,
+  or any time after that callback has returned
<constant>UTRACE_STOP</constant>.
+</para>
+
+!Finclude/asm-generic/syscall.h
+
+</sect1>
+
+</chapter>
+
+<chapter id="internals"><title>Kernel Internals</title>
+
+<para>
+  This chapter covers the interface to the tracing infrastructure
+  from the core of the kernel and the architecture-specific code.
+  This is for maintainers of the kernel and arch code, and not relevant
+  to using the tracing facilities described in preceding chapters.
+</para>
+
+<sect1 id="tracehook"><title>Core Calls In</title>
+
+<para>
+  These calls are declared in
<filename>&lt;linux/tracehook.h&gt;</filename>.
+  The core kernel calls these functions at various important places.
+</para>
+
+!Finclude/linux/tracehook.h
+
+</sect1>
+
+<sect1 id="arch"><title>Architecture Calls Out</title>
+
+<para>
+  An arch that has done all these things sets
+  <constant>CONFIG_HAVE_ARCH_TRACEHOOK</constant>.
+  This is required to enable the <application>utrace</application> code.
+</para>
+
+<sect2
id="arch-ptrace"><title><filename>&lt;asm/ptrace.h&gt;</filename></title>
+
+<para>
+  An arch defines these in <filename>&lt;asm/ptrace.h&gt;</filename>
+  if it supports hardware single-step or block-step features.
+</para>
+
+!Finclude/linux/ptrace.h arch_has_single_step arch_has_block_step
+!Finclude/linux/ptrace.h user_enable_single_step user_enable_block_step
+!Finclude/linux/ptrace.h user_disable_single_step
+
+</sect2>
+
+<sect2 id="arch-syscall">
+ 
<title><filename>&lt;asm/syscall.h&gt;</filename></title>
+
+  <para>
+    An arch provides <filename>&lt;asm/syscall.h&gt;</filename> that
+    defines these as inlines, or declares them as exported functions.
+    These interfaces are described in <xref linkend="syscall"/>.
+  </para>
+
+</sect2>
+
+<sect2 id="arch-tracehook">
+ 
<title><filename>&lt;linux/tracehook.h&gt;</filename></title>
+
+  <para>
+    An arch must define <constant>TIF_NOTIFY_RESUME</constant>
+    and <constant>TIF_SYSCALL_TRACE</constant>
+    in its <filename>&lt;asm/thread_info.h&gt;</filename>.
+    The arch code must call the following functions, all declared
+    in <filename>&lt;linux/tracehook.h&gt;</filename> and
+    described in <xref linkend="tracehook"/>:
+
+    <itemizedlist>
+      <listitem>
+	<para><function>tracehook_notify_resume</function></para>
+      </listitem>
+      <listitem>
+	<para><function>tracehook_report_syscall_entry</function></para>
+      </listitem>
+      <listitem>
+	<para><function>tracehook_report_syscall_exit</function></para>
+      </listitem>
+      <listitem>
+	<para><function>tracehook_signal_handler</function></para>
+      </listitem>
+    </itemizedlist>
+
+  </para>
+
+</sect2>
+
+</sect1>
+
+</chapter>
+
+</book>
diff --git a/fs/proc/array.c b/fs/proc/array.c
index 3a1dafd..f0c0ea2 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -81,6 +81,7 @@
 #include <linux/pid_namespace.h>
 #include <linux/ptrace.h>
 #include <linux/tracehook.h>
+#include <linux/utrace.h>
 
 #include <asm/pgtable.h>
 #include <asm/processor.h>
@@ -192,6 +193,8 @@ static inline void task_state(struct seq_file *m, struct pid_namespace
*ns,
 		cred->uid, cred->euid, cred->suid, cred->fsuid,
 		cred->gid, cred->egid, cred->sgid, cred->fsgid);
 
+	task_utrace_proc_status(m, p);
+
 	task_lock(p);
 	if (p->files)
 		fdt = files_fdtable(p->files);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 68daf4f..4d45f93 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1403,6 +1403,11 @@ struct task_struct {
 #endif
 	seccomp_t seccomp;
 
+#ifdef CONFIG_UTRACE
+	struct utrace *utrace;
+	unsigned long utrace_flags;
+#endif
+
 /* Thread group tracking */
    	u32 parent_exec_id;
    	u32 self_exec_id;
diff --git a/include/linux/utrace.h b/include/linux/utrace.h
new file mode 100644
index 0000000..f251efe
--- /dev/null
+++ b/include/linux/utrace.h
@@ -0,0 +1,692 @@
+/*
+ * utrace infrastructure interface for debugging user processes
+ *
+ * Copyright (C) 2006-2009 Red Hat, Inc.  All rights reserved.
+ *
+ * This copyrighted material is made available to anyone wishing to use,
+ * modify, copy, or redistribute it subject to the terms and conditions
+ * of the GNU General Public License v.2.
+ *
+ * Red Hat Author: Roland McGrath.
+ *
+ * This interface allows for notification of interesting events in a
+ * thread.  It also mediates access to thread state such as registers.
+ * Multiple unrelated users can be associated with a single thread.
+ * We call each of these a tracing engine.
+ *
+ * A tracing engine starts by calling utrace_attach_task() or
+ * utrace_attach_pid() on the chosen thread, passing in a set of hooks
+ * (&struct utrace_engine_ops), and some associated data.  This produces a
+ * &struct utrace_engine, which is the handle used for all other
+ * operations.  An attached engine has its ops vector, its data, and an
+ * event mask controlled by utrace_set_events().
+ *
+ * For each event bit that is set, that engine will get the
+ * appropriate ops->report_*() callback when the event occurs.  The
+ * &struct utrace_engine_ops need not provide callbacks for an event
+ * unless the engine sets one of the associated event bits.
+ */
+
+#ifndef _LINUX_UTRACE_H
+#define _LINUX_UTRACE_H	1
+
+#include <linux/list.h>
+#include <linux/kref.h>
+#include <linux/signal.h>
+#include <linux/sched.h>
+
+struct linux_binprm;
+struct pt_regs;
+struct utrace;
+struct user_regset;
+struct user_regset_view;
+
+/*
+ * Event bits passed to utrace_set_events().
+ * These appear in &struct task_struct.@utrace_flags
+ * and &struct utrace_engine.@flags.
+ */
+enum utrace_events {
+	_UTRACE_EVENT_QUIESCE,	/* Thread is available for examination.  */
+	_UTRACE_EVENT_REAP,  	/* Zombie reaped, no more tracing possible.  */
+	_UTRACE_EVENT_CLONE,	/* Successful clone/fork/vfork just done.  */
+	_UTRACE_EVENT_EXEC,	/* Successful execve just completed.  */
+	_UTRACE_EVENT_EXIT,	/* Thread exit in progress.  */
+	_UTRACE_EVENT_DEATH,	/* Thread has died.  */
+	_UTRACE_EVENT_SYSCALL_ENTRY, /* User entered kernel for system call. */
+	_UTRACE_EVENT_SYSCALL_EXIT, /* Returning to user after system call.  */
+	_UTRACE_EVENT_SIGNAL,	/* Signal delivery will run a user handler.  */
+	_UTRACE_EVENT_SIGNAL_IGN, /* No-op signal to be delivered.  */
+	_UTRACE_EVENT_SIGNAL_STOP, /* Signal delivery will suspend.  */
+	_UTRACE_EVENT_SIGNAL_TERM, /* Signal delivery will terminate.  */
+	_UTRACE_EVENT_SIGNAL_CORE, /* Signal delivery will dump core.  */
+	_UTRACE_EVENT_JCTL,	/* Job control stop or continue completed.  */
+	_UTRACE_NEVENTS
+};
+#define UTRACE_EVENT(type)	(1UL << _UTRACE_EVENT_##type)
+
+/*
+ * All the kinds of signal events.
+ * These all use the @report_signal() callback.
+ */
+#define UTRACE_EVENT_SIGNAL_ALL	(UTRACE_EVENT(SIGNAL) \
+				 | UTRACE_EVENT(SIGNAL_IGN) \
+				 | UTRACE_EVENT(SIGNAL_STOP) \
+				 | UTRACE_EVENT(SIGNAL_TERM) \
+				 | UTRACE_EVENT(SIGNAL_CORE))
+/*
+ * Both kinds of syscall events; these call the @report_syscall_entry()
+ * and @report_syscall_exit() callbacks, respectively.
+ */
+#define UTRACE_EVENT_SYSCALL	\
+	(UTRACE_EVENT(SYSCALL_ENTRY) | UTRACE_EVENT(SYSCALL_EXIT))
+
+/*
+ * The event reports triggered synchronously by task death.
+ */
+#define _UTRACE_DEATH_EVENTS (UTRACE_EVENT(DEATH) | UTRACE_EVENT(QUIESCE))
+
+/*
+ * Hooks in <linux/tracehook.h> call these entry points to the utrace dispatch.
+ */
+void utrace_free_task(struct task_struct *);
+bool utrace_interrupt_pending(void);
+void utrace_resume(struct task_struct *, struct pt_regs *);
+void utrace_finish_stop(void);
+void utrace_maybe_reap(struct task_struct *, struct utrace *, bool);
+int utrace_get_signal(struct task_struct *, struct pt_regs *,
+		      siginfo_t *, struct k_sigaction *);
+void utrace_report_clone(unsigned long, struct task_struct *);
+void utrace_finish_vfork(struct task_struct *);
+void utrace_report_exit(long *exit_code);
+void utrace_report_death(struct task_struct *, struct utrace *, bool, int);
+void utrace_report_jctl(int notify, int type);
+void utrace_report_exec(struct linux_binfmt *, struct linux_binprm *,
+			struct pt_regs *regs);
+bool utrace_report_syscall_entry(struct pt_regs *);
+void utrace_report_syscall_exit(struct pt_regs *);
+void utrace_signal_handler(struct task_struct *, int);
+
+#ifndef CONFIG_UTRACE
+
+/*
+ * <linux/tracehook.h> uses these accessors to avoid #ifdef CONFIG_UTRACE.
+ */
+static inline unsigned long task_utrace_flags(struct task_struct *task)
+{
+	return 0;
+}
+static inline struct utrace *task_utrace_struct(struct task_struct *task)
+{
+	return NULL;
+}
+static inline void utrace_init_task(struct task_struct *child)
+{
+}
+
+static inline void task_utrace_proc_status(struct seq_file *m,
+					   struct task_struct *p)
+{
+}
+
+#else  /* CONFIG_UTRACE */
+
+static inline unsigned long task_utrace_flags(struct task_struct *task)
+{
+	return task->utrace_flags;
+}
+
+static inline struct utrace *task_utrace_struct(struct task_struct *task)
+{
+	struct utrace *utrace;
+
+	/*
+	 * This barrier ensures that any prior load of task->utrace_flags
+	 * is ordered before this load of task->utrace.  We use those
+	 * utrace_flags checks in the hot path to decide to call into
+	 * the utrace code.  The first attach installs task->utrace before
+	 * setting task->utrace_flags nonzero with implicit barrier in
+	 * between, see utrace_add_engine().
+	 */
+	smp_rmb();
+	utrace = task->utrace;
+
+	smp_read_barrier_depends(); /* See utrace_task_alloc().  */
+	return utrace;
+}
+
+static inline void utrace_init_task(struct task_struct *task)
+{
+	task->utrace_flags = 0;
+	task->utrace = NULL;
+}
+
+void task_utrace_proc_status(struct seq_file *m, struct task_struct *p);
+
+
+/*
+ * Version number of the API defined in this file.  This will change
+ * whenever a tracing engine's code would need some updates to keep
+ * working.  We maintain this here for the benefit of tracing engine code
+ * that is developed concurrently with utrace API improvements before they
+ * are merged into the kernel, making LINUX_VERSION_CODE checks unwieldy.
+ */
+#define UTRACE_API_VERSION	20091216
+
+/**
+ * enum utrace_resume_action - engine's choice of action for a traced task
+ * @UTRACE_STOP:		Stay quiescent after callbacks.
+ * @UTRACE_INTERRUPT:		Make @report_signal() callback soon.
+ * @UTRACE_REPORT:		Make some callback soon.
+ * @UTRACE_SINGLESTEP:		Resume in user mode for one instruction.
+ * @UTRACE_BLOCKSTEP:		Resume in user mode until next branch.
+ * @UTRACE_RESUME:		Resume normally in user mode.
+ * @UTRACE_DETACH:		Detach my engine (implies %UTRACE_RESUME).
+ *
+ * See utrace_control() for detailed descriptions of each action.  This is
+ * encoded in the @action argument and the return value for every callback
+ * with a &u32 return value.
+ *
+ * The order of these is important.  When there is more than one engine,
+ * each supplies its choice and the smallest value prevails.
+ */
+enum utrace_resume_action {
+	UTRACE_STOP,
+	UTRACE_INTERRUPT,
+	UTRACE_REPORT,
+	UTRACE_SINGLESTEP,
+	UTRACE_BLOCKSTEP,
+	UTRACE_RESUME,
+	UTRACE_DETACH,
+	UTRACE_RESUME_MAX
+};
+#define UTRACE_RESUME_BITS	(ilog2(UTRACE_RESUME_MAX) + 1)
+#define UTRACE_RESUME_MASK	((1 << UTRACE_RESUME_BITS) - 1)
+
+/**
+ * utrace_resume_action - &enum utrace_resume_action from callback action
+ * @action:		&u32 callback @action argument or return value
+ *
+ * This extracts the &enum utrace_resume_action from @action,
+ * which is the @action argument to a &struct utrace_engine_ops
+ * callback or the return value from one.
+ */
+static inline enum utrace_resume_action utrace_resume_action(u32 action)
+{
+	return action & UTRACE_RESUME_MASK;
+}
+
+/**
+ * enum utrace_signal_action - disposition of signal
+ * @UTRACE_SIGNAL_DELIVER:	Deliver according to sigaction.
+ * @UTRACE_SIGNAL_IGN:		Ignore the signal.
+ * @UTRACE_SIGNAL_TERM:		Terminate the process.
+ * @UTRACE_SIGNAL_CORE:		Terminate with core dump.
+ * @UTRACE_SIGNAL_STOP:		Deliver as absolute stop.
+ * @UTRACE_SIGNAL_TSTP:		Deliver as job control stop.
+ * @UTRACE_SIGNAL_REPORT:	Reporting before pending signals.
+ * @UTRACE_SIGNAL_HANDLER:	Reporting after signal handler setup.
+ *
+ * This is encoded in the @action argument and the return value for
+ * a @report_signal() callback.  It says what will happen to the
+ * signal described by the &siginfo_t parameter to the callback.
+ *
+ * The %UTRACE_SIGNAL_REPORT value is used in an @action argument when
+ * a tracing report is being made before dequeuing any pending signal.
+ * If this is immediately after a signal handler has been set up, then
+ * %UTRACE_SIGNAL_HANDLER is used instead.  A @report_signal callback
+ * that uses %UTRACE_SIGNAL_DELIVER|%UTRACE_SINGLESTEP will ensure
+ * it sees a %UTRACE_SIGNAL_HANDLER report.
+ */
+enum utrace_signal_action {
+	UTRACE_SIGNAL_DELIVER	= 0x00,
+	UTRACE_SIGNAL_IGN	= 0x10,
+	UTRACE_SIGNAL_TERM	= 0x20,
+	UTRACE_SIGNAL_CORE	= 0x30,
+	UTRACE_SIGNAL_STOP	= 0x40,
+	UTRACE_SIGNAL_TSTP	= 0x50,
+	UTRACE_SIGNAL_REPORT	= 0x60,
+	UTRACE_SIGNAL_HANDLER	= 0x70
+};
+#define	UTRACE_SIGNAL_MASK	0xf0
+#define UTRACE_SIGNAL_HOLD	0x100 /* Flag, push signal back on queue.  */
+
+/**
+ * utrace_signal_action - &enum utrace_signal_action from callback action
+ * @action:		@report_signal callback @action argument or return value
+ *
+ * This extracts the &enum utrace_signal_action from @action, which
+ * is the @action argument to a @report_signal callback or the
+ * return value from one.
+ */
+static inline enum utrace_signal_action utrace_signal_action(u32 action)
+{
+	return action & UTRACE_SIGNAL_MASK;
+}
+
+/**
+ * enum utrace_syscall_action - disposition of system call attempt
+ * @UTRACE_SYSCALL_RUN:		Run the system call.
+ * @UTRACE_SYSCALL_ABORT:	Don't run the system call.
+ *
+ * This is encoded in the @action argument and the return value for
+ * a @report_syscall_entry callback.
+ */
+enum utrace_syscall_action {
+	UTRACE_SYSCALL_RUN	= 0x00,
+	UTRACE_SYSCALL_ABORT	= 0x10
+};
+#define	UTRACE_SYSCALL_MASK	0xf0
+#define	UTRACE_SYSCALL_RESUMED	0x100 /* Flag, report_syscall_entry() repeats */
+
+/**
+ * utrace_syscall_action - &enum utrace_syscall_action from callback action
+ * @action:		@report_syscall_entry callback @action or return value
+ *
+ * This extracts the &enum utrace_syscall_action from @action, which
+ * is the @action argument to a @report_syscall_entry callback or the
+ * return value from one.
+ */
+static inline enum utrace_syscall_action utrace_syscall_action(u32 action)
+{
+	return action & UTRACE_SYSCALL_MASK;
+}
+
+/*
+ * Flags for utrace_attach_task() and utrace_attach_pid().
+ */
+#define UTRACE_ATTACH_MATCH_OPS		0x0001 /* Match engines on ops.  */
+#define UTRACE_ATTACH_MATCH_DATA	0x0002 /* Match engines on data.  */
+#define UTRACE_ATTACH_MATCH_MASK	0x000f
+#define UTRACE_ATTACH_CREATE		0x0010 /* Attach a new engine.  */
+#define UTRACE_ATTACH_EXCLUSIVE		0x0020 /* Refuse if existing match.  */
+
+/**
+ * struct utrace_engine - per-engine structure
+ * @ops:	&struct utrace_engine_ops pointer passed to utrace_attach_task()
+ * @data:	engine-private &void * passed to utrace_attach_task()
+ * @flags:	event mask set by utrace_set_events() plus internal flag bits
+ *
+ * The task itself never has to worry about engines detaching while
+ * it's doing event callbacks.  These structures are removed from the
+ * task's active list only when it's stopped, or by the task itself.
+ *
+ * utrace_engine_get() and utrace_engine_put() maintain a reference count.
+ * When it drops to zero, the structure is freed.  One reference is held
+ * implicitly while the engine is attached to its task.
+ */
+struct utrace_engine {
+/* private: */
+	struct kref kref;
+	void (*release)(void *);
+	struct list_head entry;
+
+/* public: */
+	const struct utrace_engine_ops *ops;
+	void *data;
+
+	unsigned long flags;
+};
+
+/**
+ * utrace_engine_get - acquire a reference on a &struct utrace_engine
+ * @engine:	&struct utrace_engine pointer
+ *
+ * You must hold a reference on @engine, and you get another.
+ */
+static inline void utrace_engine_get(struct utrace_engine *engine)
+{
+	kref_get(&engine->kref);
+}
+
+void __utrace_engine_release(struct kref *);
+
+/**
+ * utrace_engine_put - release a reference on a &struct utrace_engine
+ * @engine:	&struct utrace_engine pointer
+ *
+ * You must hold a reference on @engine, and you lose that reference.
+ * If it was the last one, @engine becomes an invalid pointer.
+ */
+static inline void utrace_engine_put(struct utrace_engine *engine)
+{
+	kref_put(&engine->kref, __utrace_engine_release);
+}
+
+/**
+ * struct utrace_engine_ops - tracing engine callbacks
+ *
+ * Each @report_*() callback corresponds to an %UTRACE_EVENT(*) bit.
+ * utrace_set_events() calls on @engine choose which callbacks will
+ * be made to @engine from @task.
+ *
+ * Most callbacks take an @action argument, giving the resume action
+ * chosen by other tracing engines.  All callbacks take an @engine
+ * argument.  The @report_reap callback takes a @task argument that
+ * might or might not be @current.  All other @report_* callbacks
+ * report an event in the @current task.
+ *
+ * For some calls, @action also includes bits specific to that event
+ * and utrace_resume_action() is used to extract the resume action.
+ * This shows what would happen if @engine wasn't there, or will if
+ * the callback's return value uses %UTRACE_RESUME.  This always
+ * starts as %UTRACE_RESUME when no other tracing is being done on
+ * this task.
+ *
+ * All return values contain &enum utrace_resume_action bits.  For
+ * some calls, other bits specific to that kind of event are added to
+ * the resume action bits with OR.  These are the same bits used in
+ * the @action argument.  The resume action returned by a callback
+ * does not override previous engines' choices, it only says what
+ * @engine wants done.  What @current actually does is the action that's
+ * most constrained among the choices made by all attached engines.
+ * See utrace_control() for more information on the actions.
+ *
+ * When %UTRACE_STOP is used in @report_syscall_entry, then @current
+ * stops before attempting the system call.  In this case, another
+ * @report_syscall_entry callback will follow after @current resumes if
+ * %UTRACE_REPORT or %UTRACE_INTERRUPT was returned by some callback
+ * or passed to utrace_control().  In a second or later callback,
+ * %UTRACE_SYSCALL_RESUMED is set in the @action argument to indicate
+ * a repeat callback still waiting to attempt the same system call
+ * invocation.  This repeat callback gives each engine an opportunity
+ * to reexamine registers another engine might have changed while
+ * @current was held in %UTRACE_STOP.
+ *
+ * In other cases, the resume action does not take effect until @current
+ * is ready to check for signals and return to user mode.  If there
+ * are more callbacks to be made, the last round of calls determines
+ * the final action.  A @report_quiesce callback with @event zero, or
+ * a @report_signal callback, will always be the last one made before
+ * @current resumes.  Only %UTRACE_STOP is "sticky"--if @engine returned
+ * %UTRACE_STOP then @current stays stopped unless @engine returns
+ * different from a following callback.
+ *
+ * The report_death() and report_reap() callbacks do not take @action
+ * arguments, and only %UTRACE_DETACH is meaningful in the return value
+ * from a report_death() callback.  None of the resume actions applies
+ * to a dead thread.
+ *
+ * All @report_*() hooks are called with no locks held, in a generally
+ * safe environment when we will be returning to user mode soon (or just
+ * entered the kernel).  It is fine to block for memory allocation and
+ * the like, but all hooks are asynchronous and must not block on
+ * external events!  If you want the thread to block, use %UTRACE_STOP
+ * in your hook's return value; then later wake it up with utrace_control().
+ *
+ * @report_quiesce:
+ *	Requested by %UTRACE_EVENT(%QUIESCE).
+ *	This does not indicate any event, but just that @current is in a
+ *	safe place for examination.  This call is made before each specific
+ *	event callback, except for @report_reap.  The @event argument gives
+ *	the %UTRACE_EVENT(@which) value for the event occurring.  This
+ *	callback might be made for events @engine has not requested, if
+ *	some other engine is tracing the event; calling utrace_set_events()
+ *	call here can request the immediate callback for this occurrence of
+ *	@event.  @event is zero when there is no other event, @current is
+ *	now ready to check for signals and return to user mode, and some
+ *	engine has used %UTRACE_REPORT or %UTRACE_INTERRUPT to request this
+ *	callback.  For this case, if @report_signal is not %NULL, the
+ *	@report_quiesce callback may be replaced with a @report_signal
+ *	callback passing %UTRACE_SIGNAL_REPORT in its @action argument,
+ *	whenever @current is entering the signal-check path anyway.
+ *
+ * @report_signal:
+ *	Requested by %UTRACE_EVENT(%SIGNAL_*) or %UTRACE_EVENT(%QUIESCE).
+ *	Use utrace_signal_action() and utrace_resume_action() on @action.
+ *	The signal action is %UTRACE_SIGNAL_REPORT when some engine has
+ *	used %UTRACE_REPORT or %UTRACE_INTERRUPT; the callback can choose
+ *	to stop or to deliver an artificial signal, before pending signals.
+ *	It's %UTRACE_SIGNAL_HANDLER instead when signal handler setup just
+ *	finished (after a previous %UTRACE_SIGNAL_DELIVER return); this
+ *	serves in lieu of any %UTRACE_SIGNAL_REPORT callback requested by
+ *	%UTRACE_REPORT or %UTRACE_INTERRUPT, and is also implicitly
+ *	requested by %UTRACE_SINGLESTEP or %UTRACE_BLOCKSTEP into the
+ *	signal delivery.  The other signal actions indicate a signal about
+ *	to be delivered; the previous engine's return value sets the signal
+ *	action seen by the the following engine's callback.  The @info data
+ *	can be changed at will, including @info->si_signo.  The settings in
+ *	@return_ka determines what %UTRACE_SIGNAL_DELIVER does.  @orig_ka
+ *	is what was in force before other tracing engines intervened, and
+ *	it's %NULL when this report began as %UTRACE_SIGNAL_REPORT or
+ *	%UTRACE_SIGNAL_HANDLER.  For a report without a new signal, @info
+ *	is left uninitialized and must be set completely by an engine that
+ *	chooses to deliver a signal; if there was a previous @report_signal
+ *	callback ending in %UTRACE_STOP and it was just resumed using
+ *	%UTRACE_REPORT or %UTRACE_INTERRUPT, then @info is left unchanged
+ *	from the previous callback.  In this way, the original signal can
+ *	be left in @info while returning %UTRACE_STOP|%UTRACE_SIGNAL_IGN
+ *	and then found again when resuming with %UTRACE_INTERRUPT.
+ *	The %UTRACE_SIGNAL_HOLD flag bit can be OR'd into the return value,
+ *	and might be in @action if the previous engine returned it.  This
+ *	flag asks that the signal in @info be pushed back on @current's queue
+ *	so that it will be seen again after whatever action is taken now.
+ *
+ * @report_clone:
+ *	Requested by %UTRACE_EVENT(%CLONE).
+ *	Event reported for parent, before the new task @child might run.
+ *	@clone_flags gives the flags used in the clone system call, or
+ *	equivalent flags for a fork() or vfork() system call.  This
+ *	function can use utrace_attach_task() on @child.  Then passing
+ *	%UTRACE_STOP to utrace_control() on @child here keeps the child
+ *	stopped before it ever runs in user mode, %UTRACE_REPORT or
+ *	%UTRACE_INTERRUPT ensures a callback from @child before it
+ *	starts in user mode.
+ *
+ * @report_jctl:
+ *	Requested by %UTRACE_EVENT(%JCTL).
+ *	Job control event; @type is %CLD_STOPPED or %CLD_CONTINUED,
+ *	indicating whether we are stopping or resuming now.  If @notify
+ *	is nonzero, @current is the last thread to stop and so will send
+ *	%SIGCHLD to its parent after this callback; @notify reflects
+ *	what the parent's %SIGCHLD has in @si_code, which can sometimes
+ *	be %CLD_STOPPED even when @type is %CLD_CONTINUED.
+ *
+ * @report_exec:
+ *	Requested by %UTRACE_EVENT(%EXEC).
+ *	An execve system call has succeeded and the new program is about to
+ *	start running.  The initial user register state is handy to be tweaked
+ *	directly in @regs.  @fmt and @bprm gives the details of this exec.
+ *
+ * @report_syscall_entry:
+ *	Requested by %UTRACE_EVENT(%SYSCALL_ENTRY).
+ *	Thread has entered the kernel to request a system call.
+ *	The user register state is handy to be tweaked directly in @regs.
+ *	The @action argument contains an &enum utrace_syscall_action,
+ *	use utrace_syscall_action() to extract it.  The return value
+ *	overrides the last engine's action for the system call.
+ *	If the final action is %UTRACE_SYSCALL_ABORT, no system call
+ *	is made.  The details of the system call being attempted can
+ *	be fetched here with syscall_get_nr() and syscall_get_arguments().
+ *	The parameter registers can be changed with syscall_set_arguments().
+ *	See above about the %UTRACE_SYSCALL_RESUMED flag in @action.
+ *	Use %UTRACE_REPORT in the return value to guarantee you get
+ *	another callback (with %UTRACE_SYSCALL_RESUMED flag) in case
+ *	@current stops with %UTRACE_STOP before attempting the system call.
+ *
+ * @report_syscall_exit:
+ *	Requested by %UTRACE_EVENT(%SYSCALL_EXIT).
+ *	Thread is about to leave the kernel after a system call request.
+ *	The user register state is handy to be tweaked directly in @regs.
+ *	The results of the system call attempt can be examined here using
+ *	syscall_get_error() and syscall_get_return_value().  It is safe
+ *	here to call syscall_set_return_value() or syscall_rollback().
+ *
+ * @report_exit:
+ *	Requested by %UTRACE_EVENT(%EXIT).
+ *	Thread is exiting and cannot be prevented from doing so,
+ *	but all its state is still live.  The @code value will be
+ *	the wait result seen by the parent, and can be changed by
+ *	this engine or others.  The @orig_code value is the real
+ *	status, not changed by any tracing engine.  Returning %UTRACE_STOP
+ *	here keeps @current stopped before it cleans up its state and dies,
+ *	so it can be examined by other processes.  When @current is allowed
+ *	to run, it will die and get to the @report_death callback.
+ *
+ * @report_death:
+ *	Requested by %UTRACE_EVENT(%DEATH).
+ *	Thread is really dead now.  It might be reaped by its parent at
+ *	any time, or self-reap immediately.  Though the actual reaping
+ *	may happen in parallel, a report_reap() callback will always be
+ *	ordered after a report_death() callback.
+ *
+ * @report_reap:
+ *	Requested by %UTRACE_EVENT(%REAP).
+ *	Called when someone reaps the dead task (parent, init, or self).
+ *	This means the parent called wait, or else this was a detached
+ *	thread or a process whose parent ignores SIGCHLD.
+ *	No more callbacks are made after this one.
+ *	The engine is always detached.
+ *	There is nothing more a tracing engine can do about this thread.
+ *	After this callback, the @engine pointer will become invalid.
+ *	The @task pointer may become invalid if get_task_struct() hasn't
+ *	been used to keep it alive.
+ *	An engine should always request this callback if it stores the
+ *	@engine pointer or stores any pointer in @engine->data, so it
+ *	can clean up its data structures.
+ *	Unlike other callbacks, this can be called from the parent's context
+ *	rather than from the traced thread itself--it must not delay the
+ *	parent by blocking.
+ *
+ * @release:
+ *	If not %NULL, this is called after the last utrace_engine_put()
+ *	call for a &struct utrace_engine, which could be implicit after
+ *	a %UTRACE_DETACH return from another callback.  Its argument is
+ *	the engine's @data member.
+ */
+struct utrace_engine_ops {
+	u32 (*report_quiesce)(u32 action, struct utrace_engine *engine,
+			      unsigned long event);
+	u32 (*report_signal)(u32 action, struct utrace_engine *engine,
+			     struct pt_regs *regs,
+			     siginfo_t *info,
+			     const struct k_sigaction *orig_ka,
+			     struct k_sigaction *return_ka);
+	u32 (*report_clone)(u32 action, struct utrace_engine *engine,
+			    unsigned long clone_flags,
+			    struct task_struct *child);
+	u32 (*report_jctl)(u32 action, struct utrace_engine *engine,
+			   int type, int notify);
+	u32 (*report_exec)(u32 action, struct utrace_engine *engine,
+			   const struct linux_binfmt *fmt,
+			   const struct linux_binprm *bprm,
+			   struct pt_regs *regs);
+	u32 (*report_syscall_entry)(u32 action, struct utrace_engine *engine,
+				    struct pt_regs *regs);
+	u32 (*report_syscall_exit)(u32 action, struct utrace_engine *engine,
+				   struct pt_regs *regs);
+	u32 (*report_exit)(u32 action, struct utrace_engine *engine,
+			   long orig_code, long *code);
+	u32 (*report_death)(struct utrace_engine *engine,
+			    bool group_dead, int signal);
+	void (*report_reap)(struct utrace_engine *engine,
+			    struct task_struct *task);
+	void (*release)(void *data);
+};
+
+/**
+ * struct utrace_examiner - private state for using utrace_prepare_examine()
+ *
+ * The members of &struct utrace_examiner are private to the implementation.
+ * This data type holds the state from a call to utrace_prepare_examine()
+ * to be used by a call to utrace_finish_examine().
+ */
+struct utrace_examiner {
+/* private: */
+	long state;
+	unsigned long ncsw;
+};
+
+/*
+ * These are the exported entry points for tracing engines to use.
+ * See kernel/utrace.c for their kerneldoc comments with interface details.
+ */
+struct utrace_engine *utrace_attach_task(struct task_struct *, int,
+					 const struct utrace_engine_ops *,
+					 void *);
+struct utrace_engine *utrace_attach_pid(struct pid *, int,
+					const struct utrace_engine_ops *,
+					void *);
+int __must_check utrace_control(struct task_struct *,
+				struct utrace_engine *,
+				enum utrace_resume_action);
+int __must_check utrace_set_events(struct task_struct *,
+				   struct utrace_engine *,
+				   unsigned long eventmask);
+int __must_check utrace_barrier(struct task_struct *,
+				struct utrace_engine *);
+int __must_check utrace_prepare_examine(struct task_struct *,
+					struct utrace_engine *,
+					struct utrace_examiner *);
+int __must_check utrace_finish_examine(struct task_struct *,
+				       struct utrace_engine *,
+				       struct utrace_examiner *);
+
+/**
+ * utrace_control_pid - control a thread being traced by a tracing engine
+ * @pid:		thread to affect
+ * @engine:		attached engine to affect
+ * @action:		&enum utrace_resume_action for thread to do
+ *
+ * This is the same as utrace_control(), but takes a &struct pid
+ * pointer rather than a &struct task_struct pointer.  The caller must
+ * hold a ref on @pid, but does not need to worry about the task
+ * staying valid.  If it's been reaped so that @pid points nowhere,
+ * then this call returns -%ESRCH.
+ */
+static inline __must_check int utrace_control_pid(
+	struct pid *pid, struct utrace_engine *engine,
+	enum utrace_resume_action action)
+{
+	/*
+	 * We don't bother with rcu_read_lock() here to protect the
+	 * task_struct pointer, because utrace_control will return
+	 * -ESRCH without looking at that pointer if the engine is
+	 * already detached.  A task_struct pointer can't die before
+	 * all the engines are detached in release_task() first.
+	 */
+	struct task_struct *task = pid_task(pid, PIDTYPE_PID);
+	return unlikely(!task) ? -ESRCH : utrace_control(task, engine, action);
+}
+
+/**
+ * utrace_set_events_pid - choose which event reports a tracing engine gets
+ * @pid:		thread to affect
+ * @engine:		attached engine to affect
+ * @eventmask:		new event mask
+ *
+ * This is the same as utrace_set_events(), but takes a &struct pid
+ * pointer rather than a &struct task_struct pointer.  The caller must
+ * hold a ref on @pid, but does not need to worry about the task
+ * staying valid.  If it's been reaped so that @pid points nowhere,
+ * then this call returns -%ESRCH.
+ */
+static inline __must_check int utrace_set_events_pid(
+	struct pid *pid, struct utrace_engine *engine, unsigned long eventmask)
+{
+	struct task_struct *task = pid_task(pid, PIDTYPE_PID);
+	return unlikely(!task) ? -ESRCH :
+		utrace_set_events(task, engine, eventmask);
+}
+
+/**
+ * utrace_barrier_pid - synchronize with simultaneous tracing callbacks
+ * @pid:		thread to affect
+ * @engine:		engine to affect (can be detached)
+ *
+ * This is the same as utrace_barrier(), but takes a &struct pid
+ * pointer rather than a &struct task_struct pointer.  The caller must
+ * hold a ref on @pid, but does not need to worry about the task
+ * staying valid.  If it's been reaped so that @pid points nowhere,
+ * then this call returns -%ESRCH.
+ */
+static inline __must_check int utrace_barrier_pid(struct pid *pid,
+						  struct utrace_engine *engine)
+{
+	struct task_struct *task = pid_task(pid, PIDTYPE_PID);
+	return unlikely(!task) ? -ESRCH : utrace_barrier(task, engine);
+}
+
+#endif	/* CONFIG_UTRACE */
+
+#endif	/* linux/utrace.h */
diff --git a/init/Kconfig b/init/Kconfig
index 43298f9..3f670e5 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -372,6 +372,15 @@ config AUDIT_TREE
 	depends on AUDITSYSCALL
 	select FSNOTIFY
 
