arm64 uprobes support showing signs of life

[William Cohen]

Hi All,

Pratyush has been implementing uprobes support for aarch64 and has
posted a set of patches
(http://comments.gmane.org/gmane.linux.ports.arm.kernel/382237) for
review.  The really new Linux kernels requires a patch to the
systemtap runtime because the f_dentry macro has been removed.  With
the patched systemtap I was able to run the systemtap testsuite and
get some test results exercising the uprobes support:

https://web.elastic.org/~dejazilla/viewsummary.php?summary=%3D%27%3C54A9E21C...

There looked to be a localized fixes for plt support to eliminate the
unsupported systemtap.base/list.exp plt-* tests.

A number of the tests appear to fail because of userspace arguments
cannot be found for sdt probes.

The uprobe patches still need some refinement.  On a number of
systemtap "make installcheck" runs the kernel would get stuck spewing
out:

Unexpected kernel single-step exception at EL1

However, things are looking better for user-space probing on arm64.

-Will

[Peter Robinson]

On Mon, Jan 5, 2015 at 4:06 PM, William Cohen  wrote:
> Hi All,
>
> Pratyush has been implementing uprobes support for aarch64 and has
> posted a set of patches
> (http://comments.gmane.org/gmane.linux.ports.arm.kernel/382237) for
> review.  The really new Linux kernels requires a patch to the
> systemtap runtime because the f_dentry macro has been removed.  With
> the patched systemtap I was able to run the systemtap testsuite and
> get some test results exercising the uprobes support:
>
> https://web.elastic.org/~dejazilla/viewsummary.php?summary=%3D%27%3C54A9E21C...
>
> There looked to be a localized fixes for plt support to eliminate the
> unsupported systemtap.base/list.exp plt-* tests.
>
> A number of the tests appear to fail because of userspace arguments
> cannot be found for sdt probes.
>
> The uprobe patches still need some refinement.  On a number of
> systemtap "make installcheck" runs the kernel would get stuck spewing
> out:
>
> Unexpected kernel single-step exception at EL1
>
> However, things are looking better for user-space probing on arm64.

good news, thanks for the update.

Peter

[Pratyush Anand]

Hi Will,

On Monday 05 January 2015 09:36 PM, William Cohen wrote:
> Hi All,
>
> Pratyush has been implementing uprobes support for aarch64 and has
> posted a set of patches
> (http://comments.gmane.org/gmane.linux.ports.arm.kernel/382237) for
> review.  The really new Linux kernels requires a patch to the
> systemtap runtime because the f_dentry macro has been removed.  With
> the patched systemtap I was able to run the systemtap testsuite and
> get some test results exercising the uprobes support:
>
> https://web.elastic.org/~dejazilla/viewsummary.php?summary=%3D%27%3C54A9E21C...
>
> There looked to be a localized fixes for plt support to eliminate the
> unsupported systemtap.base/list.exp plt-* tests.
>
> A number of the tests appear to fail because of userspace arguments
> cannot be found for sdt probes.
>
> The uprobe patches still need some refinement.  On a number of
> systemtap "make installcheck" runs the kernel would get stuck spewing
> out:
>
> Unexpected kernel single-step exception at EL1

There are at least two functions of arm64/uprobe implementation 
uprobe_breakpoint_handler and uprobe_single_step_handler, where a kprobe 
insertion might be causing above issue. Currently I have qualified these 
functions with __kprobe, so that one can not insert a kprobe there. With 
this you should not be able to see the above message.

However, I am still investigating if a kprobe insertion be allowed to 
these functions or any other function which is called directly from 
debug exception handler (do_debug_exception)

Update code is here:

https://github.com/pratyushanand/linux.git : ml_arm64_uprobe_devel_v2

~Pratyush
> However, things are looking better for user-space probing on arm64.
>
> -Will

[William Cohen]

