Features/MiniDebugInfo

MiniDebugInfo

Summary

Install minimal debuginfo by default to increase quality of bug reports and allow easier support for profiling and user space tracing.

Owner

• Name: Alexander Larsson
• Email: alexl@redhat.com

Current status

• Targeted release: Fedora 18
• Last updated: 2012-05-07
• Percentage of completion: 100%

The changes needed to generate MiniDebugInfo has landed in rpm 4.10.0-3 and was enabled by default in redhat-rpm-config 9.1.0-32. gdb supports reading the minidebuginfo since version 7.4.50.20120703-12.fc18.

Detailed Description

The debuginfo packages in Fedora are very useful if you're a developer and you are debugging a bug on your machine. However, they are very large, so they cannot be deployed in a wider scale. In many usecases you really only need enough information to be able to create backtraces, which is far smaller and could be installed by default.

The main advantage of this is a general improvement of bug report quality (as reported via ABRT or manually), as the reports will have complete backtraces from all users. Additionally things like system wide profiling and userspace tracing would also be possible without large debuginfo installation.

Debug info for backtraces relies on two types of information, the function names in the symbol tables, and (optionally) the linenumber debug information. I wrote a script that takes our current debuginfo packages and generates minimal debuginfo files containing only the minimal function name information and optionally line number information. Here are the sizes it generates for some F17 packages:

Package	Size of executables	symbols	xz symbols	symbols+lines	xz symbols+lines
coreutils	4851 Kb	15.46%	3.92%	38.82%	14.15%
gcc	13003 Kb	3.54%	.87%	23.68%	10.13%
gdb	5062 Kb	3.98%	.85%	18.67%	8.25%
libreoffice-core	142897 Kb	17.96%	2.38%	61.22%	11.76%
libreoffice-writer	11501 Kb	18.52%	2.73%	71.27%	12.42%
cairo	808 Kb	11.95%	2.46%	33.56%	13.08%
gimp	14828 Kb	8.89%	1.89%	29.43%	7.90%
gtk2	6007 Kb	11.46%	2.22%	28.20%	8.72%
gtk3	5302 Kb	8.73%	1.64%	26.08%	8.37%
webkitgtk	31147 Kb	18.51%	2.68%	98.03%	13.75%
webkitgtk3	31143 Kb	18.50%	2.68%	97.97%	13.76%
nautilus	2039 Kb	14.59%	2.89%	33.09%	10.20%
evolution	7492 Kb	8.16%	1.82%	25.95%	7.96%
xulrunner	25533 Kb	43.78%	6.97%	108.84%	22.11%
qt3	10458 Kb	.89%	.21%	21.58%	7.18%
qt	12377 Kb	14.65%	2.17%	58.66%	12.34%
qtwebkit	25989 Kb	19.03%	2.83%	92.24%	13.29%
kdelibs	36231 Kb	10.20%	1.84%	36.91%	9.35%
kde-baseapps	8030 Kb	11.48%	2.46%	32.44%	9.22%
samba	17508 Kb	3.25%	.63%	16.90%	5.31%
httpd	2496 Kb	11.83%	2.97%	25.53%	9.94%
llvm-libs	23176 Kb	2.78%	.42%	33.83%	9.41%
emacs	12445 Kb	1.17%	.30%	4.98%	2.29%
Total	450334 Kb	15.21%	2.32%	58.24%	11.42%

On my F17 installation /usr is 8771 Mb, and 1861 Mb of that is made up of ELF binaries. Leading to the following estimations for full-distro sizes:

	symbols	xz symbols	symbols+lines	xz symbols+lines
Estimated minidebuginfo size	283Mb	43Mb	1084Mb	213Mb
Estimated percentage of /usr	3.2%	0.5%	12.3%	2.4%

To minimize space use (on the installed system but also on the installer medium) we've decided to only go with the compressed symbols.

The implementation we use is that the normal separate debuginfo object is stripped of the non-minimal debug info, compressed with xz and then injected into the main binary. Then gdb can use this as a fallback if the normal debuginfo package is not installed.

Benefit to Fedora

We get better bug reports by default, even when offline or on a bad link, without users having to wait while downloading large amounts of data, or having to send possibly privacy sensitive core information to anyone. Additionally this will work for all packages built on the system (not only the official packages, but also site-specific packages) and will work with or without ABRT (some sites may not want to send bug reports to ABRT but collect them locally).

It will also make it easier to do things like system wide profiling, userspace dynamic probes and causual debugging.

Scope

Generation of the minidebuginfo just requires a few lines in /usr/lib/rpm/find-debuginfo.sh.

Consumers of debuginfo (mainly gdb, but also profilers, etc) must be changed to look in additional places for extra debug info.

This feature requires a mass rebuild to generate the new minidebuginfo for all packages.

How To Test

Running any application under gdb you should be able to get a backtrace with function names for all frames by doing:

1. Run "gdb <program name>"
2. Type "run"
3. Do something
4. Press ctrl-c to get back to gdb
5. Type "bt" and watch the full backtrace. There should be real function names everywhere, no "??" entries

User Experience

The end-user experience should be minimally changed. The new debug information is mainly targeted towards developers.

There will be a slight increase in the size of the binaries. Custom installations (like embedded images) can avoid this size increase by stripping the binaries like this:

 strip -R .gnu_debugdata $binary_file

Dependencies / affected packages

• gdb
• rpm
• redhat-rpm-config
• libunwind
• elfutils
• ABRT

Contingency Plan

Revert the change and rebuild all packages.

Documentation

• No relevant documentation

Release Notes

• Fedora 18 ships with a small subset of debug information by default. Full debug information is still available in the debuginfo packages.

Comments and Discussion

• See Talk:Features/MiniDebugInfo