+config UTRACE
+	bool "Infrastructure for tracing and debugging user processes"
+	depends on EXPERIMENTAL
+	depends on HAVE_ARCH_TRACEHOOK
+	help
+	  Enable the utrace process tracing interface.  This is an internal
+	  kernel interface exported to kernel modules, to track events in
+	  user threads, extract and change user thread state.
+
 source "kernel/irq/Kconfig"
 
 menu "RCU Subsystem"
diff --git a/kernel/Makefile b/kernel/Makefile
index e898c5b..e43bbfb 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -68,6 +68,7 @@ obj-$(CONFIG_IKCONFIG) += configs.o
 obj-$(CONFIG_RESOURCE_COUNTERS) += res_counter.o
 obj-$(CONFIG_SMP) += stop_machine.o
 obj-$(CONFIG_KPROBES_SANITY_TEST) += test_kprobes.o
+obj-$(CONFIG_UTRACE) += utrace.o
 obj-$(CONFIG_AUDIT) += audit.o auditfilter.o
 obj-$(CONFIG_AUDITSYSCALL) += auditsc.o
 obj-$(CONFIG_AUDIT_WATCH) += audit_watch.o
diff --git a/kernel/utrace.c b/kernel/utrace.c
new file mode 100644
index 0000000..ef856c9
--- /dev/null
+++ b/kernel/utrace.c
@@ -0,0 +1,2440 @@
+/*
+ * utrace infrastructure interface for debugging user processes
+ *
+ * Copyright (C) 2006-2010 Red Hat, Inc.  All rights reserved.
+ *
+ * This copyrighted material is made available to anyone wishing to use,
+ * modify, copy, or redistribute it subject to the terms and conditions
+ * of the GNU General Public License v.2.
+ *
+ * Red Hat Author: Roland McGrath.
+ */
+
+#include <linux/utrace.h>
+#include <linux/tracehook.h>
+#include <linux/regset.h>
+#include <asm/syscall.h>
+#include <linux/ptrace.h>
+#include <linux/err.h>
+#include <linux/sched.h>
+#include <linux/freezer.h>
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/slab.h>
+#include <linux/seq_file.h>
+
+
+/*
+ * Per-thread structure private to utrace implementation.
+ * If task_struct.utrace_flags is nonzero, task_struct.utrace
+ * has always been allocated first.  Once allocated, it is
+ * never freed until free_task().
+ *
+ * The common event reporting loops are done by the task making the
+ * report without ever taking any locks.  To facilitate this, the two
+ * lists @attached and @attaching work together for smooth asynchronous
+ * attaching with low overhead.  Modifying either list requires @lock.
+ * The @attaching list can be modified any time while holding @lock.
+ * New engines being attached always go on this list.
+ *
+ * The @attached list is what the task itself uses for its reporting
+ * loops.  When the task itself is not quiescent, it can use the
+ * @attached list without taking any lock.  Nobody may modify the list
+ * when the task is not quiescent.  When it is quiescent, that means
+ * that it won't run again without taking @lock itself before using
+ * the list.
+ *
+ * At each place where we know the task is quiescent (or it's current),
+ * while holding @lock, we call splice_attaching(), below.  This moves
+ * the @attaching list members on to the end of the @attached list.
+ * Since this happens at the start of any reporting pass, any new
+ * engines attached asynchronously go on the stable @attached list
+ * in time to have their callbacks seen.
+ */
+struct utrace {
+	spinlock_t lock;
+	struct list_head attached, attaching;
+
+	struct task_struct *cloning;
+
+	struct utrace_engine *reporting;
+
+	enum utrace_resume_action resume:UTRACE_RESUME_BITS;
+	unsigned int signal_handler:1;
+	unsigned int vfork_stop:1; /* need utrace_stop() before vfork wait */
+	unsigned int death:1;	/* in utrace_report_death() now */
+	unsigned int reap:1;	/* release_task() has run */
+	unsigned int pending_attach:1; /* need splice_attaching() */
+};
+
+static struct kmem_cache *utrace_cachep;
+static struct kmem_cache *utrace_engine_cachep;
+static const struct utrace_engine_ops utrace_detached_ops; /* forward decl */
+
+static int __init utrace_init(void)
+{
+	utrace_cachep = KMEM_CACHE(utrace, SLAB_PANIC);
+	utrace_engine_cachep = KMEM_CACHE(utrace_engine, SLAB_PANIC);
+	return 0;
+}
+module_init(utrace_init);
+
+/*
+ * Set up @task.utrace for the first time.  We can have races
+ * between two utrace_attach_task() calls here.  The task_lock()
+ * governs installing the new pointer.  If another one got in first,
+ * we just punt the new one we allocated.
+ *
+ * This returns false only in case of a memory allocation failure.
+ */
+static bool utrace_task_alloc(struct task_struct *task)
+{
+	struct utrace *utrace = kmem_cache_zalloc(utrace_cachep, GFP_KERNEL);
+	if (unlikely(!utrace))
+		return false;
+	spin_lock_init(&utrace->lock);
+	INIT_LIST_HEAD(&utrace->attached);
+	INIT_LIST_HEAD(&utrace->attaching);
+	utrace->resume = UTRACE_RESUME;
+	task_lock(task);
+	if (likely(!task->utrace)) {
+		/*
+		 * This barrier makes sure the initialization of the struct
+		 * precedes the installation of the pointer.  This pairs
+		 * with smp_read_barrier_depends() in task_utrace_struct().
+		 */
+		smp_wmb();
+		task->utrace = utrace;
+	}
+	task_unlock(task);
+
+	if (unlikely(task->utrace != utrace))
+		kmem_cache_free(utrace_cachep, utrace);
+	return true;
+}
+
+/*
+ * This is called via tracehook_free_task() from free_task()
+ * when @task is being deallocated.
+ */
+void utrace_free_task(struct task_struct *task)
+{
+	kmem_cache_free(utrace_cachep, task->utrace);
+}
+
+/*
+ * This is calledwhen the task is safely quiescent, i.e. it won't consult
+ * utrace->attached without the lock.  Move any engines attached
+ * asynchronously from @utrace->attaching onto the @utrace->attached list.
+ */
+static void splice_attaching(struct utrace *utrace)
+{
+	lockdep_assert_held(&utrace->lock);
+	list_splice_tail_init(&utrace->attaching, &utrace->attached);
+	utrace->pending_attach = 0;
+}
+
+/*
+ * This is the exported function used by the utrace_engine_put() inline.
+ */
+void __utrace_engine_release(struct kref *kref)
+{
+	struct utrace_engine *engine = container_of(kref, struct utrace_engine,
+						    kref);
+	BUG_ON(!list_empty(&engine->entry));
+	if (engine->release)
+		(*engine->release)(engine->data);
+	kmem_cache_free(utrace_engine_cachep, engine);
+}
+EXPORT_SYMBOL_GPL(__utrace_engine_release);
+
+static bool engine_matches(struct utrace_engine *engine, int flags,
+			   const struct utrace_engine_ops *ops, void *data)
+{
+	if ((flags & UTRACE_ATTACH_MATCH_OPS) && engine->ops != ops)
+		return false;
+	if ((flags & UTRACE_ATTACH_MATCH_DATA) && engine->data != data)
+		return false;
+	return engine->ops && engine->ops != &utrace_detached_ops;
+}
+
+static struct utrace_engine *find_matching_engine(
+	struct utrace *utrace, int flags,
+	const struct utrace_engine_ops *ops, void *data)
+{
+	struct utrace_engine *engine;
+	list_for_each_entry(engine, &utrace->attached, entry)
+		if (engine_matches(engine, flags, ops, data))
+			return engine;
+	list_for_each_entry(engine, &utrace->attaching, entry)
+		if (engine_matches(engine, flags, ops, data))
+			return engine;
+	return NULL;
+}
+
+/*
+ * Enqueue @engine, or maybe don't if UTRACE_ATTACH_EXCLUSIVE.
+ */
+static int utrace_add_engine(struct task_struct *target,
+			     struct utrace *utrace,
+			     struct utrace_engine *engine,
+			     int flags,
+			     const struct utrace_engine_ops *ops,
+			     void *data)
+{
+	int ret;
+
+	spin_lock(&utrace->lock);
+
+	ret = -EEXIST;
+	if ((flags & UTRACE_ATTACH_EXCLUSIVE) &&
+	     unlikely(find_matching_engine(utrace, flags, ops, data)))
+		goto unlock;
+
+	/*
+	 * In case we had no engines before, make sure that
+	 * utrace_flags is not zero. Since we did unlock+lock
+	 * at least once after utrace_task_alloc() installed
+	 * ->utrace, we have the necessary barrier which pairs
+	 * with rmb() in task_utrace_struct().
+	 */
+	ret = -ESRCH;
+	if (!target->utrace_flags) {
+		target->utrace_flags = UTRACE_EVENT(REAP);
+		/*
+		 * If we race with tracehook_prepare_release_task()
+		 * make sure that either it sees utrace_flags != 0
+		 * or we see exit_state == EXIT_DEAD.
+		 */
+		smp_mb();
+		if (unlikely(target->exit_state == EXIT_DEAD)) {
+			target->utrace_flags = 0;
+			goto unlock;
+		}
+	}
+
+	/*
+	 * Put the new engine on the pending ->attaching list.
+	 * Make sure it gets onto the ->attached list by the next
+	 * time it's examined.  Setting ->pending_attach ensures
+	 * that start_report() takes the lock and splices the lists
+	 * before the next new reporting pass.
+	 *
+	 * When target == current, it would be safe just to call
+	 * splice_attaching() right here.  But if we're inside a
+	 * callback, that would mean the new engine also gets
+	 * notified about the event that precipitated its own
+	 * creation.  This is not what the user wants.
+	 */
+	list_add_tail(&engine->entry, &utrace->attaching);
+	utrace->pending_attach = 1;
+	utrace_engine_get(engine);
+	ret = 0;
+unlock:
+	spin_unlock(&utrace->lock);
+
+	return ret;
+}
+
+/**
+ * utrace_attach_task - attach new engine, or look up an attached engine
+ * @target:	thread to attach to
+ * @flags:	flag bits combined with OR, see below
+ * @ops:	callback table for new engine
+ * @data:	engine private data pointer
+ *
+ * The caller must ensure that the @target thread does not get freed,
+ * i.e. hold a ref or be its parent.  It is always safe to call this
+ * on @current, or on the @child pointer in a @report_clone callback.
+ * For most other cases, it's easier to use utrace_attach_pid() instead.
+ *
+ * UTRACE_ATTACH_CREATE:
+ * Create a new engine.  If %UTRACE_ATTACH_CREATE is not specified, you
+ * only look up an existing engine already attached to the thread.
+ *
+ * UTRACE_ATTACH_EXCLUSIVE:
+ * Attempting to attach a second (matching) engine fails with -%EEXIST.
+ *
+ * UTRACE_ATTACH_MATCH_OPS: Only consider engines matching @ops.
+ * UTRACE_ATTACH_MATCH_DATA: Only consider engines matching @data.
+ *
+ * Calls with neither %UTRACE_ATTACH_MATCH_OPS nor %UTRACE_ATTACH_MATCH_DATA
+ * match the first among any engines attached to @target.  That means that
+ * %UTRACE_ATTACH_EXCLUSIVE in such a call fails with -%EEXIST if there
+ * are any engines on @target at all.
+ */
+struct utrace_engine *utrace_attach_task(
+	struct task_struct *target, int flags,
+	const struct utrace_engine_ops *ops, void *data)
+{
+	struct utrace *utrace = task_utrace_struct(target);
+	struct utrace_engine *engine;
+	int ret;
+
+	if (!(flags & UTRACE_ATTACH_CREATE)) {
+		if (unlikely(!utrace))
+			return ERR_PTR(-ENOENT);
+		spin_lock(&utrace->lock);
+		engine = find_matching_engine(utrace, flags, ops, data);
+		if (engine)
+			utrace_engine_get(engine);
+		spin_unlock(&utrace->lock);
+		return engine ?: ERR_PTR(-ENOENT);
+	}
+
+	if (unlikely(!ops) || unlikely(ops == &utrace_detached_ops))
+		return ERR_PTR(-EINVAL);
+
+	if (unlikely(target->flags & PF_KTHREAD))
+		/*
+		 * Silly kernel, utrace is for users!
+		 */
+		return ERR_PTR(-EPERM);
+
+	if (!utrace) {
+		if (unlikely(!utrace_task_alloc(target)))
+			return ERR_PTR(-ENOMEM);
+		utrace = task_utrace_struct(target);
+	}
+
+	engine = kmem_cache_alloc(utrace_engine_cachep, GFP_KERNEL);
+	if (unlikely(!engine))
+		return ERR_PTR(-ENOMEM);
+
+	/*
+	 * Initialize the new engine structure.  It starts out with one ref
+	 * to return.  utrace_add_engine() adds another for being attached.
+	 */
+	kref_init(&engine->kref);
+	engine->flags = 0;
+	engine->ops = ops;
+	engine->data = data;
+	engine->release = ops->release;
+
+	ret = utrace_add_engine(target, utrace, engine, flags, ops, data);
+
+	if (unlikely(ret)) {
+		kmem_cache_free(utrace_engine_cachep, engine);
+		engine = ERR_PTR(ret);
+	}
+
+
+	return engine;
+}
+EXPORT_SYMBOL_GPL(utrace_attach_task);
+
+/**
+ * utrace_attach_pid - attach new engine, or look up an attached engine
+ * @pid:	&struct pid pointer representing thread to attach to
+ * @flags:	flag bits combined with OR, see utrace_attach_task()
+ * @ops:	callback table for new engine
+ * @data:	engine private data pointer
+ *
+ * This is the same as utrace_attach_task(), but takes a &struct pid
+ * pointer rather than a &struct task_struct pointer.  The caller must
+ * hold a ref on @pid, but does not need to worry about the task
+ * staying valid.  If it's been reaped so that @pid points nowhere,
+ * then this call returns -%ESRCH.
+ */
+struct utrace_engine *utrace_attach_pid(
+	struct pid *pid, int flags,
+	const struct utrace_engine_ops *ops, void *data)
+{
+	struct utrace_engine *engine = ERR_PTR(-ESRCH);
+	struct task_struct *task = get_pid_task(pid, PIDTYPE_PID);
+	if (task) {
+		engine = utrace_attach_task(task, flags, ops, data);
+		put_task_struct(task);
+	}
+	return engine;
+}
+EXPORT_SYMBOL_GPL(utrace_attach_pid);
+
+/*
+ * When an engine is detached, the target thread may still see it and
+ * make callbacks until it quiesces.  We install a special ops vector
+ * with these two callbacks.  When the target thread quiesces, it can
+ * safely free the engine itself.  For any event we will always get
+ * the report_quiesce() callback first, so we only need this one
+ * pointer to be set.  The only exception is report_reap(), so we
+ * supply that callback too.
+ */
+static u32 utrace_detached_quiesce(u32 action, struct utrace_engine *engine,
+				   unsigned long event)
+{
+	return UTRACE_DETACH;
+}
+
+static void utrace_detached_reap(struct utrace_engine *engine,
+				 struct task_struct *task)
+{
+}
+
+static const struct utrace_engine_ops utrace_detached_ops = {
+	.report_quiesce = &utrace_detached_quiesce,
+	.report_reap = &utrace_detached_reap
+};
+
+/*
+ * The caller has to hold a ref on the engine.  If the attached flag is
+ * true (all but utrace_barrier() calls), the engine is supposed to be
+ * attached.  If the attached flag is false (utrace_barrier() only),
+ * then return -ERESTARTSYS for an engine marked for detach but not yet
+ * fully detached.  The task pointer can be invalid if the engine is
+ * detached.
+ *
+ * Get the utrace lock for the target task.
+ * Returns the struct if locked, or ERR_PTR(-errno).
+ *
+ * This has to be robust against races with:
+ *	utrace_control(target, UTRACE_DETACH) calls
+ *	UTRACE_DETACH after reports
+ *	utrace_report_death
+ *	utrace_release_task
+ */
+static struct utrace *get_utrace_lock(struct task_struct *target,
+				      struct utrace_engine *engine,
+				      bool attached)
+	__acquires(utrace->lock)
+{
+	struct utrace *utrace;
+
+	rcu_read_lock();
+
+	/*
+	 * If this engine was already detached, bail out before we look at
+	 * the task_struct pointer at all.  If it's detached after this
+	 * check, then RCU is still keeping this task_struct pointer valid.
+	 *
+	 * The ops pointer is NULL when the engine is fully detached.
+	 * It's &utrace_detached_ops when it's marked detached but still
+	 * on the list.  In the latter case, utrace_barrier() still works,
+	 * since the target might be in the middle of an old callback.
+	 */
+	if (unlikely(!engine->ops)) {
+		rcu_read_unlock();
+		return ERR_PTR(-ESRCH);
+	}
+
+	if (unlikely(engine->ops == &utrace_detached_ops)) {
+		rcu_read_unlock();
+		return attached ? ERR_PTR(-ESRCH) : ERR_PTR(-ERESTARTSYS);
+	}
+
+	utrace = task_utrace_struct(target);
+	spin_lock(&utrace->lock);
+	if (unlikely(utrace->reap) || unlikely(!engine->ops) ||
+	    unlikely(engine->ops == &utrace_detached_ops)) {
+		/*
+		 * By the time we got the utrace lock,
+		 * it had been reaped or detached already.
+		 */
+		spin_unlock(&utrace->lock);
+		utrace = ERR_PTR(-ESRCH);
+		if (!attached && engine->ops == &utrace_detached_ops)
+			utrace = ERR_PTR(-ERESTARTSYS);
+	}
+	rcu_read_unlock();
+
+	return utrace;
+}
+
+/*
+ * Now that we don't hold any locks, run through any
+ * detached engines and free their references.  Each
+ * engine had one implicit ref while it was attached.
+ */
+static void put_detached_list(struct list_head *list)
+{
+	struct utrace_engine *engine, *next;
+	list_for_each_entry_safe(engine, next, list, entry) {
+		list_del_init(&engine->entry);
+		utrace_engine_put(engine);
+	}
+}
+
+/*
+ * We use an extra bit in utrace_engine.flags past the event bits,
+ * to record whether the engine is keeping the target thread stopped.
+ *
+ * This bit is set in task_struct.utrace_flags whenever it is set in any
+ * engine's flags.  Only utrace_reset() resets it in utrace_flags.
+ */
+#define ENGINE_STOP		(1UL << _UTRACE_NEVENTS)
+
+static void mark_engine_wants_stop(struct task_struct *task,
+				   struct utrace_engine *engine)
+{
+	engine->flags |= ENGINE_STOP;
+	task->utrace_flags |= ENGINE_STOP;
+}
+
+static void clear_engine_wants_stop(struct utrace_engine *engine)
+{
+	engine->flags &= ~ENGINE_STOP;
+}
+
+static bool engine_wants_stop(struct utrace_engine *engine)
+{
+	return (engine->flags & ENGINE_STOP) != 0;
+}
+
+/**
+ * utrace_set_events - choose which event reports a tracing engine gets
+ * @target:		thread to affect
+ * @engine:		attached engine to affect
+ * @events:		new event mask
+ *
+ * This changes the set of events for which @engine wants callbacks made.
+ *
+ * This fails with -%EALREADY and does nothing if you try to clear
+ * %UTRACE_EVENT(%DEATH) when the @report_death callback may already have
+ * begun, or if you try to newly set %UTRACE_EVENT(%DEATH) or
+ * %UTRACE_EVENT(%QUIESCE) when @target is already dead or dying.
+ *
+ * This fails with -%ESRCH if you try to clear %UTRACE_EVENT(%REAP) when
+ * the @report_reap callback may already have begun, or when @target has
+ * already been detached, including forcible detach on reaping.
+ *
+ * If @target was stopped before the call, then after a successful call,
+ * no event callbacks not requested in @events will be made; if
+ * %UTRACE_EVENT(%QUIESCE) is included in @events, then a
+ * @report_quiesce callback will be made when @target resumes.
+ *
+ * If @target was not stopped and @events excludes some bits that were
+ * set before, this can return -%EINPROGRESS to indicate that @target
+ * may have been making some callback to @engine.  When this returns
+ * zero, you can be sure that no event callbacks you've disabled in
+ * @events can be made.  If @events only sets new bits that were not set
+ * before on @engine, then -%EINPROGRESS will never be returned.
+ *
+ * To synchronize after an -%EINPROGRESS return, see utrace_barrier().
+ *
+ * When @target is @current, -%EINPROGRESS is not returned.  But note
+ * that a newly-created engine will not receive any callbacks related to
+ * an event notification already in progress.  This call enables @events
+ * callbacks to be made as soon as @engine becomes eligible for any
+ * callbacks, see utrace_attach_task().
+ *
+ * These rules provide for coherent synchronization based on %UTRACE_STOP,
+ * even when %SIGKILL is breaking its normal simple rules.
+ */
+int utrace_set_events(struct task_struct *target,
+		      struct utrace_engine *engine,
+		      unsigned long events)
+{
+	struct utrace *utrace;
+	unsigned long old_flags, old_utrace_flags;
+	int ret = -EALREADY;
+
+	/*
+	 * We just ignore the internal bit, so callers can use
+	 * engine->flags to seed bitwise ops for our argument.
+	 */
+	events &= ~ENGINE_STOP;
+
+	utrace = get_utrace_lock(target, engine, true);
+	if (unlikely(IS_ERR(utrace)))
+		return PTR_ERR(utrace);
+
+	old_utrace_flags = target->utrace_flags;
+	old_flags = engine->flags & ~ENGINE_STOP;
+
+	/*
+	 * If utrace_report_death() is already progress now,
+	 * it's too late to clear the death event bits.
+	 */
+	if (((old_flags & ~events) & _UTRACE_DEATH_EVENTS) && utrace->death)
+		goto unlock;
+
+	/*
+	 * When setting these flags, it's essential that we really
+	 * synchronize with exit_notify().  They cannot be set after
+	 * exit_notify() takes the tasklist_lock.  By holding the read
+	 * lock here while setting the flags, we ensure that the calls
+	 * to tracehook_notify_death() and tracehook_report_death() will
