6:13 a.m.
= Proposed System Wide Change: Parallel Installable Debuginfo =
https://fedoraproject.org/wiki/Changes/ParallelInstallableDebuginfo
Change owner(s):
* Mark Wielaard <mjw AT redhat DOT com>
debuginfo packages can be installed in parallel to make it easier to
observe what programs are doing or to debug when they have crashed.
That way debugging, tracing or profiling programs can be done
independent of whether they are 32bit, 64bit, a slightly newer or
older version than currently installed or even from a different
architecture.
== Detailed Description ==
Currently only one version of a debuginfo package can be installed for
a given package. Even on a multi-lib system you cannot install the
64-bit and 32-bit versions of a debuginfo package in parallel
(technically you sometimes can, because of RPM file coloring, the
64bit version of the .debug files win over the 32bit version - causing
lots of confusion). But there are various situation where having
multiple versions of the debuginfo package installed help with
tracing, profiling, debugging and/or crash analysis (see the Benefit
to Fedora section below). There are various things provided by a
debuginfo file that might conflict preventing parallel installation of
different versions:
* build-id file /usr/lib/debug/.build-id/xx/yyyy...yyy which is a
symlink to the main ELF file.
* build-id.debug file /usr/lib/debug/.build-id/xx/yyyy...yyy.debug
which is a symlink to the .debug ELF file.
* The .debug files under /usr/lib/debug/ with file path names
mirroring the main ELF file paths under / with .debug added.
* The source files under /usr/src/debug/<name>-<version>/
They can be made non-conflicting in the following ways:
* The main build-id file should not be in the debuginfo file, but in
the main package (this was always a problem since the package and
debuginfo package installed might not match). If we want to make
usr/lib/debug/ a network resource then we will need to move the
symlink to another location (maybe /usr/lib/.build-id). Unfortunately
this means a change will be necessary for debuginfo consumers to that
depend on the old location. We could keep the old symlink and point it
to the new location to work around it. But I will audit the consumers
to see which depend on it and discuss if we can have a new standard
location.
* build-ids are globally unique identifiers. They will be different
across arches. But might match between minor releases if the exact
same ELF image is produced. The linker will get an option to hash in
the full nvr to make sure all build-ids are always fully unique.
* The .debug file names will be changed to main ELF file
name-vr.debug. This name will also be set in the .gnu_debuglink
section of the main file by changing the options given to eu-strip in
the rpm find-debuginfo.sh script.
* The source files will be moved under
/usr/src/debug/<name>-<version>-<release>.<arch>/. This needs
changes
to the rpm debugedit program which rewrites the DWARF source file
information.
These changes will make all files in any debuginfo file unique so they
don't conflict when installed in parallel. There should be no changes
necessary to programs (gdb, perf, valgrind, systemtap,
systemd-coredump, eu-stack, abrt-hook-ccpp, etc.) that use build-ids
or .gnu_debuglink to lookup DWARF debug information and source
references for tracing, profiling and debugging.
It would be good to tweak dnf debuginfo-install to know about parallel
installable debuginfo packages and maybe have an easy option to
install the debuginfo for a core file or for the packages running in a
container.
Alternative solutions currently rejected:
* Move main ELF image build-id file under
/usr/lib/.build-id/xx/yyyy...yyy when moving into main pages. Because
existing programs probably depend on the link being under
/usr/lib/debug/.
* Since when the build-id is identifical also the ELF file is
identical we could mark all build-id.debug files as replacable in the
rpm. It isn't clear that works for symlinks though (but we could
reverse the symlink direction from debug file to build-id file). And
currently you can identify the exact package nvr installed given just
one build-id. That would be impossible if multiple packages could
contain the same build-id/ELF image file.
* Do away with the old .gnu_debuglink way of accessing files under
/usr/lib/debug and just not install .debug files and only support
build-id based debug lookups. Because it isn't clear build-ids are
100% available and all programs work with build-id lookups instead
through .gnu_debuglink names.
* Move the .debug files under a subdir like the sources.
/usr/lib/debug/<name>-<version>-<release>.<arch>/. This cannot
easily
be expressed in .gnu_debuglink, which officially only allows a
basename.
== Scope ==
* Proposal owners: Patches need to be developed against the linker
(binutils ld and gold) to accept a hash value to seed the build-id
calculation, against rpm debugedit to rewrite source paths (currently
source paths can only be smaller, this change might create larger
paths) and the rpm find-debuginfo.sh script to change the paths,
symlinks and .gnu_debuglink names as outlined in the Detailed
Description. And the dnf debuginfo-install plugin might be patches to
provide subcommands for pulling in debuginfo packages found by
build-id in core files and/or programs running in containers.
* Other developers: Upstream binutils, rpm and dnf maintainers have to
review the proposed patches. If accepted the package maintainers will
have to decide whether those patches can be backported for the next
fedora release. Once all changes are in a package debuginfo needs to
be regenerated before it becomes parallel installable.
* Release engineering: Needs to be discussed. In theory no changes
apart from those listed above are needed. But if we want to support
installing cross-architectures (not just multi-lib arch) debuginfo
then some way needs to be found to get those in the right repodata.
* List of deliverables: N/A (Still Unknown)
* Policies and guidelines: No changes, the debuginfo related rpm
macros won't change. They will just start producing parallel
installable debuginfo packages once all changes are in place.
* Trademark approval: N/A (not needed for this Change)
--
Jan Kuřík
Platform & Fedora Program Manager
Red Hat Czech s.r.o., Purkynova 99/71, 612 45 Brno, Czech Republic