On 01/07/2015 07:07 AM, Pratyush Anand wrote:
> Hi Will,
>
> On Monday 05 January 2015 09:36 PM, William Cohen wrote:
> > Hi All,
> >
> > Pratyush has been implementing uprobes support for aarch64 and has
> > posted a set of patches
> > (http://comments.gmane.org/gmane.linux.ports.arm.kernel/382237) for
> > review.  The really new Linux kernels requires a patch to the
> > systemtap runtime because the f_dentry macro has been removed.  With
> > the patched systemtap I was able to run the systemtap testsuite and
> > get some test results exercising the uprobes support:
> >
> > https://web.elastic.org/~dejazilla/viewsummary.php?summary=%3D%27%3C54A9E21C...
> >
> > There looked to be a localized fixes for plt support to eliminate the
> > unsupported systemtap.base/list.exp plt-* tests.
> >
> > A number of the tests appear to fail because of userspace arguments
> > cannot be found for sdt probes.
> >
> > The uprobe patches still need some refinement.  On a number of
> > systemtap "make installcheck" runs the kernel would get stuck spewing
> > out:
> >
> > Unexpected kernel single-step exception at EL1
>
> There are at least two functions of arm64/uprobe implementation uprobe_breakpoint_handler and uprobe_single_step_handler, where a kprobe insertion might be causing above issue. Currently I have qualified these functions with __kprobe, so that one can not insert a kprobe there. With this you should not be able to see the above message.
>
> However, I am still investigating if a kprobe insertion be allowed to these functions or any other function which is called directly from debug exception handler (do_debug_exception)

Hi Pratyush,

Are there any other places that need to be protected from kprobe probing in the arm64 code?
> Update code is here:
>
> https://github.com/pratyushanand/linux.git : ml_arm64_uprobe_devel_v2
>
> ~Pratyush

I will give the new kernel a try.

-Will
> > However, things are looking better for user-space probing on arm64.
> >
> > -Will

[Pratyush Anand]

On Wednesday 07 January 2015 06:40 PM, William Cohen wrote:
> On 01/07/2015 07:07 AM, Pratyush Anand wrote:
> > Hi Will,
> >
> > On Monday 05 January 2015 09:36 PM, William Cohen wrote:
> > > Hi All,
> > >
> > > Pratyush has been implementing uprobes support for aarch64 and has
> > > posted a set of patches
> > > (http://comments.gmane.org/gmane.linux.ports.arm.kernel/382237) for
> > > review.  The really new Linux kernels requires a patch to the
> > > systemtap runtime because the f_dentry macro has been removed.  With
> > > the patched systemtap I was able to run the systemtap testsuite and
> > > get some test results exercising the uprobes support:
> > >
> > > https://web.elastic.org/~dejazilla/viewsummary.php?summary=%3D%27%3C54A9E21C...
> > >
> > > There looked to be a localized fixes for plt support to eliminate the
> > > unsupported systemtap.base/list.exp plt-* tests.
> > >
> > > A number of the tests appear to fail because of userspace arguments
> > > cannot be found for sdt probes.
> > >
> > > The uprobe patches still need some refinement.  On a number of
> > > systemtap "make installcheck" runs the kernel would get stuck spewing
> > > out:
> > > Unexpected kernel single-step exception at EL1
> >
> > There are at least two functions of arm64/uprobe implementation uprobe_breakpoint_handler and uprobe_single_step_handler, where a kprobe insertion might be causing above issue. Currently I have qualified these functions with __kprobe, so that one can not insert a kprobe there. With this you should not be able to see the above message.
> >
> > However, I am still investigating if a kprobe insertion be allowed to these functions or any other function which is called directly from debug exception handler (do_debug_exception)
>
> Hi Pratyush,
>
> Are there any other places that need to be protected from kprobe probing in the arm64 code?

yes, I tried inserting kprobe in 
arch/arm64/kernel/debug-monitors.c:brk_handler and it failed. So, I 
think as of now, we can not probe any function which comes in the path 
of do_debug_exception.

Looking into kprobe code, it seems that it allows  REENTER, so it should 
have worked. Do not have much idea at present. Still, studying these 
part of code.

~Pratyush
> > Update code is here:
> >
> > https://github.com/pratyushanand/linux.git : ml_arm64_uprobe_devel_v2
> >
> > ~Pratyush
>
> I will give the new kernel a try.
>
> -Will
> > > However, things are looking better for user-space probing on arm64.
> > >
> > > -Will