+	 * see the new flags.  This ensures that utrace_release_task()
+	 * knows positively that utrace_report_death() will be called or
+	 * that it won't.
+	 */
+	if ((events & ~old_flags) & _UTRACE_DEATH_EVENTS) {
+		read_lock(&tasklist_lock);
+		if (unlikely(target->exit_state)) {
+			read_unlock(&tasklist_lock);
+			goto unlock;
+		}
+		target->utrace_flags |= events;
+		read_unlock(&tasklist_lock);
+	}
+
+	engine->flags = events | (engine->flags & ENGINE_STOP);
+	target->utrace_flags |= events;
+
+	if ((events & UTRACE_EVENT_SYSCALL) &&
+	    !(old_utrace_flags & UTRACE_EVENT_SYSCALL))
+		set_tsk_thread_flag(target, TIF_SYSCALL_TRACE);
+
+	ret = 0;
+	if ((old_flags & ~events) && target != current &&
+	    !task_is_stopped_or_traced(target) && !target->exit_state) {
+		/*
+		 * This barrier ensures that our engine->flags changes
+		 * have hit before we examine utrace->reporting,
+		 * pairing with the barrier in start_callback().  If
+		 * @target has not yet hit finish_callback() to clear
+		 * utrace->reporting, we might be in the middle of a
+		 * callback to @engine.
+		 */
+		smp_mb();
+		if (utrace->reporting == engine)
+			ret = -EINPROGRESS;
+	}
+unlock:
+	spin_unlock(&utrace->lock);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(utrace_set_events);
+
+/*
+ * Asynchronously mark an engine as being detached.
+ *
+ * This must work while the target thread races with us doing
+ * start_callback(), defined below.  It uses smp_rmb() between checking
+ * @engine->flags and using @engine->ops.  Here we change @engine->ops
+ * first, then use smp_wmb() before changing @engine->flags.  This ensures
+ * it can check the old flags before using the old ops, or check the old
+ * flags before using the new ops, or check the new flags before using the
+ * new ops, but can never check the new flags before using the old ops.
+ * Hence, utrace_detached_ops might be used with any old flags in place.
+ * It has report_quiesce() and report_reap() callbacks to handle all cases.
+ */
+static void mark_engine_detached(struct utrace_engine *engine)
+{
+	engine->ops = &utrace_detached_ops;
+	smp_wmb();
+	engine->flags = UTRACE_EVENT(QUIESCE);
+}
+
+/*
+ * Get @target to stop and return true if it is already stopped now.
+ * If we return false, it will make some event callback soonish.
+ * Called with @utrace locked.
+ */
+static bool utrace_do_stop(struct task_struct *target, struct utrace *utrace)
+{
+	if (task_is_stopped(target)) {
+		/*
+		 * Stopped is considered quiescent; when it wakes up, it will
+		 * go through utrace_finish_stop() before doing anything else.
+		 */
+		spin_lock_irq(&target->sighand->siglock);
+		if (likely(task_is_stopped(target)))
+			__set_task_state(target, TASK_TRACED);
+		spin_unlock_irq(&target->sighand->siglock);
+	} else if (utrace->resume > UTRACE_REPORT) {
+		utrace->resume = UTRACE_REPORT;
+		set_notify_resume(target);
+	}
+
+	return task_is_traced(target);
+}
+
+/*
+ * If the target is not dead it should not be in tracing
+ * stop any more.  Wake it unless it's in job control stop.
+ */
+static void utrace_wakeup(struct task_struct *target, struct utrace *utrace)
+{
+	lockdep_assert_held(&utrace->lock);
+	spin_lock_irq(&target->sighand->siglock);
+	if (target->signal->flags & SIGNAL_STOP_STOPPED ||
+	    target->signal->group_stop_count)
+		target->state = TASK_STOPPED;
+	else
+		wake_up_state(target, __TASK_TRACED);
+	spin_unlock_irq(&target->sighand->siglock);
+}
+
+/*
+ * This is called when there might be some detached engines on the list or
+ * some stale bits in @task->utrace_flags.  Clean them up and recompute the
+ * flags.  Returns true if we're now fully detached.
+ *
+ * Called with @utrace->lock held, returns with it released.
+ * After this returns, @utrace might be freed if everything detached.
+ */
+static bool utrace_reset(struct task_struct *task, struct utrace *utrace)
+	__releases(utrace->lock)
+{
+	struct utrace_engine *engine, *next;
+	unsigned long flags = 0;
+	LIST_HEAD(detached);
+
+	splice_attaching(utrace);
+
+	/*
+	 * Update the set of events of interest from the union
+	 * of the interests of the remaining tracing engines.
+	 * For any engine marked detached, remove it from the list.
+	 * We'll collect them on the detached list.
+	 */
+	list_for_each_entry_safe(engine, next, &utrace->attached, entry) {
+		if (engine->ops == &utrace_detached_ops) {
+			engine->ops = NULL;
+			list_move(&engine->entry, &detached);
+		} else {
+			flags |= engine->flags | UTRACE_EVENT(REAP);
+		}
+	}
+
+	if (task->exit_state) {
+		/*
+		 * Once it's already dead, we never install any flags
+		 * except REAP.  When ->exit_state is set and events
+		 * like DEATH are not set, then they never can be set.
+		 * This ensures that utrace_release_task() knows
+		 * positively that utrace_report_death() can never run.
+		 */
+		BUG_ON(utrace->death);
+		flags &= UTRACE_EVENT(REAP);
+	} else if (!(flags & UTRACE_EVENT_SYSCALL) &&
+		   test_tsk_thread_flag(task, TIF_SYSCALL_TRACE)) {
+		clear_tsk_thread_flag(task, TIF_SYSCALL_TRACE);
+	}
+
+	if (!flags) {
+		/*
+		 * No more engines, cleared out the utrace.
+		 */
+		utrace->resume = UTRACE_RESUME;
+		utrace->signal_handler = 0;
+	}
+
+	/*
+	 * If no more engines want it stopped, wake it up.
+	 */
+	if (task_is_traced(task) && !(flags & ENGINE_STOP)) {
+		/*
+		 * It just resumes, so make sure single-step
+		 * is not left set.
+		 */
+		if (utrace->resume == UTRACE_RESUME)
+			user_disable_single_step(task);
+		utrace_wakeup(task, utrace);
+	}
+
+	/*
+	 * In theory spin_lock() doesn't imply rcu_read_lock().
+	 * Once we clear ->utrace_flags this task_struct can go away
+	 * because tracehook_prepare_release_task() path does not take
+	 * utrace->lock when ->utrace_flags == 0.
+	 */
+	rcu_read_lock();
+	task->utrace_flags = flags;
+	spin_unlock(&utrace->lock);
+	rcu_read_unlock();
+
+	put_detached_list(&detached);
+
+	return !flags;
+}
+
+void utrace_finish_stop(void)
+{
+	/*
+	 * If we were task_is_traced() and then SIGKILL'ed, make
+	 * sure we do nothing until the tracer drops utrace->lock.
+	 */
+	if (unlikely(__fatal_signal_pending(current))) {
+		struct utrace *utrace = task_utrace_struct(current);
+		spin_unlock_wait(&utrace->lock);
+	}
+}
+
+/*
+ * Perform %UTRACE_STOP, i.e. block in TASK_TRACED until woken up.
+ * @task == current, @utrace == current->utrace, which is not locked.
+ * Return true if we were woken up by SIGKILL even though some utrace
+ * engine may still want us to stay stopped.
+ */
+static void utrace_stop(struct task_struct *task, struct utrace *utrace,
+			enum utrace_resume_action action)
+{
+relock:
+	spin_lock(&utrace->lock);
+
+	if (action < utrace->resume) {
+		/*
+		 * Ensure a reporting pass when we're resumed.
+		 */
+		utrace->resume = action;
+		if (action == UTRACE_INTERRUPT)
+			set_thread_flag(TIF_SIGPENDING);
+		else
+			set_thread_flag(TIF_NOTIFY_RESUME);
+	}
+
+	/*
+	 * If the ENGINE_STOP bit is clear in utrace_flags, that means
+	 * utrace_reset() ran after we processed some UTRACE_STOP return
+	 * values from callbacks to get here.  If all engines have detached
+	 * or resumed us, we don't stop.  This check doesn't require
+	 * siglock, but it should follow the interrupt/report bookkeeping
+	 * steps (this can matter for UTRACE_RESUME but not UTRACE_DETACH).
+	 */
+	if (unlikely(!(task->utrace_flags & ENGINE_STOP))) {
+		utrace_reset(task, utrace);
+		if (task->utrace_flags & ENGINE_STOP)
+			goto relock;
+		return;
+	}
+
+	/*
+	 * The siglock protects us against signals.  As well as SIGKILL
+	 * waking us up, we must synchronize with the signal bookkeeping
+	 * for stop signals and SIGCONT.
+	 */
+	spin_lock_irq(&task->sighand->siglock);
+
+	if (unlikely(__fatal_signal_pending(task))) {
+		spin_unlock_irq(&task->sighand->siglock);
+		spin_unlock(&utrace->lock);
+		return;
+	}
+
+	__set_current_state(TASK_TRACED);
+
+	/*
+	 * If there is a group stop in progress,
+	 * we must participate in the bookkeeping.
+	 */
+	if (unlikely(task->signal->group_stop_count) &&
+			!--task->signal->group_stop_count)
+		task->signal->flags = SIGNAL_STOP_STOPPED;
+
+	spin_unlock_irq(&task->sighand->siglock);
+	spin_unlock(&utrace->lock);
+
+	schedule();
+
+	utrace_finish_stop();
+
+	/*
+	 * While in TASK_TRACED, we were considered "frozen enough".
+	 * Now that we woke up, it's crucial if we're supposed to be
+	 * frozen that we freeze now before running anything substantial.
+	 */
+	try_to_freeze();
+
+	/*
+	 * While we were in TASK_TRACED, complete_signal() considered
+	 * us "uninterested" in signal wakeups.  Now make sure our
+	 * TIF_SIGPENDING state is correct for normal running.
+	 */
+	spin_lock_irq(&task->sighand->siglock);
+	recalc_sigpending();
+	spin_unlock_irq(&task->sighand->siglock);
+}
+
+/*
+ * Called by release_task() with @reap set to true.
+ * Called by utrace_report_death() with @reap set to false.
+ * On reap, make report_reap callbacks and clean out @utrace
+ * unless still making callbacks.  On death, update bookkeeping
+ * and handle the reap work if release_task() came in first.
+ */
+void utrace_maybe_reap(struct task_struct *target, struct utrace *utrace,
+		       bool reap)
+{
+	struct utrace_engine *engine, *next;
+	struct list_head attached;
+
+	spin_lock(&utrace->lock);
+
+	if (reap) {
+		/*
+		 * If the target will do some final callbacks but hasn't
+		 * finished them yet, we know because it clears these event
+		 * bits after it's done.  Instead of cleaning up here and
+		 * requiring utrace_report_death() to cope with it, we
+		 * delay the REAP report and the teardown until after the
+		 * target finishes its death reports.
+		 */
+		utrace->reap = 1;
+
+		if (target->utrace_flags & _UTRACE_DEATH_EVENTS) {
+			spin_unlock(&utrace->lock);
+			return;
+		}
+	} else {
+		/*
+		 * After we unlock with this flag clear, any competing
+		 * utrace_control/utrace_set_events calls know that we've
+		 * finished our callbacks and any detach bookkeeping.
+		 */
+		utrace->death = 0;
+
+		if (!utrace->reap) {
+			/*
+			 * We're just dead, not reaped yet.  This will
+			 * reset @target->utrace_flags so the later call
+			 * with @reap set won't hit the check above.
+			 */
+			utrace_reset(target, utrace);
+			return;
+		}
+	}
+
+	/*
+	 * utrace_add_engine() checks ->utrace_flags != 0.  Since
+	 * @utrace->reap is set, nobody can set or clear UTRACE_EVENT(REAP)
+	 * in @engine->flags or change @engine->ops and nobody can change
+	 * @utrace->attached after we drop the lock.
+	 */
+	target->utrace_flags = 0;
+
+	/*
+	 * We clear out @utrace->attached before we drop the lock so
+	 * that find_matching_engine() can't come across any old engine
+	 * while we are busy tearing it down.
+	 */
+	list_replace_init(&utrace->attached, &attached);
+	list_splice_tail_init(&utrace->attaching, &attached);
+
+	spin_unlock(&utrace->lock);
+
+	list_for_each_entry_safe(engine, next, &attached, entry) {
+		if (engine->flags & UTRACE_EVENT(REAP))
+			engine->ops->report_reap(engine, target);
+
+		engine->ops = NULL;
+		engine->flags = 0;
+		list_del_init(&engine->entry);
+
+		utrace_engine_put(engine);
+	}
+}
+
+/*
+ * You can't do anything to a dead task but detach it.
+ * If release_task() has been called, you can't do that.
+ *
+ * On the exit path, DEATH and QUIESCE event bits are set only
+ * before utrace_report_death() has taken the lock.  At that point,
+ * the death report will come soon, so disallow detach until it's
+ * done.  This prevents us from racing with it detaching itself.
+ *
+ * Called only when @target->exit_state is nonzero.
+ */
+static inline int utrace_control_dead(struct task_struct *target,
+				      struct utrace *utrace,
+				      enum utrace_resume_action action)
+{
+	lockdep_assert_held(&utrace->lock);
+
+	if (action != UTRACE_DETACH || unlikely(utrace->reap))
+		return -ESRCH;
+
+	if (unlikely(utrace->death))
+		/*
+		 * We have already started the death report.  We can't
+		 * prevent the report_death and report_reap callbacks,
+		 * so tell the caller they will happen.
+		 */
+		return -EALREADY;
+
+	return 0;
+}
+
+/**
+ * utrace_control - control a thread being traced by a tracing engine
+ * @target:		thread to affect
+ * @engine:		attached engine to affect
+ * @action:		&enum utrace_resume_action for thread to do
+ *
+ * This is how a tracing engine asks a traced thread to do something.
+ * This call is controlled by the @action argument, which has the
+ * same meaning as the &enum utrace_resume_action value returned by
+ * event reporting callbacks.
+ *
+ * If @target is already dead (@target->exit_state nonzero),
+ * all actions except %UTRACE_DETACH fail with -%ESRCH.
+ *
+ * The following sections describe each option for the @action argument.
+ *
+ * UTRACE_DETACH:
+ *
+ * After this, the @engine data structure is no longer accessible,
+ * and the thread might be reaped.  The thread will start running
+ * again if it was stopped and no longer has any attached engines
+ * that want it stopped.
+ *
+ * If the @report_reap callback may already have begun, this fails
+ * with -%ESRCH.  If the @report_death callback may already have
+ * begun, this fails with -%EALREADY.
+ *
+ * If @target is not already stopped, then a callback to this engine
+ * might be in progress or about to start on another CPU.  If so,
+ * then this returns -%EINPROGRESS; the detach happens as soon as
+ * the pending callback is finished.  To synchronize after an
+ * -%EINPROGRESS return, see utrace_barrier().
+ *
+ * If @target is properly stopped before utrace_control() is called,
+ * then after successful return it's guaranteed that no more callbacks
+ * to the @engine->ops vector will be made.
+ *
+ * The only exception is %SIGKILL (and exec or group-exit by another
+ * thread in the group), which can cause asynchronous @report_death
+ * and/or @report_reap callbacks even when %UTRACE_STOP was used.
+ * (In that event, this fails with -%ESRCH or -%EALREADY, see above.)
+ *
+ * UTRACE_STOP:
+ *
+ * This asks that @target stop running.  This returns 0 only if
+ * @target is already stopped, either for tracing or for job
+ * control.  Then @target will remain stopped until another
+ * utrace_control() call is made on @engine; @target can be woken
+ * only by %SIGKILL (or equivalent, such as exec or termination by
+ * another thread in the same thread group).
+ *
+ * This returns -%EINPROGRESS if @target is not already stopped.
+ * Then the effect is like %UTRACE_REPORT.  A @report_quiesce or
+ * @report_signal callback will be made soon.  Your callback can
+ * then return %UTRACE_STOP to keep @target stopped.
+ *
+ * This does not interrupt system calls in progress, including ones
+ * that sleep for a long time.  For that, use %UTRACE_INTERRUPT.
+ * To interrupt system calls and then keep @target stopped, your
+ * @report_signal callback can return %UTRACE_STOP.
+ *
+ * UTRACE_RESUME:
+ *
+ * Just let @target continue running normally, reversing the effect
+ * of a previous %UTRACE_STOP.  If another engine is keeping @target
+ * stopped, then it remains stopped until all engines let it resume.
+ * If @target was not stopped, this has no effect.
+ *
+ * UTRACE_REPORT:
+ *
+ * This is like %UTRACE_RESUME, but also ensures that there will be
+ * a @report_quiesce or @report_signal callback made soon.  If
+ * @target had been stopped, then there will be a callback before it
+ * resumes running normally.  If another engine is keeping @target
+ * stopped, then there might be no callbacks until all engines let
+ * it resume.
+ *
+ * Since this is meaningless unless @report_quiesce callbacks will
+ * be made, it returns -%EINVAL if @engine lacks %UTRACE_EVENT(%QUIESCE).
+ *
+ * UTRACE_INTERRUPT:
+ *
+ * This is like %UTRACE_REPORT, but ensures that @target will make a
+ * @report_signal callback before it resumes or delivers signals.
+ * If @target was in a system call or about to enter one, work in
+ * progress will be interrupted as if by %SIGSTOP.  If another
+ * engine is keeping @target stopped, then there might be no
+ * callbacks until all engines let it resume.
+ *
+ * This gives @engine an opportunity to introduce a forced signal
+ * disposition via its @report_signal callback.
+ *
+ * UTRACE_SINGLESTEP:
+ *
+ * It's invalid to use this unless arch_has_single_step() returned true.
+ * This is like %UTRACE_RESUME, but resumes for one user instruction only.
+ *
+ * Note that passing %UTRACE_SINGLESTEP or %UTRACE_BLOCKSTEP to
+ * utrace_control() or returning it from an event callback alone does
+ * not necessarily ensure that stepping will be enabled.  If there are
+ * more callbacks made to any engine before returning to user mode,
+ * then the resume action is chosen only by the last set of callbacks.
+ * To be sure, enable %UTRACE_EVENT(%QUIESCE) and look for the
+ * @report_quiesce callback with a zero event mask, or the
+ * @report_signal callback with %UTRACE_SIGNAL_REPORT.
+ *
+ * Since this is not robust unless @report_quiesce callbacks will
+ * be made, it returns -%EINVAL if @engine lacks %UTRACE_EVENT(%QUIESCE).
+ *
+ * UTRACE_BLOCKSTEP:
+ *
+ * It's invalid to use this unless arch_has_block_step() returned true.
+ * This is like %UTRACE_SINGLESTEP, but resumes for one whole basic
+ * block of user instructions.
+ *
+ * Since this is not robust unless @report_quiesce callbacks will
+ * be made, it returns -%EINVAL if @engine lacks %UTRACE_EVENT(%QUIESCE).
+ *
+ * %UTRACE_BLOCKSTEP devolves to %UTRACE_SINGLESTEP when another
+ * tracing engine is using %UTRACE_SINGLESTEP at the same time.
+ */
+int utrace_control(struct task_struct *target,
+		   struct utrace_engine *engine,
+		   enum utrace_resume_action action)
+{
+	struct utrace *utrace;
+	bool reset;
+	int ret;
+
+	if (unlikely(action >= UTRACE_RESUME_MAX)) {
+		WARN(1, "invalid action argument to utrace_control()!");
+		return -EINVAL;
+	}
+
+	/*
+	 * This is a sanity check for a programming error in the caller.
+	 * Their request can only work properly in all cases by relying on
+	 * a follow-up callback, but they didn't set one up!  This check
+	 * doesn't do locking, but it shouldn't matter.  The caller has to
+	 * be synchronously sure the callback is set up to be operating the
+	 * interface properly.
+	 */
+	if (action >= UTRACE_REPORT && action < UTRACE_RESUME &&
+	    unlikely(!(engine->flags & UTRACE_EVENT(QUIESCE)))) {
+		WARN(1, "utrace_control() with no QUIESCE callback in place!");
+		return -EINVAL;
+	}
+
+	utrace = get_utrace_lock(target, engine, true);
+	if (unlikely(IS_ERR(utrace)))
+		return PTR_ERR(utrace);
+
+	reset = task_is_traced(target);
+	ret = 0;
+
+	/*
+	 * ->exit_state can change under us, this doesn't matter.
+	 * We do not care about ->exit_state in fact, but we do
+	 * care about ->reap and ->death. If either flag is set,
+	 * we must also see ->exit_state != 0.
+	 */
+	if (unlikely(target->exit_state)) {
+		ret = utrace_control_dead(target, utrace, action);
+		if (ret) {
+			spin_unlock(&utrace->lock);
+			return ret;
+		}
+		reset = true;
+	}
+
+	switch (action) {
+	case UTRACE_STOP:
+		mark_engine_wants_stop(target, engine);
+		if (!reset && !utrace_do_stop(target, utrace))
+			ret = -EINPROGRESS;
+		reset = false;
+		break;
+
+	case UTRACE_DETACH:
+		if (engine_wants_stop(engine))
+			target->utrace_flags &= ~ENGINE_STOP;
+		mark_engine_detached(engine);
+		reset = reset || utrace_do_stop(target, utrace);
+		if (!reset) {
+			/*
+			 * As in utrace_set_events(), this barrier ensures
+			 * that our engine->flags changes have hit before we
+			 * examine utrace->reporting, pairing with the barrier
+			 * in start_callback().  If @target has not yet hit
+			 * finish_callback() to clear utrace->reporting, we
+			 * might be in the middle of a callback to @engine.
+			 */
+			smp_mb();
+			if (utrace->reporting == engine)
+				ret = -EINPROGRESS;
+		}
+		break;
+
+	case UTRACE_RESUME:
+		clear_engine_wants_stop(engine);
+		break;
+
+	case UTRACE_BLOCKSTEP:
+		/*
+		 * Resume from stopped, step one block.
+		 * We fall through to treat it like UTRACE_SINGLESTEP.
+		 */
+		if (unlikely(!arch_has_block_step())) {
+			WARN(1, "UTRACE_BLOCKSTEP when !arch_has_block_step()");
+			action = UTRACE_SINGLESTEP;
+		}
+
+	case UTRACE_SINGLESTEP:
+		/*
+		 * Resume from stopped, step one instruction.
+		 * We fall through to the UTRACE_REPORT case.
+		 */
+		if (unlikely(!arch_has_single_step())) {
+			WARN(1,
+			     "UTRACE_SINGLESTEP when !arch_has_single_step()");
+			reset = false;
+			ret = -EOPNOTSUPP;
+			break;
+		}
+
+	case UTRACE_REPORT:
+		/*
+		 * Make the thread call tracehook_notify_resume() soon.
+		 * But don't bother if it's already been interrupted.
+		 * In that case, utrace_get_signal() will be reporting soon.
+		 */
+		clear_engine_wants_stop(engine);
+		if (action < utrace->resume) {
+			utrace->resume = action;
+			set_notify_resume(target);
+		}
+		break;
+
+	case UTRACE_INTERRUPT:
+		/*
+		 * Make the thread call tracehook_get_signal() soon.
+		 */
+		clear_engine_wants_stop(engine);
+		if (utrace->resume == UTRACE_INTERRUPT)
+			break;
+		utrace->resume = UTRACE_INTERRUPT;
+
+		/*
+		 * If it's not already stopped, interrupt it now.  We need
+		 * the siglock here in case it calls recalc_sigpending()
+		 * and clears its own TIF_SIGPENDING.  By taking the lock,
+		 * we've serialized any later recalc_sigpending() after our
+		 * setting of utrace->resume to force it on.
+		 */
+		if (reset) {
+			/*
+			 * This is really just to keep the invariant that
+			 * TIF_SIGPENDING is set with UTRACE_INTERRUPT.
+			 * When it's stopped, we know it's always going
+			 * through utrace_get_signal() and will recalculate.
+			 */
+			set_tsk_thread_flag(target, TIF_SIGPENDING);
+		} else {
+			struct sighand_struct *sighand;
+			unsigned long irqflags;
+			sighand = lock_task_sighand(target, &irqflags);
+			if (likely(sighand)) {
+				signal_wake_up(target, 0);
+				unlock_task_sighand(target, &irqflags);
+			}
+		}
+		break;
+
+	default:
+		BUG();		/* We checked it on entry.  */
+	}
+
+	/*
+	 * Let the thread resume running.  If it's not stopped now,
+	 * there is nothing more we need to do.
+	 */
+	if (reset)
+		utrace_reset(target, utrace);
+	else
+		spin_unlock(&utrace->lock);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(utrace_control);
+
+/**
+ * utrace_barrier - synchronize with simultaneous tracing callbacks
+ * @target:		thread to affect
+ * @engine:		engine to affect (can be detached)
+ *
+ * This blocks while @target might be in the midst of making a callback to
+ * @engine.  It can be interrupted by signals and will return -%ERESTARTSYS.
+ * A return value of zero means no callback from @target to @engine was
+ * in progress.  Any effect of its return value (such as %UTRACE_STOP) has
+ * already been applied to @engine.
+ *
+ * It's not necessary to keep the @target pointer alive for this call.
+ * It's only necessary to hold a ref on @engine.  This will return
+ * safely even if @target has been reaped and has no task refs.
+ *
+ * A successful return from utrace_barrier() guarantees its ordering
+ * with respect to utrace_set_events() and utrace_control() calls.  If
+ * @target was not properly stopped, event callbacks just disabled might
+ * still be in progress; utrace_barrier() waits until there is no chance
+ * an unwanted callback can be in progress.
+ */
+int utrace_barrier(struct task_struct *target, struct utrace_engine *engine)
+{
+	struct utrace *utrace;
+	int ret = -ERESTARTSYS;
+
+	if (unlikely(target == current))
+		return 0;
+
+	do {
+		utrace = get_utrace_lock(target, engine, false);
+		if (unlikely(IS_ERR(utrace))) {
+			ret = PTR_ERR(utrace);
+			if (ret != -ERESTARTSYS)
+				break;
+		} else {
+			/*
+			 * All engine state changes are done while
+			 * holding the lock, i.e. before we get here.
+			 * Since we have the lock, we only need to
+			 * worry about @target making a callback.
+			 * When it has entered start_callback() but
+			 * not yet gotten to finish_callback(), we
+			 * will see utrace->reporting == @engine.
+			 * When @target doesn't take the lock, it uses
+			 * barriers to order setting utrace->reporting
+			 * before it examines the engine state.
+			 */
+			if (utrace->reporting != engine)
+				ret = 0;
+			spin_unlock(&utrace->lock);
+			if (!ret)
+				break;
+		}
+		schedule_timeout_interruptible(1);
+	} while (!signal_pending(current));
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(utrace_barrier);
+
+/*
+ * This is local state used for reporting loops, perhaps optimized away.
+ */
+struct utrace_report {
+	u32 result;
+	enum utrace_resume_action action;
+	enum utrace_resume_action resume_action;
+	bool detaches;
+	bool spurious;
+};
+
+#define INIT_REPORT(var)			\
+	struct utrace_report var = {		\
+		.action = UTRACE_RESUME,	\
+		.resume_action = UTRACE_RESUME,	\
+		.spurious = true 		\
+	}
+
+/*
+ * We are now making the report, so clear the flag saying we need one.
+ * When there is a new attach, ->pending_attach is set just so we will
+ * know to do splice_attaching() here before the callback loop.
+ */
+static enum utrace_resume_action start_report(struct utrace *utrace)
+{
+	enum utrace_resume_action resume = utrace->resume;
+	if (utrace->pending_attach ||
+	    (resume > UTRACE_INTERRUPT && resume < UTRACE_RESUME)) {
+		spin_lock(&utrace->lock);
+		splice_attaching(utrace);
+		resume = utrace->resume;
+		if (resume > UTRACE_INTERRUPT)
+			utrace->resume = UTRACE_RESUME;
+		spin_unlock(&utrace->lock);
+	}
+	return resume;
+}
+
+static inline void finish_report_reset(struct task_struct *task,
+				       struct utrace *utrace,
+				       struct utrace_report *report)
+{
+	if (unlikely(report->spurious || report->detaches)) {
+		spin_lock(&utrace->lock);
+		if (utrace_reset(task, utrace))
+			report->action = UTRACE_RESUME;
+	}
+}
+
+/*
+ * Complete a normal reporting pass, pairing with a start_report() call.
+ * This handles any UTRACE_DETACH or UTRACE_REPORT or UTRACE_INTERRUPT
+ * returns from engine callbacks.  If @will_not_stop is true and any
+ * engine's last callback used UTRACE_STOP, we do UTRACE_REPORT here to
+ * ensure we stop before user mode.  If there were no callbacks made, it
+ * will recompute @task->utrace_flags to avoid another false-positive.
+ */
+static void finish_report(struct task_struct *task, struct utrace *utrace,
+			  struct utrace_report *report, bool will_not_stop)
+{
+	enum utrace_resume_action resume = report->action;
+
+	if (resume == UTRACE_STOP)
+		resume = will_not_stop ? UTRACE_REPORT : UTRACE_RESUME;
+
+	if (resume < utrace->resume) {
+		spin_lock(&utrace->lock);
+		utrace->resume = resume;
+		if (resume == UTRACE_INTERRUPT)
+			set_tsk_thread_flag(task, TIF_SIGPENDING);
+		else
+			set_tsk_thread_flag(task, TIF_NOTIFY_RESUME);
+		spin_unlock(&utrace->lock);
+	}
+
+	finish_report_reset(task, utrace, report);
+}
+
+static void finish_callback_report(struct task_struct *task,
+				   struct utrace *utrace,
+				   struct utrace_report *report,
+				   struct utrace_engine *engine,
+				   enum utrace_resume_action action)
+{
+	if (action == UTRACE_DETACH) {
+		/*
+		 * By holding the lock here, we make sure that
+		 * utrace_barrier() (really get_utrace_lock()) sees the
+		 * effect of this detach.  Otherwise utrace_barrier() could
+		 * return 0 after this callback had returned UTRACE_DETACH.
+		 * This way, a 0 return is an unambiguous indicator that any
+		 * callback returning UTRACE_DETACH has indeed caused detach.
+		 */
+		spin_lock(&utrace->lock);
+		engine->ops = &utrace_detached_ops;
+		spin_unlock(&utrace->lock);
+	}
+
+	/*
+	 * If utrace_control() was used, treat that like UTRACE_DETACH here.
+	 */
+	if (engine->ops == &utrace_detached_ops) {
+		report->detaches = true;
+		return;
+	}
+
+	if (action < report->action)
+		report->action = action;
+
+	if (action != UTRACE_STOP) {
+		if (action < report->resume_action)
+			report->resume_action = action;
+
+		if (engine_wants_stop(engine)) {
+			spin_lock(&utrace->lock);
+			clear_engine_wants_stop(engine);
+			spin_unlock(&utrace->lock);
+		}
+
+		return;
+	}
+
+	if (!engine_wants_stop(engine)) {
+		spin_lock(&utrace->lock);
+		/*
+		 * If utrace_control() came in and detached us
+		 * before we got the lock, we must not stop now.
+		 */
+		if (unlikely(engine->ops == &utrace_detached_ops))
+			report->detaches = true;
+		else
+			mark_engine_wants_stop(task, engine);
+		spin_unlock(&utrace->lock);
+	}
+}
+
+/*
+ * Apply the return value of one engine callback to @report.
+ * Returns true if @engine detached and should not get any more callbacks.
+ */
+static bool finish_callback(struct task_struct *task, struct utrace *utrace,
+			    struct utrace_report *report,
+			    struct utrace_engine *engine,
+			    u32 ret)
+{
+	report->result = ret & ~UTRACE_RESUME_MASK;
+	finish_callback_report(task, utrace, report, engine,
+			       utrace_resume_action(ret));
+
+	/*
+	 * Now that we have applied the effect of the return value,
+	 * clear this so that utrace_barrier() can stop waiting.
+	 * A subsequent utrace_control() can stop or resume @engine
+	 * and know this was ordered after its callback's action.
+	 *
+	 * We don't need any barriers here because utrace_barrier()
+	 * takes utrace->lock.  If we touched engine->flags above,
+	 * the lock guaranteed this change was before utrace_barrier()
+	 * examined utrace->reporting.
+	 */
+	utrace->reporting = NULL;
+
+	/*
+	 * We've just done an engine callback.  These are allowed to sleep,
+	 * though all well-behaved ones restrict that to blocking kalloc()
+	 * or quickly-acquired mutex_lock() and the like.  This is a good
+	 * place to make sure tracing engines don't introduce too much
+	 * latency under voluntary preemption.
+	 */
+	might_sleep();
+
+	return engine->ops == &utrace_detached_ops;
+}
+
+/*
+ * Start the callbacks for @engine to consider @event (a bit mask).
+ * This makes the report_quiesce() callback first.  If @engine wants
+ * a specific callback for @event, we return the ops vector to use.
+ * If not, we return NULL.  The return value from the ops->callback
+ * function called should be passed to finish_callback().
+ */
+static const struct utrace_engine_ops *start_callback(
+	struct utrace *utrace, struct utrace_report *report,
+	struct utrace_engine *engine, struct task_struct *task,
+	unsigned long event)
+{
+	const struct utrace_engine_ops *ops;
+	unsigned long want;
+
+	/*
+	 * This barrier ensures that we've set utrace->reporting before
+	 * we examine engine->flags or engine->ops.  utrace_barrier()
+	 * relies on this ordering to indicate that the effect of any
+	 * utrace_control() and utrace_set_events() calls is in place
+	 * by the time utrace->reporting can be seen to be NULL.
+	 */
+	utrace->reporting = engine;
+	smp_mb();
+
+	/*
+	 * This pairs with the barrier in mark_engine_detached().
+	 * It makes sure that we never see the old ops vector with
+	 * the new flags, in case the original vector had no report_quiesce.
+	 */
+	want = engine->flags;
+	smp_rmb();
+	ops = engine->ops;
+
+	if ((want & UTRACE_EVENT(QUIESCE)) || ops == &utrace_detached_ops) {
+		if (finish_callback(task, utrace, report, engine,
+				    (*ops->report_quiesce)(report->action,
+							   engine, event)))
+			return NULL;
+
+		if (!event) {
+			/* We only got here to report QUIESCE */
+			report->spurious = false;
+			return NULL;
+		}
+
+		/*
+		 * finish_callback() reset utrace->reporting after the
+		 * quiesce callback.  Now we set it again (as above)
+		 * before re-examining engine->flags, which could have
+		 * been changed synchronously by ->report_quiesce or
+		 * asynchronously by utrace_control() or utrace_set_events().
+		 */
+		utrace->reporting = engine;
+		smp_mb();
+		want = engine->flags;
+	}
+
+	if (want & ENGINE_STOP)
+		report->action = UTRACE_STOP;
+
+	if (want & event) {
+		report->spurious = false;
+		return ops;
+	}
+
+	utrace->reporting = NULL;
+	return NULL;
+}
+
+/*
+ * Do a normal reporting pass for engines interested in @event.
+ * @callback is the name of the member in the ops vector, and remaining
+ * args are the extras it takes after the standard three args.
+ */
+#define REPORT_CALLBACKS(rev, task, utrace, report, event, callback, ...)     \
+	do {								      \
+		struct utrace_engine *engine;				      \
+		const struct utrace_engine_ops *ops;			      \
+		list_for_each_entry##rev(engine, &utrace->attached, entry) {  \
+			ops = start_callback(utrace, report, engine, task,    \
+					     event);			      \
+			if (!ops)					      \
+				continue;				      \
+			finish_callback(task, utrace, report, engine,	      \
+					(*ops->callback)(__VA_ARGS__));	      \
+		}							      \
+	} while (0)
+#define REPORT(task, utrace, report, event, callback, ...)		      \
+	do {								      \
+		start_report(utrace);					      \
+		REPORT_CALLBACKS(, task, utrace, report, event, callback,     \
+				 (report)->action, engine, ## __VA_ARGS__);   \
+		finish_report(task, utrace, report, true);		      \
+	} while (0)
+
+/*
+ * Called iff UTRACE_EVENT(EXEC) flag is set.
+ */
+void utrace_report_exec(struct linux_binfmt *fmt, struct linux_binprm *bprm,
+			struct pt_regs *regs)
+{
+	struct task_struct *task = current;
+	struct utrace *utrace = task_utrace_struct(task);
+	INIT_REPORT(report);
+
+	REPORT(task, utrace, &report, UTRACE_EVENT(EXEC),
+	       report_exec, fmt, bprm, regs);
+}
+
+static u32 do_report_syscall_entry(struct pt_regs *regs,
+				   struct task_struct *task,
+				   struct utrace *utrace,
+				   struct utrace_report *report,
+				   u32 resume_report)
+{
+	start_report(utrace);
+	REPORT_CALLBACKS(_reverse, task, utrace, report,
+			 UTRACE_EVENT(SYSCALL_ENTRY), report_syscall_entry,
+			 resume_report | report->result | report->action,
+			 engine, regs);
+	finish_report(task, utrace, report, false);
+
+	if (report->action != UTRACE_STOP)
+		return 0;
+
+	utrace_stop(task, utrace, report->resume_action);
+
+	if (fatal_signal_pending(task)) {
+		/*
+		 * We are continuing despite UTRACE_STOP because of a
+		 * SIGKILL.  Don't let the system call actually proceed.
+		 */
+		report->result = UTRACE_SYSCALL_ABORT;
+	} else if (utrace->resume <= UTRACE_REPORT) {
+		/*
+		 * If we've been asked for another report after our stop,
+		 * go back to report (and maybe stop) again before we run
+		 * the system call.  The second (and later) reports are
+		 * marked with the UTRACE_SYSCALL_RESUMED flag so that
+		 * engines know this is a second report at the same
+		 * entry.  This gives them the chance to examine the
+		 * registers anew after they might have been changed
+		 * while we were stopped.
+		 */
+		report->detaches = false;
+		report->spurious = true;
+		report->action = report->resume_action = UTRACE_RESUME;
+		return UTRACE_SYSCALL_RESUMED;
+	}
+
+	return 0;
+}
+
+/*
+ * Called iff UTRACE_EVENT(SYSCALL_ENTRY) flag is set.
+ * Return true to prevent the system call.
+ */
+bool utrace_report_syscall_entry(struct pt_regs *regs)
+{
+	struct task_struct *task = current;
+	struct utrace *utrace = task_utrace_struct(task);
+	INIT_REPORT(report);
+	u32 resume_report = 0;
+
+	do {
+		resume_report = do_report_syscall_entry(regs, task, utrace,
+							&report, resume_report);
+	} while (resume_report);
+
+	return utrace_syscall_action(report.result) == UTRACE_SYSCALL_ABORT;
+}
+
+/*
+ * Called iff UTRACE_EVENT(SYSCALL_EXIT) flag is set.
+ */
+void utrace_report_syscall_exit(struct pt_regs *regs)
+{
+	struct task_struct *task = current;
+	struct utrace *utrace = task_utrace_struct(task);
+	INIT_REPORT(report);
+
+	REPORT(task, utrace, &report, UTRACE_EVENT(SYSCALL_EXIT),
+	       report_syscall_exit, regs);
+}
+
+/*
+ * Called iff UTRACE_EVENT(CLONE) flag is set.
+ * This notification call blocks the wake_up_new_task call on the child.
+ * So we must not quiesce here.  tracehook_report_clone_complete will do
+ * a quiescence check momentarily.
+ */
+void utrace_report_clone(unsigned long clone_flags, struct task_struct *child)
+{
+	struct task_struct *task = current;
+	struct utrace *utrace = task_utrace_struct(task);
+	INIT_REPORT(report);
+
+	/*
+	 * We don't use the REPORT() macro here, because we need
+	 * to clear utrace->cloning before finish_report().
+	 * After finish_report(), utrace can be a stale pointer
+	 * in cases when report.action is still UTRACE_RESUME.
+	 */
+	start_report(utrace);
+	utrace->cloning = child;
+
+	REPORT_CALLBACKS(, task, utrace, &report,
+			 UTRACE_EVENT(CLONE), report_clone,
+			 report.action, engine, clone_flags, child);
+
+	utrace->cloning = NULL;
+	finish_report(task, utrace, &report, !(clone_flags & CLONE_VFORK));
+
+	/*
+	 * For a vfork, we will go into an uninterruptible block waiting
+	 * for the child.  We need UTRACE_STOP to happen before this, not
+	 * after.  For CLONE_VFORK, utrace_finish_vfork() will be called.
+	 */
+	if (report.action == UTRACE_STOP && (clone_flags & CLONE_VFORK)) {
+		spin_lock(&utrace->lock);
+		utrace->vfork_stop = 1;
+		spin_unlock(&utrace->lock);
+	}
+}
+
+/*
+ * We're called after utrace_report_clone() for a CLONE_VFORK.
+ * If UTRACE_STOP was left from the clone report, we stop here.
+ * After this, we'll enter the uninterruptible wait_for_completion()
+ * waiting for the child.
+ */
+void utrace_finish_vfork(struct task_struct *task)
+{
+	struct utrace *utrace = task_utrace_struct(task);
+
+	if (utrace->vfork_stop) {
+		spin_lock(&utrace->lock);
+		utrace->vfork_stop = 0;
+		spin_unlock(&utrace->lock);
+		utrace_stop(task, utrace, UTRACE_RESUME); /* XXX */
+	}
+}
+
+/*
+ * Called iff UTRACE_EVENT(JCTL) flag is set.
+ *
+ * Called with siglock held.
+ */
+void utrace_report_jctl(int notify, int what)
+{
+	struct task_struct *task = current;
+	struct utrace *utrace = task_utrace_struct(task);
+	INIT_REPORT(report);
+
+	spin_unlock_irq(&task->sighand->siglock);
+
+	REPORT(task, utrace, &report, UTRACE_EVENT(JCTL),
+	       report_jctl, what, notify);
+
+	spin_lock_irq(&task->sighand->siglock);
+}
+
+/*
+ * Called iff UTRACE_EVENT(EXIT) flag is set.
+ */
+void utrace_report_exit(long *exit_code)
+{
+	struct task_struct *task = current;
+	struct utrace *utrace = task_utrace_struct(task);
+	INIT_REPORT(report);
+	long orig_code = *exit_code;
+
+	REPORT(task, utrace, &report, UTRACE_EVENT(EXIT),
+	       report_exit, orig_code, exit_code);
+
+	if (report.action == UTRACE_STOP)
+		utrace_stop(task, utrace, report.resume_action);
+}
+
+/*
+ * Called iff UTRACE_EVENT(DEATH) or UTRACE_EVENT(QUIESCE) flag is set.
+ *
+ * It is always possible that we are racing with utrace_release_task here.
+ * For this reason, utrace_release_task checks for the event bits that get
+ * us here, and delays its cleanup for us to do.
+ */
+void utrace_report_death(struct task_struct *task, struct utrace *utrace,
+			 bool group_dead, int signal)
+{
+	INIT_REPORT(report);
+
+	BUG_ON(!task->exit_state);
+
+	/*
+	 * We are presently considered "quiescent"--which is accurate
+	 * inasmuch as we won't run any more user instructions ever again.
+	 * But for utrace_control and utrace_set_events to be robust, they
+	 * must be sure whether or not we will run any more callbacks.  If
+	 * a call comes in before we do, taking the lock here synchronizes
+	 * us so we don't run any callbacks just disabled.  Calls that come
+	 * in while we're running the callbacks will see the exit.death
+	 * flag and know that we are not yet fully quiescent for purposes
+	 * of detach bookkeeping.
+	 */
+	spin_lock(&utrace->lock);
+	BUG_ON(utrace->death);
+	utrace->death = 1;
+	utrace->resume = UTRACE_RESUME;
+	splice_attaching(utrace);
+	spin_unlock(&utrace->lock);
+
+	REPORT_CALLBACKS(, task, utrace, &report, UTRACE_EVENT(DEATH),
+			 report_death, engine, group_dead, signal);
+
+	utrace_maybe_reap(task, utrace, false);
+}
+
+/*
+ * Finish the last reporting pass before returning to user mode.
+ */
+static void finish_resume_report(struct task_struct *task,
+				 struct utrace *utrace,
+				 struct utrace_report *report)
+{
+	finish_report_reset(task, utrace, report);
+
+	switch (report->action) {
+	case UTRACE_STOP:
+		utrace_stop(task, utrace, report->resume_action);
+		break;
+
+	case UTRACE_INTERRUPT:
+		if (!signal_pending(task))
+			set_tsk_thread_flag(task, TIF_SIGPENDING);
+		break;
+
+	case UTRACE_BLOCKSTEP:
+		if (likely(arch_has_block_step())) {
+			user_enable_block_step(task);
+			break;
+		}
+
+		/*
+		 * This means some callback is to blame for failing
+		 * to check arch_has_block_step() itself.  Warn and
+		 * then fall through to treat it as SINGLESTEP.
+		 */
+		WARN(1, "UTRACE_BLOCKSTEP when !arch_has_block_step()");
+
+	case UTRACE_SINGLESTEP:
+		if (likely(arch_has_single_step())) {
+			user_enable_single_step(task);
+		} else {
+			/*
+			 * This means some callback is to blame for failing
+			 * to check arch_has_single_step() itself.  Spew
+			 * about it so the loser will fix his module.
+			 */
+			WARN(1,
+			     "UTRACE_SINGLESTEP when !arch_has_single_step()");
+		}
+		break;
+
+	case UTRACE_REPORT:
+	case UTRACE_RESUME:
+	default:
+		user_disable_single_step(task);
+		break;
+	}
+}
+
+/*
+ * This is called when TIF_NOTIFY_RESUME had been set (and is now clear).
+ * We are close to user mode, and this is the place to report or stop.
+ * When we return, we're going to user mode or into the signals code.
+ */
+void utrace_resume(struct task_struct *task, struct pt_regs *regs)
+{
+	struct utrace *utrace = task_utrace_struct(task);
+	INIT_REPORT(report);
+	struct utrace_engine *engine;
+
+	/*
+	 * Some machines get here with interrupts disabled.  The same arch
+	 * code path leads to calling into get_signal_to_deliver(), which
+	 * implicitly reenables them by virtue of spin_unlock_irq.
+	 */
+	local_irq_enable();
+
+	/*
+	 * If this flag is still set it's because there was a signal
+	 * handler setup done but no report_signal following it.  Clear
+	 * the flag before we get to user so it doesn't confuse us later.
+	 */
+	if (unlikely(utrace->signal_handler)) {
+		spin_lock(&utrace->lock);
+		utrace->signal_handler = 0;
+		spin_unlock(&utrace->lock);
+	}
+
+	/*
+	 * Update our bookkeeping even if there are no callbacks made here.
+	 */
+	report.action = start_report(utrace);
+
+	switch (report.action) {
+	case UTRACE_RESUME:
+		/*
+		 * Anything we might have done was already handled by
+		 * utrace_get_signal(), or this is an entirely spurious
+		 * call.  (The arch might use TIF_NOTIFY_RESUME for other
+		 * purposes as well as calling us.)
+		 */
+		return;
+	case UTRACE_REPORT:
+		if (unlikely(!(task->utrace_flags & UTRACE_EVENT(QUIESCE))))
+			break;
+		/*
+		 * Do a simple reporting pass, with no specific
+		 * callback after report_quiesce.
+		 */
+		report.action = UTRACE_RESUME;
+		list_for_each_entry(engine, &utrace->attached, entry)
+			start_callback(utrace, &report, engine, task, 0);
+		break;
+	default:
+		/*
+		 * Even if this report was truly spurious, there is no need
+		 * for utrace_reset() now.  TIF_NOTIFY_RESUME was already
+		 * cleared--it doesn't stay spuriously set.
+		 */
+		report.spurious = false;
+		break;
+	}
+
+	/*
+	 * Finish the report and either stop or get ready to resume.
+	 * If utrace->resume was not UTRACE_REPORT, this applies its
+	 * effect now (i.e. step or interrupt).
+	 */
+	finish_resume_report(task, utrace, &report);
+}
+
+/*
+ * Return true if current has forced signal_pending().
+ *
+ * This is called only when current->utrace_flags is nonzero, so we know
+ * that current->utrace must be set.  It's not inlined in tracehook.h
+ * just so that struct utrace can stay opaque outside this file.
+ */
+bool utrace_interrupt_pending(void)
+{
+	return task_utrace_struct(current)->resume == UTRACE_INTERRUPT;
+}
+
+/*
+ * Take the siglock and push @info back on our queue.
+ * Returns with @task->sighand->siglock held.
+ */
+static void push_back_signal(struct task_struct *task, siginfo_t *info)
+	__acquires(task->sighand->siglock)
+{
+	struct sigqueue *q;
+
+	if (unlikely(!info->si_signo)) { /* Oh, a wise guy! */
+		spin_lock_irq(&task->sighand->siglock);
+		return;
+	}
+
+	q = sigqueue_alloc();
+	if (likely(q)) {
+		q->flags = 0;
+		copy_siginfo(&q->info, info);
+	}
+
+	spin_lock_irq(&task->sighand->siglock);
+
+	sigaddset(&task->pending.signal, info->si_signo);
+	if (likely(q))
+		list_add(&q->list, &task->pending.list);
+
+	set_tsk_thread_flag(task, TIF_SIGPENDING);
+}
+
+/*
+ * This is the hook from the signals code, called with the siglock held.
+ * Here is the ideal place to stop.  We also dequeue and intercept signals.
+ */
+int utrace_get_signal(struct task_struct *task, struct pt_regs *regs,
+		      siginfo_t *info, struct k_sigaction *return_ka)
+	__releases(task->sighand->siglock)
+	__acquires(task->sighand->siglock)
+{
+	struct utrace *utrace;
+	struct k_sigaction *ka;
+	INIT_REPORT(report);
+	struct utrace_engine *engine;
+	const struct utrace_engine_ops *ops;
+	unsigned long event, want;
+	u32 ret;
+	int signr;
+
+	utrace = task_utrace_struct(task);
+	if (utrace->resume < UTRACE_RESUME ||
+	    utrace->pending_attach || utrace->signal_handler) {
+		enum utrace_resume_action resume;
+
+		/*
+		 * We've been asked for an explicit report before we
+		 * even check for pending signals.
+		 */
+
+		spin_unlock_irq(&task->sighand->siglock);
+
+		spin_lock(&utrace->lock);
+
+		splice_attaching(utrace);
+
+		report.result = utrace->signal_handler ?
+			UTRACE_SIGNAL_HANDLER : UTRACE_SIGNAL_REPORT;
+		utrace->signal_handler = 0;
+
+		resume = utrace->resume;
+		utrace->resume = UTRACE_RESUME;
+
+		spin_unlock(&utrace->lock);
+
+		/*
+		 * Make sure signal_pending() only returns true
+		 * if there are real signals pending.
+		 */
+		if (signal_pending(task)) {
+			spin_lock_irq(&task->sighand->siglock);
+			recalc_sigpending();
+			spin_unlock_irq(&task->sighand->siglock);
+		}
+
+		if (resume > UTRACE_REPORT) {
+			/*
+			 * We only got here to process utrace->resume.
+			 * Despite no callbacks, this report is not spurious.
+			 */
+			report.action = resume;
+			report.spurious = false;
+			finish_resume_report(task, utrace, &report);
+			return -1;
+		} else if (!(task->utrace_flags & UTRACE_EVENT(QUIESCE))) {
+			/*
+			 * We only got here to clear utrace->signal_handler.
+			 */
+			return -1;
+		}
+
+		/*
+		 * Do a reporting pass for no signal, just for EVENT(QUIESCE).
+		 * The engine callbacks can fill in *info and *return_ka.
+		 * We'll pass NULL for the @orig_ka argument to indicate
+		 * that there was no original signal.
+		 */
+		event = 0;
+		ka = NULL;
+		memset(return_ka, 0, sizeof *return_ka);
+	} else if (!(task->utrace_flags & UTRACE_EVENT_SIGNAL_ALL) ||
+		   unlikely(task->signal->group_stop_count)) {
+		/*
+		 * If no engine is interested in intercepting signals or
+		 * we must stop, let the caller just dequeue them normally
+		 * or participate in group-stop.
+		 */
+		return 0;
+	} else {
+		/*
+		 * Steal the next signal so we can let tracing engines
+		 * examine it.  From the signal number and sigaction,
+		 * determine what normal delivery would do.  If no
+		 * engine perturbs it, we'll do that by returning the
+		 * signal number after setting *return_ka.
+		 */
+		signr = dequeue_signal(task, &task->blocked, info);
+		if (signr == 0)
+			return signr;
+		BUG_ON(signr != info->si_signo);
+
+		ka = &task->sighand->action[signr - 1];
+		*return_ka = *ka;
+
+		/*
+		 * We are never allowed to interfere with SIGKILL.
+		 * Just punt after filling in *return_ka for our caller.
+		 */
+		if (signr == SIGKILL)
+			return signr;
+
+		if (ka->sa.sa_handler == SIG_IGN) {
+			event = UTRACE_EVENT(SIGNAL_IGN);
+			report.result = UTRACE_SIGNAL_IGN;
+		} else if (ka->sa.sa_handler != SIG_DFL) {
+			event = UTRACE_EVENT(SIGNAL);
+			report.result = UTRACE_SIGNAL_DELIVER;
+		} else if (sig_kernel_coredump(signr)) {
+			event = UTRACE_EVENT(SIGNAL_CORE);
+			report.result = UTRACE_SIGNAL_CORE;
+		} else if (sig_kernel_ignore(signr)) {
+			event = UTRACE_EVENT(SIGNAL_IGN);
+			report.result = UTRACE_SIGNAL_IGN;
+		} else if (signr == SIGSTOP) {
+			event = UTRACE_EVENT(SIGNAL_STOP);
+			report.result = UTRACE_SIGNAL_STOP;
+		} else if (sig_kernel_stop(signr)) {
+			event = UTRACE_EVENT(SIGNAL_STOP);
+			report.result = UTRACE_SIGNAL_TSTP;
+		} else {
+			event = UTRACE_EVENT(SIGNAL_TERM);
+			report.result = UTRACE_SIGNAL_TERM;
+		}
+
+		/*
+		 * Now that we know what event type this signal is, we
+		 * can short-circuit if no engines care about those.
+		 */
+		if ((task->utrace_flags & (event | UTRACE_EVENT(QUIESCE))) == 0)
+			return signr;
+
+		/*
+		 * We have some interested engines, so tell them about
+		 * the signal and let them change its disposition.
+		 */
+		spin_unlock_irq(&task->sighand->siglock);
+	}
+
+	/*
+	 * This reporting pass chooses what signal disposition we'll act on.
+	 */
+	list_for_each_entry(engine, &utrace->attached, entry) {
+		/*
+		 * See start_callback() comment about this barrier.
+		 */
+		utrace->reporting = engine;
+		smp_mb();
+
+		/*
+		 * This pairs with the barrier in mark_engine_detached(),
+		 * see start_callback() comments.
+		 */
+		want = engine->flags;
+		smp_rmb();
+		ops = engine->ops;
+
+		if ((want & (event | UTRACE_EVENT(QUIESCE))) == 0) {
+			utrace->reporting = NULL;
+			continue;
+		}
+
+		if (ops->report_signal)
+			ret = (*ops->report_signal)(
+				report.result | report.action, engine,
+				regs, info, ka, return_ka);
+		else
+			ret = (report.result | (*ops->report_quiesce)(
+				       report.action, engine, event));
+
+		/*
+		 * Avoid a tight loop reporting again and again if some
+		 * engine is too stupid.
+		 */
+		switch (utrace_resume_action(ret)) {
+		default:
+			break;
+		case UTRACE_INTERRUPT:
+		case UTRACE_REPORT:
+			ret = (ret & ~UTRACE_RESUME_MASK) | UTRACE_RESUME;
+			break;
+		}
+
+		finish_callback(task, utrace, &report, engine, ret);
+	}
+
+	/*
+	 * We express the chosen action to the signals code in terms
+	 * of a representative signal whose default action does it.
+	 * Our caller uses our return value (signr) to decide what to
+	 * do, but uses info->si_signo as the signal number to report.
+	 */
+	switch (utrace_signal_action(report.result)) {
+	case UTRACE_SIGNAL_TERM:
+		signr = SIGTERM;
+		break;
+
+	case UTRACE_SIGNAL_CORE:
+		signr = SIGQUIT;
+		break;
+
+	case UTRACE_SIGNAL_STOP:
+		signr = SIGSTOP;
+		break;
+
+	case UTRACE_SIGNAL_TSTP:
+		signr = SIGTSTP;
+		break;
+
+	case UTRACE_SIGNAL_DELIVER:
+		signr = info->si_signo;
+
+		if (return_ka->sa.sa_handler == SIG_DFL) {
+			/*
+			 * We'll do signr's normal default action.
+			 * For ignore, we'll fall through below.
+			 * For stop/death, break locks and returns it.
+			 */
+			if (likely(signr) && !sig_kernel_ignore(signr))
+				break;
+		} else if (return_ka->sa.sa_handler != SIG_IGN &&
+			   likely(signr)) {
+			/*
+			 * Complete the bookkeeping after the report.
+			 * The handler will run.  If an engine wanted to
+			 * stop or step, then make sure we do another
+			 * report after signal handler setup.
+			 */
+			if (report.action != UTRACE_RESUME)
+				report.action = UTRACE_INTERRUPT;
+			finish_report(task, utrace, &report, true);
+
+			if (unlikely(report.result & UTRACE_SIGNAL_HOLD))
+				push_back_signal(task, info);
+			else
+				spin_lock_irq(&task->sighand->siglock);
+
+			/*
+			 * We do the SA_ONESHOT work here since the
+			 * normal path will only touch *return_ka now.
+			 */
+			if (unlikely(return_ka->sa.sa_flags & SA_ONESHOT)) {
+				return_ka->sa.sa_flags &= ~SA_ONESHOT;
+				if (likely(valid_signal(signr))) {
+					ka = &task->sighand->action[signr - 1];
+					ka->sa.sa_handler = SIG_DFL;
+				}
+			}
+
+			return signr;
+		}
+
+		/* Fall through for an ignored signal.  */
+
+	case UTRACE_SIGNAL_IGN:
+	case UTRACE_SIGNAL_REPORT:
+	default:
+		/*
+		 * If the signal is being ignored, then we are on the way
+		 * directly back to user mode.  We can stop here, or step,
+		 * as in utrace_resume(), above.  After we've dealt with that,
+		 * our caller will relock and come back through here.
+		 */
+		finish_resume_report(task, utrace, &report);
+
+		if (unlikely(fatal_signal_pending(task))) {
+			/*
+			 * The only reason we woke up now was because of a
+			 * SIGKILL.  Don't do normal dequeuing in case it
+			 * might get a signal other than SIGKILL.  That would
+			 * perturb the death state so it might differ from
+			 * what the debugger would have allowed to happen.
+			 * Instead, pluck out just the SIGKILL to be sure
+			 * we'll die immediately with nothing else different
+			 * from the quiescent state the debugger wanted us in.
+			 */
+			sigset_t sigkill_only;
+			siginitsetinv(&sigkill_only, sigmask(SIGKILL));
+			spin_lock_irq(&task->sighand->siglock);
+			signr = dequeue_signal(task, &sigkill_only, info);
+			BUG_ON(signr != SIGKILL);
+			*return_ka = task->sighand->action[SIGKILL - 1];
+			return signr;
+		}
+
+		if (unlikely(report.result & UTRACE_SIGNAL_HOLD)) {
+			push_back_signal(task, info);
+			spin_unlock_irq(&task->sighand->siglock);
+		}
+
+		return -1;
+	}
+
+	/*
+	 * Complete the bookkeeping after the report.
+	 * This sets utrace->resume if UTRACE_STOP was used.
+	 */
+	finish_report(task, utrace, &report, true);
+
+	return_ka->sa.sa_handler = SIG_DFL;
+
+	/*
+	 * If this signal is fatal, si_signo gets through as exit_code.
+	 * We can't allow a completely bogus value there or else core
+	 * kernel code can freak out.  (If an engine wants to control
+	 * the exit_code value exactly, it can do so in report_exit.)
+	 * We'll produce a big complaint in dmesg, but won't crash.
+	 * That's nicer for debugging your utrace engine.
+	 */
+	if (unlikely(info->si_signo & 0x80)) {
+		WARN(1, "utrace engine left bogus si_signo value!");
+		info->si_signo = SIGTRAP;
+	}
+
+	if (unlikely(report.result & UTRACE_SIGNAL_HOLD))
+		push_back_signal(task, info);
+	else
+		spin_lock_irq(&task->sighand->siglock);
+
+	if (sig_kernel_stop(signr))
+		task->jobctl |= JOBCTL_STOP_DEQUEUED;
+
+	return signr;
+}
+
+/*
+ * This gets called after a signal handler has been set up.
+ * We set a flag so the next report knows it happened.
+ * If we're already stepping, make sure we do a report_signal.
+ * If not, make sure we get into utrace_resume() where we can
+ * clear the signal_handler flag before resuming.
+ */
+void utrace_signal_handler(struct task_struct *task, int stepping)
+{
+	struct utrace *utrace = task_utrace_struct(task);
+
+	spin_lock(&utrace->lock);
+
+	utrace->signal_handler = 1;
+	if (utrace->resume > UTRACE_INTERRUPT) {
+		if (stepping) {
+			utrace->resume = UTRACE_INTERRUPT;
+			set_tsk_thread_flag(task, TIF_SIGPENDING);
+		} else if (utrace->resume == UTRACE_RESUME) {
+			set_tsk_thread_flag(task, TIF_NOTIFY_RESUME);
+		}
+	}
+
+	spin_unlock(&utrace->lock);
+}
+
+/**
+ * utrace_prepare_examine - prepare to examine thread state
+ * @target:		thread of interest, a &struct task_struct pointer
+ * @engine:		engine pointer returned by utrace_attach_task()
+ * @exam:		temporary state, a &struct utrace_examiner pointer
+ *
+ * This call prepares to safely examine the thread @target using
+ * &struct user_regset calls, or direct access to thread-synchronous fields.
+ *
+ * When @target is current, this call is superfluous.  When @target is
+ * another thread, it must be held stopped via %UTRACE_STOP by @engine.
+ *
+ * This call may block the caller until @target stays stopped, so it must
+ * be called only after the caller is sure @target is about to unschedule.
+ * This means a zero return from a utrace_control() call on @engine giving
+ * %UTRACE_STOP, or a report_quiesce() or report_signal() callback to
+ * @engine that used %UTRACE_STOP in its return value.
+ *
+ * Returns -%ESRCH if @target is dead or -%EINVAL if %UTRACE_STOP was
+ * not used.  If @target has started running again despite %UTRACE_STOP
+ * (for %SIGKILL or a spurious wakeup), this call returns -%EAGAIN.
+ *
+ * When this call returns zero, it's safe to use &struct user_regset
+ * calls and task_user_regset_view() on @target and to examine some of
+ * its fields directly.  When the examination is complete, a
+ * utrace_finish_examine() call must follow to check whether it was
+ * completed safely.
+ */
+int utrace_prepare_examine(struct task_struct *target,
+			   struct utrace_engine *engine,
+			   struct utrace_examiner *exam)
+{
+	int ret = 0;
+
+	if (unlikely(target == current))
+		return 0;
+
+	rcu_read_lock();
+	if (unlikely(!engine_wants_stop(engine)))
+		ret = -EINVAL;
+	else if (unlikely(target->exit_state))
+		ret = -ESRCH;
+	else {
+		exam->state = target->state;
+		if (unlikely(exam->state == TASK_RUNNING))
+			ret = -EAGAIN;
+		else
+			get_task_struct(target);
+	}
+	rcu_read_unlock();
+
+	if (likely(!ret)) {
+		exam->ncsw = wait_task_inactive(target, exam->state);
+		put_task_struct(target);
+		if (unlikely(!exam->ncsw))
+			ret = -EAGAIN;
+	}
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(utrace_prepare_examine);
+
+/**
+ * utrace_finish_examine - complete an examination of thread state
+ * @target:		thread of interest, a &struct task_struct pointer
+ * @engine:		engine pointer returned by utrace_attach_task()
+ * @exam:		pointer passed to utrace_prepare_examine() call
+ *
+ * This call completes an examination on the thread @target begun by a
+ * paired utrace_prepare_examine() call with the same arguments that
+ * returned success (zero).
+ *
+ * When @target is current, this call is superfluous.  When @target is
+ * another thread, this returns zero if @target has remained unscheduled
+ * since the paired utrace_prepare_examine() call returned zero.
+ *
+ * When this returns an error, any examination done since the paired
+ * utrace_prepare_examine() call is unreliable and the data extracted
+ * should be discarded.  The error is -%EINVAL if @engine is not
+ * keeping @target stopped, or -%EAGAIN if @target woke up unexpectedly.
+ */
+int utrace_finish_examine(struct task_struct *target,
+			  struct utrace_engine *engine,
+			  struct utrace_examiner *exam)
+{
+	int ret = 0;
+
+	if (unlikely(target == current))
+		return 0;
+
+	rcu_read_lock();
+	if (unlikely(!engine_wants_stop(engine)))
+		ret = -EINVAL;
+	else if (unlikely(target->state != exam->state))
+		ret = -EAGAIN;
+	else
+		get_task_struct(target);
+	rcu_read_unlock();
+
+	if (likely(!ret)) {
+		unsigned long ncsw = wait_task_inactive(target, exam->state);
+		if (unlikely(ncsw != exam->ncsw))
+			ret = -EAGAIN;
+		put_task_struct(target);
+	}
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(utrace_finish_examine);
+
+/*
+ * This is declared in linux/regset.h and defined in machine-dependent
+ * code.  We put the export here to ensure no machine forgets it.
+ */
+EXPORT_SYMBOL_GPL(task_user_regset_view);
+
+/*
+ * Called with rcu_read_lock() held.
+ */
+void task_utrace_proc_status(struct seq_file *m, struct task_struct *p)
+{
+	seq_printf(m, "Utrace:\t%lx\n", p->utrace_flags);
+}
-- 
1.5.5.1

[PATCH 02/33] utrace: add utrace_init_task/utrace_free_task calls

[Oleg Nesterov]

Add the necessary copy_process()->utrace_init_task() and
free_task()->utrace_free_task() calls.

Originally this was the part of "utrace core" patch, but since
tracehooks are dying it doesn't make sense to reintroduce them.
Instead, just call the utrace_ helpers directly. This is fine
even without CONFIG_UTRACE, gcc is smart enough to optimize out
the code in free_task().

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 kernel/fork.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index ba0d172..b8719c2 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -66,6 +66,7 @@
 #include <linux/user-return-notifier.h>
 #include <linux/oom.h>
 #include <linux/khugepaged.h>
+#include <linux/utrace.h>
 
 #include <asm/pgtable.h>
 #include <asm/pgalloc.h>
@@ -167,6 +168,8 @@ void free_task(struct task_struct *tsk)
 	free_thread_info(tsk->stack);
 	rt_mutex_debug_task_free(tsk);
 	ftrace_graph_exit_task(tsk);
+	if (task_utrace_struct(tsk))
+		utrace_free_task(tsk);
 	free_task_struct(tsk);
 }
 EXPORT_SYMBOL(free_task);
@@ -1093,6 +1096,8 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	if (!p)
 		goto fork_out;
 
+	utrace_init_task(p);
+
 	ftrace_graph_init_task(p);
 
 	rt_mutex_init_task(p);
-- 
1.5.5.1

[PATCH 03/33] tracehooks: add utrace hooks

[Oleg Nesterov]

Add the necessary utrace hooks in the tracehooks which were not
removed yet.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 include/linux/tracehook.h |   22 +++++++++++++++++++++-
 1 files changed, 21 insertions(+), 1 deletions(-)

diff --git a/include/linux/tracehook.h b/include/linux/tracehook.h
index a71a292..8cc28bc 100644
--- a/include/linux/tracehook.h
+++ b/include/linux/tracehook.h
@@ -49,6 +49,7 @@
 #include <linux/sched.h>
 #include <linux/ptrace.h>
 #include <linux/security.h>
+#include <linux/utrace.h>
 struct linux_binprm;
 
 /*
@@ -96,6 +97,9 @@ static inline void ptrace_report_syscall(struct pt_regs *regs)
 static inline __must_check int tracehook_report_syscall_entry(
 	struct pt_regs *regs)
 {
+	if ((task_utrace_flags(current) & UTRACE_EVENT(SYSCALL_ENTRY)) &&
+	    utrace_report_syscall_entry(regs))
+		return 1;
 	ptrace_report_syscall(regs);
 	return 0;
 }
@@ -119,6 +123,9 @@ static inline __must_check int tracehook_report_syscall_entry(
  */
 static inline void tracehook_report_syscall_exit(struct pt_regs *regs, int step)
 {
+	if (task_utrace_flags(current) & UTRACE_EVENT(SYSCALL_EXIT))
+		utrace_report_syscall_exit(regs);
+
 	if (step) {
 		siginfo_t info;
 		user_single_step_siginfo(current, regs, &info);
@@ -148,6 +155,8 @@ static inline void tracehook_signal_handler(int sig, siginfo_t *info,
 					    const struct k_sigaction *ka,
 					    struct pt_regs *regs, int stepping)
 {
+	if (task_utrace_flags(current))
+		utrace_signal_handler(current, stepping);
 	if (stepping)
 		ptrace_notify(SIGTRAP);
 }
@@ -179,10 +188,21 @@ static inline void set_notify_resume(struct task_struct *task)
  * asynchronously, this will be called again before we return to
  * user mode.
  *
- * Called without locks.
+ * Called without locks.  However, on some machines this may be
+ * called with interrupts disabled.
  */
 static inline void tracehook_notify_resume(struct pt_regs *regs)
 {
+	struct task_struct *task = current;
+	/*
+	 * Prevent the following store/load from getting ahead of the
+	 * caller which clears TIF_NOTIFY_RESUME. This pairs with the
+	 * implicit mb() before setting TIF_NOTIFY_RESUME in
+	 * set_notify_resume().
+	 */
+	smp_mb();
+	if (task_utrace_flags(task))
+		utrace_resume(task, regs);
 }
 #endif	/* TIF_NOTIFY_RESUME */
 
-- 
1.5.5.1

[PATCH 04/33] tracehooks: reintroduce tracehook_consider_fatal_signal()

[Oleg Nesterov]

Add the killed tracehook_consider_fatal_signal() back. It has multiple
callers and it is not easy add the necessary checks inline.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 arch/s390/kernel/traps.c  |    4 ++--
 include/linux/tracehook.h |   22 ++++++++++++++++++++++
 kernel/signal.c           |    4 ++--
 3 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/arch/s390/kernel/traps.c b/arch/s390/kernel/traps.c
index a9807dd..f506e1b 100644
--- a/arch/s390/kernel/traps.c
+++ b/arch/s390/kernel/traps.c
@@ -329,7 +329,7 @@ void __kprobes do_per_trap(struct pt_regs *regs)
 
 	if (notify_die(DIE_SSTEP, "sstep", regs, 0, 0, SIGTRAP) == NOTIFY_STOP)
 		return;
-	if (!current->ptrace)
+	if (!tracehook_consider_fatal_signal(current, SIGTRAP))
 		return;
 	info.si_signo = SIGTRAP;
 	info.si_errno = 0;
@@ -429,7 +429,7 @@ static void __kprobes illegal_op(struct pt_regs *regs, long
pgm_int_code,
 		if (get_user(*((__u16 *) opcode), (__u16 __user *) location))
 			return;
 		if (*((__u16 *) opcode) == S390_BREAKPOINT_U16) {
-			if (current->ptrace) {
+			if (tracehook_consider_fatal_signal(current, SIGTRAP)) {
 				info.si_signo = SIGTRAP;
 				info.si_errno = 0;
 				info.si_code = TRAP_BRKPT;
diff --git a/include/linux/tracehook.h b/include/linux/tracehook.h
index 8cc28bc..ec2af67 100644
--- a/include/linux/tracehook.h
+++ b/include/linux/tracehook.h
@@ -161,6 +161,28 @@ static inline void tracehook_signal_handler(int sig, siginfo_t
*info,
 		ptrace_notify(SIGTRAP);
 }
 
+/**
+ * tracehook_consider_fatal_signal - suppress special handling of fatal signal
+ * @task:		task receiving the signal
+ * @sig:		signal number being sent
+ *
+ * Return nonzero to prevent special handling of this termination signal.
+ * Normally handler for signal is %SIG_DFL.  It can be %SIG_IGN if @sig is
+ * ignored, in which case force_sig() is about to reset it to %SIG_DFL.
+ * When this returns zero, this signal might cause a quick termination
+ * that does not give the debugger a chance to intercept the signal.
+ *
+ * Called with or without @task->sighand->siglock held.
+ */
+static inline int tracehook_consider_fatal_signal(struct task_struct *task,
+						  int sig)
+{
+	if (unlikely(task_utrace_flags(task) & (UTRACE_EVENT(SIGNAL_TERM) |
+						UTRACE_EVENT(SIGNAL_CORE))))
+		return 1;
+	return task->ptrace != 0;
+}
+
 #ifdef TIF_NOTIFY_RESUME
 /**
  * set_notify_resume - cause tracehook_notify_resume() to be called
diff --git a/kernel/signal.c b/kernel/signal.c
index b3f78d0..d7b90cd 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -494,7 +494,7 @@ int unhandled_signal(struct task_struct *tsk, int sig)
 	if (handler != SIG_IGN && handler != SIG_DFL)
 		return 0;
 	/* if ptraced, let the tracer determine */
-	return !tsk->ptrace;
+	return !tracehook_consider_fatal_signal(tsk, sig);
 }
 
 /*
@@ -982,7 +982,7 @@ static void complete_signal(int sig, struct task_struct *p, int
group)
 	if (sig_fatal(p, sig) &&
 	    !(signal->flags & (SIGNAL_UNKILLABLE | SIGNAL_GROUP_EXIT)) &&
 	    !sigismember(&t->real_blocked, sig) &&
-	    (sig == SIGKILL || !t->ptrace)) {
+	    (sig == SIGKILL || !tracehook_consider_fatal_signal(t, sig))) {
 		/*
 		 * This signal will be fatal to the whole group.
 		 */
-- 
1.5.5.1

[PATCH 05/33] add utrace hooks into sig_ignored() and recalc_sigpending()

[Oleg Nesterov]

Add the necessary and somewhat "special" hooks into sig_ignored() and
recalc_sigpending(). Basically this restores _force_sigpending() and
_consider_ignored_signal() tracehook logic.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 include/linux/utrace.h |    2 ++
 kernel/signal.c        |    7 ++++++-
 2 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/include/linux/utrace.h b/include/linux/utrace.h
index f251efe..1b8da1c 100644
--- a/include/linux/utrace.h
+++ b/include/linux/utrace.h
@@ -107,6 +107,8 @@ bool utrace_report_syscall_entry(struct pt_regs *);
 void utrace_report_syscall_exit(struct pt_regs *);
 void utrace_signal_handler(struct task_struct *, int);
 
+#define UTRACE_FLAG(task, ev)	(task_utrace_flags(task) & UTRACE_EVENT(ev))
+
 #ifndef CONFIG_UTRACE
 
 /*
diff --git a/kernel/signal.c b/kernel/signal.c
index d7b90cd..8594cb2 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -87,7 +87,7 @@ static int sig_ignored(struct task_struct *t, int sig, int
from_ancestor_ns)
 	/*
 	 * Tracers may want to know about even ignored signals.
 	 */
-	return !t->ptrace;
+	return !t->ptrace && !UTRACE_FLAG(t, SIGNAL_IGN);
 }
 
 /*
@@ -150,6 +150,11 @@ void recalc_sigpending_and_wake(struct task_struct *t)
 
 void recalc_sigpending(void)
 {
+	if (task_utrace_flags(current) && utrace_interrupt_pending()) {
+		set_thread_flag(TIF_SIGPENDING);
+		return;
+	}
+
 	if (!recalc_sigpending_tsk(current) && !freezing(current))
 		clear_thread_flag(TIF_SIGPENDING);
 
-- 
1.5.5.1

[PATCH 06/33] restore the EXEC/EXIT/CLONE utrace hooks

[Oleg Nesterov]

Restore the "trivial" EXEC/EXIT/CLONE utrace hooks. Add the
simple helper, UTRACE_HOOK(), to minimize the changes.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 fs/exec.c              |    5 ++++-
 include/linux/utrace.h |    6 ++++++
 kernel/exit.c          |    1 +
 kernel/fork.c          |    4 ++++
 4 files changed, 15 insertions(+), 1 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 3625464..7d8f8bd 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1397,9 +1397,12 @@ int search_binary_handler(struct linux_binprm *bprm,struct pt_regs
*regs)
 			 */
 			bprm->recursion_depth = depth;
 			if (retval >= 0) {
-				if (depth == 0)
+				if (depth == 0) {
+					UTRACE_HOOK(current, EXEC,
+						report_exec(fmt, bprm, regs));
 					ptrace_event(PTRACE_EVENT_EXEC,
 							old_pid);
+				}
 				put_binfmt(fmt);
 				allow_write_access(bprm->file);
 				if (bprm->file)
diff --git a/include/linux/utrace.h b/include/linux/utrace.h
index 1b8da1c..9ac0b1b 100644
--- a/include/linux/utrace.h
+++ b/include/linux/utrace.h
@@ -109,6 +109,12 @@ void utrace_signal_handler(struct task_struct *, int);
 
 #define UTRACE_FLAG(task, ev)	(task_utrace_flags(task) & UTRACE_EVENT(ev))
 
+#define UTRACE_HOOK(task, ev, callback)			\
+	do {						\
+		if (UTRACE_FLAG(task, ev))		\
+			utrace_ ## callback;		\
+	} while (0)
+
 #ifndef CONFIG_UTRACE
 
 /*
diff --git a/kernel/exit.c b/kernel/exit.c
index d0b7d98..f5eba3b 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -911,6 +911,7 @@ NORET_TYPE void do_exit(long code)
 	 */
 	set_fs(USER_DS);
 
+	UTRACE_HOOK(current, EXIT, report_exit(&code));
 	ptrace_event(PTRACE_EVENT_EXIT, code);
 
 	validate_creds_for_do_exit(tsk);
diff --git a/kernel/fork.c b/kernel/fork.c
index b8719c2..09d5049 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1535,6 +1535,8 @@ long do_fork(unsigned long clone_flags,
 
 		audit_finish_fork(p);
 
+		UTRACE_HOOK(current, CLONE, report_clone(clone_flags, p));
+
 		/*
 		 * We set PF_STARTING at creation in case tracing wants to
 		 * use this to distinguish a fully live task from one that
@@ -1546,6 +1548,8 @@ long do_fork(unsigned long clone_flags,
 		wake_up_new_task(p);
 
 		/* forking complete and child started to run, tell ptracer */
+		if (clone_flags & CLONE_VFORK)
+			UTRACE_HOOK(current, CLONE, finish_vfork(current));
 		if (unlikely(trace))
 			ptrace_event(trace, nr);
 
-- 
1.5.5.1

[PATCH 07/33] utrace: utrace_report_death() can use task_utrace_struct()

[Oleg Nesterov]

utrace_report_death() assumes that the caller (exit_notify) should
pass task->utrace as an argument. This is no longer needed, it can
safely do task_utrace_struct(). This way we avoid the nasty changes
in exit_notify().

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 include/linux/utrace.h |    2 +-
 kernel/utrace.c        |    5 +++--
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/linux/utrace.h b/include/linux/utrace.h
index 9ac0b1b..9a2e2f4 100644
--- a/include/linux/utrace.h
+++ b/include/linux/utrace.h
@@ -99,7 +99,7 @@ int utrace_get_signal(struct task_struct *, struct pt_regs *,
 void utrace_report_clone(unsigned long, struct task_struct *);
 void utrace_finish_vfork(struct task_struct *);
 void utrace_report_exit(long *exit_code);
-void utrace_report_death(struct task_struct *, struct utrace *, bool, int);
+void utrace_report_death(struct task_struct *, bool, int);
 void utrace_report_jctl(int notify, int type);
 void utrace_report_exec(struct linux_binfmt *, struct linux_binprm *,
 			struct pt_regs *regs);
diff --git a/kernel/utrace.c b/kernel/utrace.c
index ef856c9..1e750ad 100644
--- a/kernel/utrace.c
+++ b/kernel/utrace.c
@@ -1759,9 +1759,10 @@ void utrace_report_exit(long *exit_code)
  * For this reason, utrace_release_task checks for the event bits that get
  * us here, and delays its cleanup for us to do.
  */
-void utrace_report_death(struct task_struct *task, struct utrace *utrace,
-			 bool group_dead, int signal)
+void utrace_report_death(struct task_struct *task, bool group_dead, int signal)
 {
+	struct utrace *utrace = task_utrace_struct(task);
+
 	INIT_REPORT(report);
 
 	BUG_ON(!task->exit_state);
-- 
1.5.5.1

[PATCH 08/33] restore the DEATH/REAP utrace hooks

[Oleg Nesterov]

Restore the necessary hooks in release_task() and exit_notify(),
add the corresponding helpers into utrace.h.

Note: the @signal argument passed to ->report_death() does not
match the previous behaviour. I think this shouldn't affect the
current users, and I bet nobody can really understand what this
magic argument should actually mean anyway.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 include/linux/utrace.h |   22 ++++++++++++++++++++++
 kernel/exit.c          |    4 ++++
 2 files changed, 26 insertions(+), 0 deletions(-)

diff --git a/include/linux/utrace.h b/include/linux/utrace.h
index 9a2e2f4..cf13839 100644
--- a/include/linux/utrace.h
+++ b/include/linux/utrace.h
@@ -697,4 +697,26 @@ static inline __must_check int utrace_barrier_pid(struct pid *pid,
 
 #endif	/* CONFIG_UTRACE */
 
+static inline void utrace_release_task(struct task_struct *task)
+{
+	/* see utrace_add_engine() about this barrier */
+	smp_mb();
+	if (task_utrace_flags(task))
+		utrace_maybe_reap(task, task_utrace_struct(task), true);
+}
+
+static inline void utrace_exit_notify(struct task_struct *task,
+					  int signal, int group_dead)
+{
+	/*
+	 * If utrace_set_events() was just called to enable
+	 * UTRACE_EVENT(DEATH), then we are obliged to call
+	 * utrace_report_death() and not miss it.  utrace_set_events()
+	 * checks @task->exit_state under tasklist_lock to synchronize
+	 * with exit_notify(), the caller.
+	 */
+	if (task_utrace_flags(task) & _UTRACE_DEATH_EVENTS)
+		utrace_report_death(task, group_dead, signal);
+}
+
 #endif	/* linux/utrace.h */
diff --git a/kernel/exit.c b/kernel/exit.c
index f5eba3b..746c5df 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -168,6 +168,8 @@ void release_task(struct task_struct * p)
 	struct task_struct *leader;
 	int zap_leader;
 repeat:
+	utrace_release_task(p);
+
 	/* don't need to get the RCU readlock here - the process is dead and
 	 * can't be modifying its own credentials. But shut RCU-lockdep up */
 	rcu_read_lock();
@@ -858,6 +860,8 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
 		wake_up_process(tsk->signal->group_exit_task);
 	write_unlock_irq(&tasklist_lock);
 
+	utrace_exit_notify(tsk, autoreap ? -1 : SIGCHLD, group_dead);
+
 	/* If the process is dead, release it - nobody will wait for it */
 	if (autoreap)
 		release_task(tsk);
-- 
1.5.5.1

[PATCH 09/33] utrace: remove jobctl bits

[Oleg Nesterov]

- change utrace_get_signal() to check JOBCTL_STOP_PENDING instead of
  signal->group_stop_count. With the recent changes group_stop_count
  doesn't necessarily mean this task should participate in group stop.

- remove the "participate in group stop" code from utrace_wakeup() and
  utrace_stop(), this is no longer needed and wrong.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 kernel/utrace.c |   16 ++--------------
 1 files changed, 2 insertions(+), 14 deletions(-)

diff --git a/kernel/utrace.c b/kernel/utrace.c
index 1e750ad..5d3974e 100644
--- a/kernel/utrace.c
+++ b/kernel/utrace.c
@@ -648,11 +648,7 @@ static void utrace_wakeup(struct task_struct *target, struct utrace
*utrace)
 {
 	lockdep_assert_held(&utrace->lock);
 	spin_lock_irq(&target->sighand->siglock);
-	if (target->signal->flags & SIGNAL_STOP_STOPPED ||
-	    target->signal->group_stop_count)
-		target->state = TASK_STOPPED;
-	else
-		wake_up_state(target, __TASK_TRACED);
+	wake_up_state(target, __TASK_TRACED);
 	spin_unlock_irq(&target->sighand->siglock);
 }
 
@@ -805,14 +801,6 @@ relock:
 
 	__set_current_state(TASK_TRACED);
 
-	/*
-	 * If there is a group stop in progress,
-	 * we must participate in the bookkeeping.
-	 */
-	if (unlikely(task->signal->group_stop_count) &&
-			!--task->signal->group_stop_count)
-		task->signal->flags = SIGNAL_STOP_STOPPED;
-
 	spin_unlock_irq(&task->sighand->siglock);
 	spin_unlock(&utrace->lock);
 
@@ -2037,7 +2025,7 @@ int utrace_get_signal(struct task_struct *task, struct pt_regs
*regs,
 		ka = NULL;
 		memset(return_ka, 0, sizeof *return_ka);
 	} else if (!(task->utrace_flags & UTRACE_EVENT_SIGNAL_ALL) ||
-		   unlikely(task->signal->group_stop_count)) {
+		   unlikely(task->jobctl & JOBCTL_STOP_PENDING)) {
 		/*
 		 * If no engine is interested in intercepting signals or
 		 * we must stop, let the caller just dequeue them normally
-- 
1.5.5.1

[PATCH 10/33] ptrace: take ->siglock around s/TRACED/RUNNING/

[Oleg Nesterov]

change ptrace_resume() and ptrace_stop() to take ->siglock around changing
task->state from TRACED to RUNNING.

With this patch __TASK_TRACED/STOPPED bits are fully protected by ->siglock,
nobody can set or clear these bits without ->siglock held.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 kernel/ptrace.c |    8 +++++++-
 kernel/signal.c |    3 +++
 2 files changed, 10 insertions(+), 1 deletions(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 24d0447..daf47bc 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -589,6 +589,8 @@ static int ptrace_setsiginfo(struct task_struct *child, const
siginfo_t *info)
 static int ptrace_resume(struct task_struct *child, long request,
 			 unsigned long data)
 {
+	unsigned long flags;
+
 	if (!valid_signal(data))
 		return -EIO;
 
@@ -617,7 +619,11 @@ static int ptrace_resume(struct task_struct *child, long request,
 	}
 
 	child->exit_code = data;
-	wake_up_state(child, __TASK_TRACED);
+
+	if (lock_task_sighand(child, &flags)) {
+		wake_up_state(child, __TASK_TRACED);
+		unlock_task_sighand(child, &flags);
+	}
 
 	return 0;
 }
diff --git a/kernel/signal.c b/kernel/signal.c
index 8594cb2..4512bb4 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1886,7 +1886,10 @@ static void ptrace_stop(int exit_code, int why, int clear_code,
siginfo_t *info)
 		if (gstop_done)
 			do_notify_parent_cldstop(current, false, why);
 
+		spin_lock_irq(&current->sighand->siglock);
 		__set_current_state(TASK_RUNNING);
+		spin_unlock_irq(&current->sighand->siglock);
+
 		if (clear_code)
 			current->exit_code = 0;
 		read_unlock(&tasklist_lock);
-- 
1.5.5.1

[PATCH 11/33] introduce wake_up_quiescent()

[Oleg Nesterov]

No functional changes. Add the new helper, wake_up_quiescent(task, state),
which simply returns wake_up_state(task, state). Change all callers which
do wake_up_state(STOPPED/TRACED) to use the new helper. ptrace_stop() is
a bit special, it does __set_current_state(RUNNING) in the very unlikely
case, change it as well.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 include/linux/signal.h |    2 ++
 kernel/ptrace.c        |    2 +-
 kernel/signal.c        |   12 ++++++++++--
 kernel/utrace.c        |    2 +-
 4 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/include/linux/signal.h b/include/linux/signal.h
index a822300..2be3712 100644
--- a/include/linux/signal.h
+++ b/include/linux/signal.h
@@ -239,6 +239,8 @@ static inline int valid_signal(unsigned long sig)
 struct timespec;
 struct pt_regs;
 
+extern int wake_up_quiescent(struct task_struct *p, unsigned int state);
+
 extern int next_signal(struct sigpending *pending, sigset_t *mask);
 extern int do_send_sig_info(int sig, struct siginfo *info,
 				struct task_struct *p, bool group);
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index daf47bc..8439ef1 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -621,7 +621,7 @@ static int ptrace_resume(struct task_struct *child, long request,
 	child->exit_code = data;
 
 	if (lock_task_sighand(child, &flags)) {
-		wake_up_state(child, __TASK_TRACED);
+		wake_up_quiescent(child, __TASK_TRACED);
 		unlock_task_sighand(child, &flags);
 	}
 
diff --git a/kernel/signal.c b/kernel/signal.c
index 4512bb4..99a6008 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -702,6 +702,14 @@ void signal_wake_up(struct task_struct *t, int resume)
 }
 
 /*
+ * wakes up the STOPPED/TRACED task, must be called with ->siglock held.
+ */
+int wake_up_quiescent(struct task_struct *p, unsigned int state)
+{
+	return wake_up_state(p, state);
+}
+
+/*
  * Remove signals in mask from the pending set and queue.
  * Returns 1 if any signals were found.
  *
@@ -888,7 +896,7 @@ static int prepare_signal(int sig, struct task_struct *p, int
from_ancestor_ns)
 			task_clear_jobctl_pending(t, JOBCTL_STOP_PENDING);
 			rm_from_queue(SIG_KERNEL_STOP_MASK, &t->pending);
 			if (likely(!(t->ptrace & PT_SEIZED)))
-				wake_up_state(t, __TASK_STOPPED);
+				wake_up_quiescent(t, __TASK_STOPPED);
 			else
 				ptrace_trap_notify(t);
 		} while_each_thread(p, t);
@@ -1887,7 +1895,7 @@ static void ptrace_stop(int exit_code, int why, int clear_code,
siginfo_t *info)
 			do_notify_parent_cldstop(current, false, why);
 
 		spin_lock_irq(&current->sighand->siglock);
-		__set_current_state(TASK_RUNNING);
+		wake_up_quiescent(current, __TASK_TRACED);
 		spin_unlock_irq(&current->sighand->siglock);
 
 		if (clear_code)
diff --git a/kernel/utrace.c b/kernel/utrace.c
index 5d3974e..cebc390 100644
--- a/kernel/utrace.c
+++ b/kernel/utrace.c
@@ -648,7 +648,7 @@ static void utrace_wakeup(struct task_struct *target, struct utrace
*utrace)
 {
 	lockdep_assert_held(&utrace->lock);
 	spin_lock_irq(&target->sighand->siglock);
-	wake_up_state(target, __TASK_TRACED);
+	wake_up_quiescent(target, __TASK_TRACED);
 	spin_unlock_irq(&target->sighand->siglock);
 }
 
-- 
1.5.5.1

[PATCH 12/33] introduce ptrace_signal_wake_up()

[Oleg Nesterov]

Add the new helper, ptrace_signal_wake_up(), change ptrace.c/signal.c
to use it instead of signal_wake_up() to wake up a STOPPED/TRACED task.

The new helper does almost the same, except:

	- it doesn't use the TASK_WAKEKILL bit to wake up the TRACED
	  or STOPPED task, it uses __TASK_STOPPED | __TASK_TRACED
	  explicitly. This is what ptrace actually wants, it should
	  never wake up a TASK_KILLABLE task.

	  This should be cleanuped upatream, signal_wake_up() should
	  take the state as an argument, not a boolean. Until then
	  we add a new static helper.

	- it uses wake_up_quiescent() instead of wake_up_state().

Thereafter every change from STOPPED/TRACED to RUNNING is done via
wake_up_quiescent().

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 include/linux/ptrace.h |    1 +
 kernel/ptrace.c        |   20 ++++++++++++++++----
 kernel/signal.c        |    2 +-
 3 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index 800f113..6d9282a 100644
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -113,6 +113,7 @@
 #include <linux/compiler.h>		/* For unlikely.  */
 #include <linux/sched.h>		/* For struct task_struct.  */
 
+extern void ptrace_signal_wake_up(struct task_struct *p, int quiescent);
 
 extern long arch_ptrace(struct task_struct *child, long request,
 			unsigned long addr, unsigned long data);
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 8439ef1..a464ab5 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -25,6 +25,18 @@
 #include <linux/hw_breakpoint.h>
 #include <linux/cn_proc.h>
 
+void ptrace_signal_wake_up(struct task_struct *p, int quiescent)
+{
+	unsigned int state;
+
+	set_tsk_thread_flag(p, TIF_SIGPENDING);
+
+	state = TASK_INTERRUPTIBLE;
+	if (quiescent)
+		state |= (__TASK_STOPPED | __TASK_TRACED);
+	if (!wake_up_quiescent(p, state))
+		kick_process(p);
+}
 
 static int ptrace_trapping_sleep_fn(void *flags)
 {
@@ -106,7 +118,7 @@ void __ptrace_unlink(struct task_struct *child)
 	 * TASK_KILLABLE sleeps.
 	 */
 	if (child->jobctl & JOBCTL_STOP_PENDING || task_is_traced(child))
-		signal_wake_up(child, task_is_traced(child));
+		ptrace_signal_wake_up(child, task_is_traced(child));
 
 	spin_unlock(&child->sighand->siglock);
 }
@@ -296,7 +308,7 @@ static int ptrace_attach(struct task_struct *task, long request,
 	 */
 	if (task_is_stopped(task) &&
 	    task_set_jobctl_pending(task, JOBCTL_TRAP_STOP | JOBCTL_TRAPPING))
-		signal_wake_up(task, 1);
+		ptrace_signal_wake_up(task, 1);
 
 	spin_unlock(&task->sighand->siglock);
 
@@ -731,7 +743,7 @@ int ptrace_request(struct task_struct *child, long request,
 		 * tracee into STOP.
 		 */
 		if (likely(task_set_jobctl_pending(child, JOBCTL_TRAP_STOP)))
-			signal_wake_up(child, child->jobctl & JOBCTL_LISTENING);
+			ptrace_signal_wake_up(child, child->jobctl & JOBCTL_LISTENING);
 
 		unlock_task_sighand(child, &flags);
 		ret = 0;
@@ -757,7 +769,7 @@ int ptrace_request(struct task_struct *child, long request,
 			 * start of this trap and now.  Trigger re-trap.
 			 */
 			if (child->jobctl & JOBCTL_TRAP_NOTIFY)
-				signal_wake_up(child, true);
+				ptrace_signal_wake_up(child, true);
 			ret = 0;
 		}
 		unlock_task_sighand(child, &flags);
diff --git a/kernel/signal.c b/kernel/signal.c
index 99a6008..7a47a93 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -854,7 +854,7 @@ static void ptrace_trap_notify(struct task_struct *t)
 	assert_spin_locked(&t->sighand->siglock);
 
 	task_set_jobctl_pending(t, JOBCTL_TRAP_NOTIFY);
-	signal_wake_up(t, t->jobctl & JOBCTL_LISTENING);
+	ptrace_signal_wake_up(t, t->jobctl & JOBCTL_LISTENING);
 }
 
 /*
-- 
1.5.5.1

[PATCH 13/33] wait_task_inactive: treat task->state and match_state as bitmasks

[Oleg Nesterov]

Change wait_task_inactive() to check "state & match_state" instead of
"state == match_state". This should not make any difference, but this
allows us to add more "stopped" bits which can be set or cleared
independently.

IOW. wait_task_inactive() assumes that if task->state != 0, it can
only be changed to TASK_RUNNING. Currently this is true, and in this
case "state & match_state" continues to work. But, unlike the current
check, it also works if task->state has other bits set while the caller
is only interested in, say, __TASK_TRACED.

Note: I think wait_task_inactive() should be cleanuped upstrean anyway,
nowadays we have TASK_WAKING and task->state != 0 doesn't necessarily
mean it is TASK_RUNNING. It also makes sense to exclude the !TASK_REPORT
bits during the check. Finally, probably this patch makes sense anyway
even without utrace. For example, a stopped _and_ traced thread could
have task->state = TASK_STOPPED | TASK_TRACED, this can be useful.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 kernel/sched.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 0e9344a..84ecc0a 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2443,7 +2443,7 @@ unsigned long wait_task_inactive(struct task_struct *p, long
match_state)
 		 * is actually now running somewhere else!
 		 */
 		while (task_running(rq, p)) {
-			if (match_state && unlikely(p->state != match_state))
+			if (match_state && !likely(p->state & match_state))
 				return 0;
 			cpu_relax();
 		}
-- 
1.5.5.1

[PATCH 14/33] introduce TASK_UTRACED state

[Oleg Nesterov]

Introduce TASK_UTRACED state, will be used by utrace instead of TASK_TRACED.

Note: this state is reported as "t (tracing stop)" to the user-space to
avoid the confusion. IOW, it looks like TASK_TRACED in /proc/pid/status.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 fs/proc/array.c       |   11 ++++++-----
 include/linux/sched.h |   20 +++++++++++---------
 2 files changed, 17 insertions(+), 14 deletions(-)

diff --git a/fs/proc/array.c b/fs/proc/array.c
index f0c0ea2..e0daec4 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -138,11 +138,12 @@ static const char * const task_state_array[] = {
 	"D (disk sleep)",	/*   2 */
 	"T (stopped)",		/*   4 */
 	"t (tracing stop)",	/*   8 */
-	"Z (zombie)",		/*  16 */
-	"X (dead)",		/*  32 */
-	"x (dead)",		/*  64 */
-	"K (wakekill)",		/* 128 */
-	"W (waking)",		/* 256 */
+	"t (tracing stop)",	/*  16 (stopped by utrace) */
+	"Z (zombie)",		/*  32 */
+	"X (dead)",		/*  64 */
+	"x (dead)",		/* 128 */
+	"K (wakekill)",		/* 256 */
+	"W (waking)",		/* 512 */
 };
 
 static inline const char *get_task_state(struct task_struct *tsk)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4d45f93..746b4d3 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -185,16 +185,17 @@ print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
 #define TASK_UNINTERRUPTIBLE	2
 #define __TASK_STOPPED		4
 #define __TASK_TRACED		8
+#define __TASK_UTRACED		16
 /* in tsk->exit_state */
-#define EXIT_ZOMBIE		16
-#define EXIT_DEAD		32
+#define EXIT_ZOMBIE		32
+#define EXIT_DEAD		64
 /* in tsk->state again */
-#define TASK_DEAD		64
-#define TASK_WAKEKILL		128
-#define TASK_WAKING		256
-#define TASK_STATE_MAX		512
+#define TASK_DEAD		128
+#define TASK_WAKEKILL		256
+#define TASK_WAKING		512
+#define TASK_STATE_MAX		1024
 
-#define TASK_STATE_TO_CHAR_STR "RSDTtZXxKW"
+#define TASK_STATE_TO_CHAR_STR "RSDTtUZXxKW"
 
 extern char ___assert_task_state[1 - 2*!!(
 		sizeof(TASK_STATE_TO_CHAR_STR)-1 != ilog2(TASK_STATE_MAX)+1)];
@@ -203,15 +204,16 @@ extern char ___assert_task_state[1 - 2*!!(
 #define TASK_KILLABLE		(TASK_WAKEKILL | TASK_UNINTERRUPTIBLE)
 #define TASK_STOPPED		(TASK_WAKEKILL | __TASK_STOPPED)
 #define TASK_TRACED		(TASK_WAKEKILL | __TASK_TRACED)
+#define TASK_UTRACED		(TASK_WAKEKILL | __TASK_UTRACED)
 
 /* Convenience macros for the sake of wake_up */
 #define TASK_NORMAL		(TASK_INTERRUPTIBLE | TASK_UNINTERRUPTIBLE)
-#define TASK_ALL		(TASK_NORMAL | __TASK_STOPPED | __TASK_TRACED)
+#define TASK_ALL		(TASK_NORMAL | __TASK_STOPPED | __TASK_TRACED | __TASK_UTRACED)
 
 /* get_task_state() */
 #define TASK_REPORT		(TASK_RUNNING | TASK_INTERRUPTIBLE | \
 				 TASK_UNINTERRUPTIBLE | __TASK_STOPPED | \
-				 __TASK_TRACED)
+				 __TASK_TRACED | __TASK_UTRACED)
 
 #define task_is_traced(task)	((task->state & __TASK_TRACED) != 0)
 #define task_is_stopped(task)	((task->state & __TASK_STOPPED) != 0)
-- 
1.5.5.1

[PATCH 15/33] utrace: use TASK_UTRACED instead of TASK_TRACED

[Oleg Nesterov]

Change utrace.c to use TASK_UTRACED instead of TASK_TRACED.

- utrace_stop/utrace_wakeup: simply use the new state

- utrace_do_stop: do not clear STOPPED/TRACED, but add the new
  __TASK_UTRACED bit to state the fact that both ptrace and utrace
  want this task to be stopped

- naturally, do not use task_is_traced() to check if this task was
  stopped by utrace, use the new task_is_utraced() helper which
  checks __TASK_UTRACED.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 kernel/utrace.c |   26 ++++++++++++++------------
 1 files changed, 14 insertions(+), 12 deletions(-)

diff --git a/kernel/utrace.c b/kernel/utrace.c
index cebc390..2097103 100644
--- a/kernel/utrace.c
+++ b/kernel/utrace.c
@@ -462,6 +462,8 @@ static void put_detached_list(struct list_head *list)
  */
 #define ENGINE_STOP		(1UL << _UTRACE_NEVENTS)
 
+#define task_is_utraced(task)	((task->state & __TASK_UTRACED) != 0)
+
 static void mark_engine_wants_stop(struct task_struct *task,
 				   struct utrace_engine *engine)
 {
@@ -576,7 +578,7 @@ int utrace_set_events(struct task_struct *target,
 
 	ret = 0;
 	if ((old_flags & ~events) && target != current &&
-	    !task_is_stopped_or_traced(target) && !target->exit_state) {
+	    !task_is_utraced(target) && !target->exit_state) {
 		/*
 		 * This barrier ensures that our engine->flags changes
 		 * have hit before we examine utrace->reporting,
@@ -623,21 +625,21 @@ static void mark_engine_detached(struct utrace_engine *engine)
  */
 static bool utrace_do_stop(struct task_struct *target, struct utrace *utrace)
 {
-	if (task_is_stopped(target)) {
+	if (task_is_stopped_or_traced(target)) {
 		/*
 		 * Stopped is considered quiescent; when it wakes up, it will
 		 * go through utrace_finish_stop() before doing anything else.
 		 */
 		spin_lock_irq(&target->sighand->siglock);
-		if (likely(task_is_stopped(target)))
-			__set_task_state(target, TASK_TRACED);
+		if (likely(task_is_stopped_or_traced(target)))
+			target->state |= TASK_UTRACED;
 		spin_unlock_irq(&target->sighand->siglock);
 	} else if (utrace->resume > UTRACE_REPORT) {
 		utrace->resume = UTRACE_REPORT;
 		set_notify_resume(target);
 	}
 
-	return task_is_traced(target);
+	return task_is_utraced(target);
 }
 
 /*
@@ -648,7 +650,7 @@ static void utrace_wakeup(struct task_struct *target, struct utrace
*utrace)
 {
 	lockdep_assert_held(&utrace->lock);
 	spin_lock_irq(&target->sighand->siglock);
-	wake_up_quiescent(target, __TASK_TRACED);
+	wake_up_quiescent(target, __TASK_UTRACED);
 	spin_unlock_irq(&target->sighand->siglock);
 }
 
@@ -710,7 +712,7 @@ static bool utrace_reset(struct task_struct *task, struct utrace
*utrace)
 	/*
 	 * If no more engines want it stopped, wake it up.
 	 */
-	if (task_is_traced(task) && !(flags & ENGINE_STOP)) {
+	if (task_is_utraced(task) && !(flags & ENGINE_STOP)) {
 		/*
 		 * It just resumes, so make sure single-step
 		 * is not left set.
@@ -749,7 +751,7 @@ void utrace_finish_stop(void)
 }
 
 /*
- * Perform %UTRACE_STOP, i.e. block in TASK_TRACED until woken up.
+ * Perform %UTRACE_STOP, i.e. block in TASK_UTRACED until woken up.
  * @task == current, @utrace == current->utrace, which is not locked.
  * Return true if we were woken up by SIGKILL even though some utrace
  * engine may still want us to stay stopped.
@@ -799,7 +801,7 @@ relock:
 		return;
 	}
 
-	__set_current_state(TASK_TRACED);
+	__set_current_state(TASK_UTRACED);
 
 	spin_unlock_irq(&task->sighand->siglock);
 	spin_unlock(&utrace->lock);
@@ -809,14 +811,14 @@ relock:
 	utrace_finish_stop();
 
 	/*
-	 * While in TASK_TRACED, we were considered "frozen enough".
+	 * While in TASK_UTRACED, we were considered "frozen enough".
 	 * Now that we woke up, it's crucial if we're supposed to be
 	 * frozen that we freeze now before running anything substantial.
 	 */
 	try_to_freeze();
 
 	/*
-	 * While we were in TASK_TRACED, complete_signal() considered
+	 * While we were in TASK_UTRACED, complete_signal() considered
 	 * us "uninterested" in signal wakeups.  Now make sure our
 	 * TIF_SIGPENDING state is correct for normal running.
 	 */
@@ -1087,7 +1089,7 @@ int utrace_control(struct task_struct *target,
 	if (unlikely(IS_ERR(utrace)))
 		return PTR_ERR(utrace);
 
-	reset = task_is_traced(target);
+	reset = task_is_utraced(target);
 	ret = 0;
 
 	/*
-- 
1.5.5.1

[PATCH 16/33] reintroduce tracehook_finish_jctl() as utrace_end_stop()

[Oleg Nesterov]

utrace_finish_stop() is needed to avoid the races with SIGKILL which
wakes up UTRACED task, and thus it should be called every time after
the STOPPED/TRACED/UTRACED returns from schedule(), remember that
TASK_UTRACED can be added while the task is STOPPED/UTRACED.

- change do_signal_state() to call this helper right after schedule(),
  otherwise this logic is broken by the upstream changes

- now that utrace doesn't control TASK_TRACED bit, ptrace_stop() must
  call this helper too.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 include/linux/utrace.h |   11 +++++++++++
 kernel/signal.c        |    5 +++++
 kernel/utrace.c        |    2 +-
 3 files changed, 17 insertions(+), 1 deletions(-)

diff --git a/include/linux/utrace.h b/include/linux/utrace.h
index cf13839..0279c74 100644
--- a/include/linux/utrace.h
+++ b/include/linux/utrace.h
@@ -719,4 +719,15 @@ static inline void utrace_exit_notify(struct task_struct *task,
 		utrace_report_death(task, group_dead, signal);
 }
 
+/**
+ * utrace_end_stop - report about return from STOPPED/TRACED
+ *
+ * This is called by do_signal_stop() and ptrace_stop after wakeup.
+ */
+static inline void utrace_end_stop(void)
+{
+	if (task_utrace_flags(current))
+		utrace_finish_stop();
+}
+
 #endif	/* linux/utrace.h */
diff --git a/kernel/signal.c b/kernel/signal.c
index 7a47a93..ba46eab 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1903,6 +1903,8 @@ static void ptrace_stop(int exit_code, int why, int clear_code,
siginfo_t *info)
 		read_unlock(&tasklist_lock);
 	}
 
+	utrace_end_stop();
+
 	/*
 	 * While in TASK_TRACED, we were considered "frozen enough".
 	 * Now that we woke up, it's crucial if we're supposed to be
@@ -2067,6 +2069,9 @@ static bool do_signal_stop(int signr)
 
 		/* Now we don't run again until woken by SIGCONT or SIGKILL */
 		schedule();
+
+		utrace_end_stop();
+
 		return true;
 	} else {
 		/*
diff --git a/kernel/utrace.c b/kernel/utrace.c
index 2097103..d41b982 100644
--- a/kernel/utrace.c
+++ b/kernel/utrace.c
@@ -741,7 +741,7 @@ static bool utrace_reset(struct task_struct *task, struct utrace
*utrace)
 void utrace_finish_stop(void)
 {
 	/*
-	 * If we were task_is_traced() and then SIGKILL'ed, make
+	 * If we were task_is_utraced() and then SIGKILL'ed, make
 	 * sure we do nothing until the tracer drops utrace->lock.
 	 */
 	if (unlikely(__fatal_signal_pending(current))) {
-- 
1.5.5.1

[PATCH 17/33] teach wake_up_quiescent() to do "selective" wake_up

[Oleg Nesterov]

Both utrace and ptrace can want the same thread to be quiescent, in this
case its state is TASK_TRACED | TASK_UTRACED. And this also means that
this task must not run unless both utrace and ptrace resume it.

Change wake_up_quiescent(p, state) to do "p->state &= ~state" and return
false unless there is no more "quiescent" bits in task->state.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 kernel/signal.c |   15 +++++++++++++++
 1 files changed, 15 insertions(+), 0 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index ba46eab..e06f795 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -701,11 +701,26 @@ void signal_wake_up(struct task_struct *t, int resume)
 		kick_process(t);
 }
 
+#define STATE_QUIESCENT	(__TASK_STOPPED | __TASK_TRACED | __TASK_UTRACED)
 /*
  * wakes up the STOPPED/TRACED task, must be called with ->siglock held.
  */
 int wake_up_quiescent(struct task_struct *p, unsigned int state)
 {
+	unsigned int quiescent = (p->state & STATE_QUIESCENT);
+
+	WARN_ON(state & ~(STATE_QUIESCENT | TASK_INTERRUPTIBLE));
+
+	if (quiescent) {
+		state &= ~TASK_INTERRUPTIBLE;
+		if ((quiescent & ~state) != 0) {
+			p->state &= ~state;
+			WARN_ON(!(p->state & STATE_QUIESCENT));
+			WARN_ON(!(p->state & TASK_WAKEKILL));
+			return 0;
+		}
+	}
+
 	return wake_up_state(p, state);
 }
 
-- 
1.5.5.1

[PATCH 18/33] ptrace_stop: do not assume the task is running after wake_up_quiescent()

[Oleg Nesterov]

If ptrace_stop() sets TASK_TRACED and then detects we should not stop,
it can race with utrace_do_stop() which can see TASK_TRACED and add
TASK_UTRACED. In this case we should stop for utrace needs.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 kernel/signal.c |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index e06f795..9348da6 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1916,6 +1916,14 @@ static void ptrace_stop(int exit_code, int why, int clear_code,
siginfo_t *info)
 		if (clear_code)
 			current->exit_code = 0;
 		read_unlock(&tasklist_lock);
+
+		/*
+		 * It is possible that __TASK_UTRACED was added by utrace
+		 * while we were __TASK_TRACED and before we take ->siglock
+		 * for wake_up_quiescent(), we need to block in this case.
+		 * Otherwise this is unnecessary but absolutely harmless.
+		 */
+		schedule();
 	}
 
 	utrace_end_stop();
-- 
1.5.5.1

[PATCH 19/33] get_signal_to_deliver: restore/restructure utrace/ptrace signal reporting

[Oleg Nesterov]

- Reintroduce tracehook_get_signal() as utrace_hook_signal().

- Change get_signal_to_deliver() to call utrace_hook_signal() first,
  before dequeue_signal()

- Always call ptrace_signal() if signal != SIGKILL, no matter whether
  this signal comes from utrace or not.

  Since this can change signr again, update "struct k_sigaction *ka"
  in this case.

IOW, roughly, ptrace acts as if it is the last attached engine, it
takes the final decision about the signal.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 include/linux/utrace.h |   31 +++++++++++++++++++++++++++++++
 kernel/signal.c        |   30 ++++++++++++++++++++----------
 2 files changed, 51 insertions(+), 10 deletions(-)

diff --git a/include/linux/utrace.h b/include/linux/utrace.h
index 0279c74..63103e2 100644
--- a/include/linux/utrace.h
+++ b/include/linux/utrace.h
@@ -730,4 +730,35 @@ static inline void utrace_end_stop(void)
 		utrace_finish_stop();
 }
 
+/**
+ * utrace_hook_signal - deliver synthetic signal to traced task
+ * @task:		@current
+ * @regs:		task_pt_regs(@current)
+ * @info:		details of synthetic signal
+ * @return_ka:		sigaction for synthetic signal
+ *
+ * Return zero to check for a real pending signal normally.
+ * Return -1 after releasing the siglock to repeat the check.
+ * Return a signal number to induce an artificial signal delivery,
+ * setting *@info and *@return_ka to specify its details and behavior.
+ *
+ * The @return_ka->sa_handler value controls the disposition of the
+ * signal, no matter the signal number.  For %SIG_DFL, the return value
+ * is a representative signal to indicate the behavior (e.g. %SIGTERM
+ * for death, %SIGQUIT for core dump, %SIGSTOP for job control stop,
+ * %SIGTSTP for stop unless in an orphaned pgrp), but the signal number
+ * reported will be @info->si_signo instead.
+ *
+ * Called with @task->sighand->siglock held, before dequeuing pending signals.
+ */
+static inline int utrace_hook_signal(struct task_struct *task,
+				       struct pt_regs *regs,
+				       siginfo_t *info,
+				       struct k_sigaction *return_ka)
+{
+	if (unlikely(task_utrace_flags(task)))
+		return utrace_get_signal(task, regs, info, return_ka);
+	return 0;
+}
+
 #endif	/* linux/utrace.h */
diff --git a/kernel/signal.c b/kernel/signal.c
index 9348da6..38ea4e6 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2242,17 +2242,27 @@ relock:
 	for (;;) {
 		struct k_sigaction *ka;
 
-		if (unlikely(current->jobctl & JOBCTL_STOP_PENDING) &&
-		    do_signal_stop(0))
+		signr = utrace_hook_signal(current, regs, info, return_ka);
+		if (unlikely(signr < 0))
 			goto relock;
 
-		if (unlikely(current->jobctl & JOBCTL_TRAP_MASK)) {
-			do_jobctl_trap();
-			spin_unlock_irq(&sighand->siglock);
-			goto relock;
-		}
+		if (unlikely(signr != 0))
+			ka = return_ka;
+		else {
+			if (unlikely(current->jobctl & JOBCTL_STOP_PENDING) &&
+			    do_signal_stop(0))
+				goto relock;
 
-		signr = dequeue_signal(current, &current->blocked, info);
+			if (unlikely(current->jobctl & JOBCTL_TRAP_MASK)) {
+				do_jobctl_trap();
+				spin_unlock_irq(&sighand->siglock);
+				goto relock;
+			}
+
+			signr = dequeue_signal(current, &current->blocked, info);
+
+			ka = &sighand->action[signr-1];
+		}
 
 		if (!signr)
 			break; /* will return 0 */
@@ -2262,9 +2272,9 @@ relock:
 					      regs, cookie);
 			if (!signr)
 				continue;
-		}
 
-		ka = &sighand->action[signr-1];
+			ka = &sighand->action[signr-1];
+		}
 
 		/* Trace actually delivered signals. */
 		trace_signal_deliver(signr, info, ka);
-- 
1.5.5.1

[PATCH 20/33] utrace_get_signal: s/JOBCTL_STOP_PENDING/JOBCTL_PENDING_MASK/

[Oleg Nesterov]

utrace_get_signal() checks JOBCTL_STOP_PENDING to detect the
case when we should not try to dequeue the signal but should
try to participate in the group-stop.

With the recent changes this is not enough, everything which
contrbutes to recalc_sigpending_tsk() should be respected.

Check JOBCTL_PENDING_MASK instead. This matches the
JOBCTL_STOP_PENDING | JOBCTL_TRAP_MASK code in the caller,
get_signal_to_deliver(). Note that this code won't run if
utrace_get_signal() returns signr > 0.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 kernel/utrace.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/utrace.c b/kernel/utrace.c
index d41b982..0bb0a06 100644
--- a/kernel/utrace.c
+++ b/kernel/utrace.c
@@ -2027,7 +2027,7 @@ int utrace_get_signal(struct task_struct *task, struct pt_regs
*regs,
 		ka = NULL;
 		memset(return_ka, 0, sizeof *return_ka);
 	} else if (!(task->utrace_flags & UTRACE_EVENT_SIGNAL_ALL) ||
-		   unlikely(task->jobctl & JOBCTL_STOP_PENDING)) {
+		   unlikely(task->jobctl & JOBCTL_PENDING_MASK)) {
 		/*
 		 * If no engine is interested in intercepting signals or
 		 * we must stop, let the caller just dequeue them normally
-- 
1.5.5.1

[PATCH 21/33] introduce ptrace_set_syscall_trace()

[Oleg Nesterov]

No functional changes. Add the new helper, ptrace_set_syscall_trace(),
which should be used to set/clear TIF_SYSCALL_TRACE in ptrace code.
Currently it does nothing more.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 kernel/ptrace.c |   15 ++++++++++-----
 1 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index a464ab5..43357e5 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -38,6 +38,14 @@ void ptrace_signal_wake_up(struct task_struct *p, int quiescent)
 		kick_process(p);
 }
 
+static void ptrace_set_syscall_trace(struct task_struct *p, bool on)
+{
+	if (on)
+		set_tsk_thread_flag(p, TIF_SYSCALL_TRACE);
+	else
+		clear_tsk_thread_flag(p, TIF_SYSCALL_TRACE);
+}
+
 static int ptrace_trapping_sleep_fn(void *flags)
 {
 	schedule();
@@ -418,7 +426,7 @@ static int ptrace_detach(struct task_struct *child, unsigned int
data)
 
 	/* Architecture-specific hardware disable .. */
 	ptrace_disable(child);
-	clear_tsk_thread_flag(child, TIF_SYSCALL_TRACE);
+	ptrace_set_syscall_trace(child, false);
 
 	write_lock_irq(&tasklist_lock);
 	/*
@@ -606,10 +614,7 @@ static int ptrace_resume(struct task_struct *child, long request,
 	if (!valid_signal(data))
 		return -EIO;
 
-	if (request == PTRACE_SYSCALL)
-		set_tsk_thread_flag(child, TIF_SYSCALL_TRACE);
-	else
-		clear_tsk_thread_flag(child, TIF_SYSCALL_TRACE);
+	ptrace_set_syscall_trace(child, request == PTRACE_SYSCALL);
 
 #ifdef TIF_SYSCALL_EMU
 	if (request == PTRACE_SYSEMU || request == PTRACE_SYSEMU_SINGLESTEP)
-- 
1.5.5.1

[PATCH 22/33] introduce PT_SYSCALL_TRACE flag

[Oleg Nesterov]

Currently tracehooks assume that if the ptraced task has
TIF_SYSCALL_TRACE set, the tracee should report the syscall.
This is not true, this thread flag can be set by utrace.

Add the new internal ptrace flag, PT_SYSCALL_TRACE. Change
ptrace_set_syscall_trace() to set/clear this bit along with
TIF_SYSCALL_TRACE, change ptrace_report_syscall() to check
this flag instead of PT_PTRACED.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 include/linux/ptrace.h    |    3 +++
 include/linux/tracehook.h |    2 +-
 kernel/ptrace.c           |    7 +++++--
 3 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index 6d9282a..c10f610 100644
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -104,6 +104,8 @@
 
 #define PT_TRACE_MASK	0x000003f4
 
+#define PT_SYSCALL_TRACE	0x00020000
+
 /* single stepping state bits (used on ARM and PA-RISC) */
 #define PT_SINGLESTEP_BIT	31
 #define PT_SINGLESTEP		(1<<PT_SINGLESTEP_BIT)
@@ -227,6 +229,7 @@ static inline void ptrace_init_task(struct task_struct *child, bool
ptrace)
 
 	if (unlikely(ptrace) && current->ptrace) {
 		child->ptrace = current->ptrace;
+		child->ptrace &= ~PT_SYSCALL_TRACE;
 		__ptrace_link(child, current->parent);
 
 		if (child->ptrace & PT_SEIZED)
diff --git a/include/linux/tracehook.h b/include/linux/tracehook.h
index ec2af67..eb9fe30 100644
--- a/include/linux/tracehook.h
+++ b/include/linux/tracehook.h
@@ -59,7 +59,7 @@ static inline void ptrace_report_syscall(struct pt_regs *regs)
 {
 	int ptrace = current->ptrace;
 
-	if (!(ptrace & PT_PTRACED))
+	if (!(ptrace & PT_SYSCALL_TRACE))
 		return;
 
 	ptrace_notify(SIGTRAP | ((ptrace & PT_TRACESYSGOOD) ? 0x80 : 0));
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 43357e5..1ac03eb 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -40,10 +40,13 @@ void ptrace_signal_wake_up(struct task_struct *p, int quiescent)
 
 static void ptrace_set_syscall_trace(struct task_struct *p, bool on)
 {
-	if (on)
+	if (on) {
+		p->ptrace |= PT_SYSCALL_TRACE;
 		set_tsk_thread_flag(p, TIF_SYSCALL_TRACE);
-	else
+	} else {
+		p->ptrace &= ~PT_SYSCALL_TRACE;
 		clear_tsk_thread_flag(p, TIF_SYSCALL_TRACE);
+	}
 }
 
 static int ptrace_trapping_sleep_fn(void *flags)
-- 
1.5.5.1

[PATCH 23/33] utrace: don't clear TIF_SYSCALL_TRACE if it is needed by ptrace

[Oleg Nesterov]

TIF_SYSCALL_TRACE should be cleared only if both ptrace and utrace do
not want it, change utrace_reset() to check PT_SYSCALL_TRACE before
clear_tsk_thread_flag(TIF_SYSCALL_TRACE).

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 kernel/utrace.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/kernel/utrace.c b/kernel/utrace.c
index 0bb0a06..bebf6de 100644
--- a/kernel/utrace.c
+++ b/kernel/utrace.c
@@ -697,6 +697,7 @@ static bool utrace_reset(struct task_struct *task, struct utrace
*utrace)
 		BUG_ON(utrace->death);
 		flags &= UTRACE_EVENT(REAP);
 	} else if (!(flags & UTRACE_EVENT_SYSCALL) &&
+		   !(task->ptrace & PT_SYSCALL_TRACE) &&
 		   test_tsk_thread_flag(task, TIF_SYSCALL_TRACE)) {
 		clear_tsk_thread_flag(task, TIF_SYSCALL_TRACE);
 	}
-- 
1.5.5.1

[PATCH 24/33] introduce task_utrace_lock/task_utrace_unlock

[Oleg Nesterov]

- Add task_utrace_lock(task). It simply takes task->utrace->lock if
  this task was ever utraced. Otherwise it takes task_lock(), this
  serializes with utrace_attach_task()->utrace_task_alloc(). In both
  case the caller can be sure it can't race with anything which needs
  utrace->lock.

- Add task_utrace_unlock(task), it releases the corresponding lock.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 include/linux/utrace.h |    9 +++++++++
 kernel/utrace.c        |   26 ++++++++++++++++++++++++++
 2 files changed, 35 insertions(+), 0 deletions(-)

diff --git a/include/linux/utrace.h b/include/linux/utrace.h
index 63103e2..f37373b 100644
--- a/include/linux/utrace.h
+++ b/include/linux/utrace.h
@@ -117,6 +117,12 @@ void utrace_signal_handler(struct task_struct *, int);
 
 #ifndef CONFIG_UTRACE
 
+static inline void task_utrace_lock(struct task_struct *task)
+{
+}
+static inline void task_utrace_unlock(struct task_struct *task)
+{
+}
 /*
  * <linux/tracehook.h> uses these accessors to avoid #ifdef CONFIG_UTRACE.
  */
@@ -139,6 +145,9 @@ static inline void task_utrace_proc_status(struct seq_file *m,
 
 #else  /* CONFIG_UTRACE */
 
+extern void task_utrace_lock(struct task_struct *task);
+extern void task_utrace_unlock(struct task_struct *task);
+
 static inline unsigned long task_utrace_flags(struct task_struct *task)
 {
 	return task->utrace_flags;
diff --git a/kernel/utrace.c b/kernel/utrace.c
index bebf6de..960dd9e 100644
--- a/kernel/utrace.c
+++ b/kernel/utrace.c
@@ -79,6 +79,32 @@ static int __init utrace_init(void)
 }
 module_init(utrace_init);
 
+void task_utrace_lock(struct task_struct *task)
+{
+	struct utrace *utrace = task_utrace_struct(task);
+
+	if (!utrace) {
+		task_lock(task);
+		utrace = task_utrace_struct(task);
+		if (!utrace)
+			return;
+
+		task_unlock(task);
+	}
+
+	spin_lock(&utrace->lock);
+}
+
+void task_utrace_unlock(struct task_struct *task)
+{
+	struct utrace *utrace = task_utrace_struct(task);
+
+	if (utrace)
+		spin_unlock(&utrace->lock);
+	else
+		task_unlock(task);
+}
+
 /*
  * Set up @task.utrace for the first time.  We can have races
  * between two utrace_attach_task() calls here.  The task_lock()
-- 
1.5.5.1

[PATCH 25/33] teach ptrace_set_syscall_trace() to play well with utrace

[Oleg Nesterov]

1. ptrace_set_syscall_trace(true)->set_tsk_thread_flag(TIF_SYSCALL_TRACE)
   can race with utrace_control()->utrace_reset() path which can miss
   PT_SYSCALL_TRACE and clear TIF_SYSCALL_TRACE after it was already set.

2. ptrace_set_syscall_trace(false) clears TIF_SYSCALL_TRACE and this is
   not utrace-friendly, it can need this flag.

Change ptrace_set_syscall_trace() to take task_utrace_lock(), this is
enough to fix the 1st problem. Check task_utrace_flags() before clearing
TIF_SYSCALL_TRACE, this fixes 2.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 kernel/ptrace.c |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 1ac03eb..739183a 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -24,6 +24,7 @@
 #include <linux/regset.h>
 #include <linux/hw_breakpoint.h>
 #include <linux/cn_proc.h>
+#include <linux/utrace.h>
 
 void ptrace_signal_wake_up(struct task_struct *p, int quiescent)
 {
@@ -40,13 +41,16 @@ void ptrace_signal_wake_up(struct task_struct *p, int quiescent)
 
 static void ptrace_set_syscall_trace(struct task_struct *p, bool on)
 {
+	task_utrace_lock(p);
 	if (on) {
 		p->ptrace |= PT_SYSCALL_TRACE;
 		set_tsk_thread_flag(p, TIF_SYSCALL_TRACE);
 	} else {
 		p->ptrace &= ~PT_SYSCALL_TRACE;
-		clear_tsk_thread_flag(p, TIF_SYSCALL_TRACE);
+		if (!(task_utrace_flags(p) & UTRACE_EVENT_SYSCALL))
+			clear_tsk_thread_flag(p, TIF_SYSCALL_TRACE);
 	}
+	task_utrace_unlock(p);
 }
 
 static int ptrace_trapping_sleep_fn(void *flags)
-- 
1.5.5.1

[PATCH 26/33] introduce PT_SINGLE_STEP and PT_SINGLE_BLOCK

[Oleg Nesterov]

Add the new internal ptrace flags, PT_SINGLE_STEP and PT_SINGLE_BLOCK.

Like PT_SYSCALL_TRACE, this is needed to avoid the unnecessary ptrace
reports when TIF_SINGLESTEP was set by another engine, not by ptrace.
Also, we need these bits to coordinate the user_*_single_step() calls
from ptrace and utrace.

TODO: update the !x86 ptrace code which does user_disable_single_step().

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 arch/x86/kernel/ptrace.c  |    1 +
 include/linux/ptrace.h    |    5 ++++-
 include/linux/tracehook.h |    7 +++++--
 kernel/ptrace.c           |    3 +++
 4 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 8252879..d1557dc 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -808,6 +808,7 @@ static int ioperm_get(struct task_struct *target,
  */
 void ptrace_disable(struct task_struct *child)
 {
+	child->ptrace &= ~(PT_SINGLE_STEP | PT_SINGLE_BLOCK);
 	user_disable_single_step(child);
 #ifdef TIF_SYSCALL_EMU
 	clear_tsk_thread_flag(child, TIF_SYSCALL_EMU);
diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index c10f610..2743315 100644
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -105,6 +105,8 @@
 #define PT_TRACE_MASK	0x000003f4
 
 #define PT_SYSCALL_TRACE	0x00020000
+#define PT_SINGLE_STEP		0x00040000
+#define PT_SINGLE_BLOCK		0x00080000
 
 /* single stepping state bits (used on ARM and PA-RISC) */
 #define PT_SINGLESTEP_BIT	31
@@ -229,7 +231,8 @@ static inline void ptrace_init_task(struct task_struct *child, bool
ptrace)
 
 	if (unlikely(ptrace) && current->ptrace) {
 		child->ptrace = current->ptrace;
-		child->ptrace &= ~PT_SYSCALL_TRACE;
+		child->ptrace &=
+			~(PT_SYSCALL_TRACE | PT_SINGLE_STEP | PT_SINGLE_BLOCK);
 		__ptrace_link(child, current->parent);
 
 		if (child->ptrace & PT_SEIZED)
diff --git a/include/linux/tracehook.h b/include/linux/tracehook.h
index eb9fe30..21c8ca2 100644
--- a/include/linux/tracehook.h
+++ b/include/linux/tracehook.h
@@ -104,6 +104,9 @@ static inline __must_check int tracehook_report_syscall_entry(
 	return 0;
 }
 
+#define ptrace_wants_step()	\
+	(current->ptrace & (PT_SINGLE_STEP | PT_SINGLE_BLOCK))
+
 /**
  * tracehook_report_syscall_exit - task has just finished a system call
  * @regs:		user register state of current task
@@ -126,7 +129,7 @@ static inline void tracehook_report_syscall_exit(struct pt_regs *regs,
int step)
 	if (task_utrace_flags(current) & UTRACE_EVENT(SYSCALL_EXIT))
 		utrace_report_syscall_exit(regs);
 
-	if (step) {
+	if (step && ptrace_wants_step()) {
 		siginfo_t info;
 		user_single_step_siginfo(current, regs, &info);
 		force_sig_info(SIGTRAP, &info, current);
@@ -157,7 +160,7 @@ static inline void tracehook_signal_handler(int sig, siginfo_t *info,
 {
 	if (task_utrace_flags(current))
 		utrace_signal_handler(current, stepping);
-	if (stepping)
+	if (stepping && ptrace_wants_step())
 		ptrace_notify(SIGTRAP);
 }
 
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 739183a..792080d 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -630,13 +630,16 @@ static int ptrace_resume(struct task_struct *child, long request,
 		clear_tsk_thread_flag(child, TIF_SYSCALL_EMU);
 #endif
 
+	child->ptrace &= ~(PT_SINGLE_STEP | PT_SINGLE_BLOCK);
 	if (is_singleblock(request)) {
 		if (unlikely(!arch_has_block_step()))
 			return -EIO;
+		child->ptrace |= PT_SINGLE_BLOCK;
 		user_enable_block_step(child);
 	} else if (is_singlestep(request) || is_sysemu_singlestep(request)) {
 		if (unlikely(!arch_has_single_step()))
 			return -EIO;
+		child->ptrace |= PT_SINGLE_STEP;
 		user_enable_single_step(child);
 	} else {
 		user_disable_single_step(child);
-- 
1.5.5.1

[PATCH 27/33] utrace: finish_resume_report: don't do user_xxx_step() if ptrace_wants_step()

[Oleg Nesterov]

finish_resume_report() should not enable/disable the stepping if
ptrace_wants_step() == T. If ptrace wants block_step while utrace
wants single_step we could "promote" the stepping, but I do not
think this really makes sense.

Unless the tracee is killed this can't race with ptrace, this is
called by the tracee itself. If it is killed we do not care.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 include/linux/tracehook.h |    8 ++++----
 kernel/utrace.c           |    9 ++++++---
 2 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/include/linux/tracehook.h b/include/linux/tracehook.h
index 21c8ca2..b6812d4 100644
--- a/include/linux/tracehook.h
+++ b/include/linux/tracehook.h
@@ -104,8 +104,8 @@ static inline __must_check int tracehook_report_syscall_entry(
 	return 0;
 }
 
-#define ptrace_wants_step()	\
-	(current->ptrace & (PT_SINGLE_STEP | PT_SINGLE_BLOCK))
+#define ptrace_wants_step(task)	\
+	((task)->ptrace & (PT_SINGLE_STEP | PT_SINGLE_BLOCK))
 
 /**
  * tracehook_report_syscall_exit - task has just finished a system call
@@ -129,7 +129,7 @@ static inline void tracehook_report_syscall_exit(struct pt_regs *regs,
int step)
 	if (task_utrace_flags(current) & UTRACE_EVENT(SYSCALL_EXIT))
 		utrace_report_syscall_exit(regs);
 
-	if (step && ptrace_wants_step()) {
+	if (step && ptrace_wants_step(current)) {
 		siginfo_t info;
 		user_single_step_siginfo(current, regs, &info);
 		force_sig_info(SIGTRAP, &info, current);
@@ -160,7 +160,7 @@ static inline void tracehook_signal_handler(int sig, siginfo_t *info,
 {
 	if (task_utrace_flags(current))
 		utrace_signal_handler(current, stepping);
-	if (stepping && ptrace_wants_step())
+	if (stepping && ptrace_wants_step(current))
 		ptrace_notify(SIGTRAP);
 }
 
diff --git a/kernel/utrace.c b/kernel/utrace.c
index 960dd9e..05e8532 100644
--- a/kernel/utrace.c
+++ b/kernel/utrace.c
@@ -1829,7 +1829,8 @@ static void finish_resume_report(struct task_struct *task,
 
 	case UTRACE_BLOCKSTEP:
 		if (likely(arch_has_block_step())) {
-			user_enable_block_step(task);
+			if (!ptrace_wants_step(task))
+				user_enable_block_step(task);
 			break;
 		}
 
@@ -1842,7 +1843,8 @@ static void finish_resume_report(struct task_struct *task,
 
 	case UTRACE_SINGLESTEP:
 		if (likely(arch_has_single_step())) {
-			user_enable_single_step(task);
+			if (!ptrace_wants_step(task))
+				user_enable_single_step(task);
 		} else {
 			/*
 			 * This means some callback is to blame for failing
@@ -1857,7 +1859,8 @@ static void finish_resume_report(struct task_struct *task,
 	case UTRACE_REPORT:
 	case UTRACE_RESUME:
 	default:
-		user_disable_single_step(task);
+		if (!ptrace_wants_step(task))
+			user_disable_single_step(task);
 		break;
 	}
 }
-- 
1.5.5.1

[PATCH 28/33] ptrace: shift user_*_step() from ptrace_resume() to ptrace_stop()

[Oleg Nesterov]

1. ptrace_resume() plays with the stopped task which can be also
   task_is_utraced(). In this case user_enable_xxx_step() can race
   with utrace_reset()->user_disable_single_step().

   We could change utrace_reset() to check ptrace_wants_step() and
   add the task_utrace_lock + unlock barrier after ptrace_resume()
   sets PT_SINGLE_STEP.

   But it is better to reassign enable/desable from the tracer to
   the tracee, it can check its PT_SINGLE_ bits after wakeup. This
   also makes sense because enable_step(task) can be faster if
   task == current.

2. ptrace can do user_disable_single_step() while utrace needs the
   stepping.

   Set TIF_NOTIFY_RESUME after user_disable_single_step() to ensure
   the tracee can't return to the user-space without utrace_resume().
   Any correct engine which wants the stepping should reassert it.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 kernel/ptrace.c |    4 ----
 kernel/signal.c |   11 +++++++++++
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 792080d..eba9a22 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -635,14 +635,10 @@ static int ptrace_resume(struct task_struct *child, long request,
 		if (unlikely(!arch_has_block_step()))
 			return -EIO;
 		child->ptrace |= PT_SINGLE_BLOCK;
-		user_enable_block_step(child);
 	} else if (is_singlestep(request) || is_sysemu_singlestep(request)) {
 		if (unlikely(!arch_has_single_step()))
 			return -EIO;
 		child->ptrace |= PT_SINGLE_STEP;
-		user_enable_single_step(child);
-	} else {
-		user_disable_single_step(child);
 	}
 
 	child->exit_code = data;
diff --git a/kernel/signal.c b/kernel/signal.c
index 38ea4e6..b13d2bc 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1928,6 +1928,17 @@ static void ptrace_stop(int exit_code, int why, int clear_code,
siginfo_t *info)
 
 	utrace_end_stop();
 
+	if (current->ptrace & PT_SINGLE_BLOCK)
+		user_enable_block_step(current);
+	else if (current->ptrace & PT_SINGLE_STEP)
+		user_enable_single_step(current);
+	else {
+		user_disable_single_step(current);
+		/* if utrace needs the stepping it should reassert */
+		if (task_utrace_flags(current))
+			set_thread_flag(TIF_NOTIFY_RESUME);
+	}
+
 	/*
 	 * While in TASK_TRACED, we were considered "frozen enough".
 	 * Now that we woke up, it's crucial if we're supposed to be
-- 
1.5.5.1

[PATCH 29/33] ptrace_disable: no need to disable stepping

[Oleg Nesterov]

ptrace_disable() is called when the tracee is quiescent and we
are going to untrace. This means we are going to clear ->ptrace
and wake up the tracee. Now that ptrace_stop() checks PT_ bits
and does user_disable_single_step() we can remove this code from
ptrace_disable(), it is unneeded and not utrace-friendly.

TODO: change !x86 code.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 arch/x86/kernel/ptrace.c |    2 --
 1 files changed, 0 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index d1557dc..96d315a 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -808,8 +808,6 @@ static int ioperm_get(struct task_struct *target,
  */
 void ptrace_disable(struct task_struct *child)
 {
-	child->ptrace &= ~(PT_SINGLE_STEP | PT_SINGLE_BLOCK);
-	user_disable_single_step(child);
 #ifdef TIF_SYSCALL_EMU
 	clear_tsk_thread_flag(child, TIF_SYSCALL_EMU);
 #endif
-- 
1.5.5.1

[PATCH 30/33] ptrace_report_syscall: check TIF_SYSCALL_EMU

[Oleg Nesterov]

4d16a64 "introduce PT_SYSCALL_TRACE flag" breaks PTRACE_SYSEMU
which doesn't set PT_SYSCALL_TRACE.

Change ptrace_report_syscall() to check TIF_SYSCALL_EMU as well.
This can't conflict with utrace, this flag can only be set by
ptrace.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 include/linux/tracehook.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/include/linux/tracehook.h b/include/linux/tracehook.h
index b6812d4..90ca578 100644
--- a/include/linux/tracehook.h
+++ b/include/linux/tracehook.h
@@ -59,7 +59,7 @@ static inline void ptrace_report_syscall(struct pt_regs *regs)
 {
 	int ptrace = current->ptrace;
 
-	if (!(ptrace & PT_SYSCALL_TRACE))
+	if (!(ptrace & PT_SYSCALL_TRACE) && !test_thread_flag(TIF_SYSCALL_EMU))
 		return;
 
 	ptrace_notify(SIGTRAP | ((ptrace & PT_TRACESYSGOOD) ? 0x80 : 0));
-- 
1.5.5.1

[PATCH 31/33] utrace_resume: check irqs_disabled() to shut up lockdep

[Oleg Nesterov]

utrace_resume() enables irqs unconditionally. With the recent changes
in lockdep.c this triggers the warning. Check irqs_disabled() before
local_irq_enable().

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 kernel/utrace.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/kernel/utrace.c b/kernel/utrace.c
index 05e8532..c817a46 100644
--- a/kernel/utrace.c
+++ b/kernel/utrace.c
@@ -1881,7 +1881,8 @@ void utrace_resume(struct task_struct *task, struct pt_regs *regs)
 	 * code path leads to calling into get_signal_to_deliver(), which
 	 * implicitly reenables them by virtue of spin_unlock_irq.
 	 */
-	local_irq_enable();
+	if (irqs_disabled())	/* make trace_hardirqs_on() happy */
+		local_irq_enable();
 
 	/*
 	 * If this flag is still set it's because there was a signal
-- 
1.5.5.1

[PATCH 32/33] ptrace_report_syscall: check if TIF_SYSCALL_EMU is defined

[Oleg Nesterov]

From: Tony Breeds <tony(a)bakeyournoodle.com&gt;

TIF_SYSCALL_EMU is x86 only, add ifdef into ptrace_report_syscall().

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 include/linux/tracehook.h |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/include/linux/tracehook.h b/include/linux/tracehook.h
index 90ca578..a1bac95 100644
--- a/include/linux/tracehook.h
+++ b/include/linux/tracehook.h
@@ -59,8 +59,12 @@ static inline void ptrace_report_syscall(struct pt_regs *regs)
 {
 	int ptrace = current->ptrace;
 
-	if (!(ptrace & PT_SYSCALL_TRACE) && !test_thread_flag(TIF_SYSCALL_EMU))
-		return;
+	if (!(ptrace & PT_SYSCALL_TRACE)) {
+#ifdef TIF_SYSCALL_EMU
+		if (!test_thread_flag(TIF_SYSCALL_EMU))
+#endif
+			return;
+	}
 
 	ptrace_notify(SIGTRAP | ((ptrace & PT_TRACESYSGOOD) ? 0x80 : 0));
 
-- 
1.5.5.1

[PATCH 33/33] utrace: s390: fix the compile problem with traps.c

[Oleg Nesterov]

d99e60e5 "tracehooks: reintroduce tracehook_consider_fatal_signal()"
breaks the compilation of arch/s390/kernel/traps.c. Restore the
necessary include removed by upstream 73b7d40f commit.

Signed-off-by: Oleg Nesterov <oleg(a)redhat.com&gt;
---
 arch/s390/kernel/traps.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/arch/s390/kernel/traps.c b/arch/s390/kernel/traps.c
index f506e1b..3132498 100644
--- a/arch/s390/kernel/traps.c
+++ b/arch/s390/kernel/traps.c
@@ -18,7 +18,7 @@
 #include <linux/kernel.h>
 #include <linux/string.h>
 #include <linux/errno.h>
-#include <linux/ptrace.h>
+#include <linux/tracehook.h>
 #include <linux/timer.h>
 #include <linux/mm.h>
 #include <linux/smp.h>
-- 
1.5.5.1

[Josh Boyer]

On Mon, Nov 21, 2011 at 09:00:39PM +0100, Oleg Nesterov wrote:
>  Hello,
>  
>  utrace patches for 3.2 kernel. 
Thanks.  I'll update the utrace.patch we have in rawhide shortly.

josh

[Dave Jones]

On Mon, Nov 21, 2011 at 03:09:48PM -0500, Josh Boyer wrote:
>  On Mon, Nov 21, 2011 at 09:00:39PM +0100, Oleg Nesterov wrote:
>  > Hello,
>  > 
>  > utrace patches for 3.2 kernel.
>  
>  Thanks.  I'll update the utrace.patch we have in rawhide shortly. 
What's the latest situation of the work happening upstream ?

	Dave

[Frank Ch. Eigler]

Hi -

On Mon, Nov 21, 2011 at 03:58:51PM -0500, Dave Jones wrote:
>   > > utrace patches for 3.2 kernel.
>   > Thanks.  I'll update the utrace.patch we have in rawhide shortly.
>  What's the latest situation of the work happening upstream ? 
You're probably referring to this:  http://lkml.org/lkml/2011/11/10/408
and associated systemtap code to use it:
http://sourceware.org/ml/systemtap/2011-q4/msg00185.html

- FChE

[Josh Boyer]

On Mon, Nov 21, 2011 at 04:03:55PM -0500, Frank Ch. Eigler wrote:
>  Hi -
>  
>  On Mon, Nov 21, 2011 at 03:58:51PM -0500, Dave Jones wrote:
>  >  > > utrace patches for 3.2 kernel.
>  >  > Thanks.  I'll update the utrace.patch we have in rawhide shortly.
>  > What's the latest situation of the work happening upstream ?
>  
>  You're probably referring to this:  http://lkml.org/lkml/2011/11/10/408
>  and associated systemtap code to use it:
> http://sourceware.org/ml/systemtap/2011-q4/msg00185.html 
If that means we don't carry utrace.patch at all anymore once it's
accepted, then maybe yes.  Speaking bluntly, that's the immediate goal
to get us one step closer to upstream.

However, utrace and uprobes are different things.  Are we going to drop
the utrace parts as soon as uprobes are in and basically give up on
that effort?

josh

[Frank Ch. Eigler]

Hi, Joshn -

>  > >  > Thanks.  I'll update the utrace.patch we have in
> rawhide shortly.
>  > > What's the latest situation of the work happening upstream ?
>  > 
>  > You're probably referring to this:  http://lkml.org/lkml/2011/11/10/408 [...]
>  If that means we don't carry utrace.patch at all anymore once
> it's
>  accepted, then maybe yes.  [...]  Are we going to drop the utrace
>  parts as soon as uprobes are in and basically give up on that
>  effort? 
For a long time, our plan has been to get uprobes upstream, and at
some point thereafter drop utrace.  They are not identical, so in fact
we'll suffer some functionality/performance regressions, until further
improvements to upstream-bound uprobes are made.  Perhaps that might
justify keeping utrace around a little while after uprobes is merged.
But we anticipate no further upstream utrace effort.

- FChE