Debugging experience in Debian

published 2019-02-15 in tag debian

Recently, a user reported that they don’t see window titles in i3 when running i3 on a Raspberry Pi with Debian.

I copied the latest Raspberry Pi Debian image onto an SD card, booted it, and was able to reproduce the issue.

Conceptually, at this point, I should be able to install and start gdb, set a break point and step through the code.

Enabling debug symbols in Debian

Debian, by default, strips debug symbols when building packages to conserve disk space and network bandwidth. The motivation is very reasonable: most users will never need the debug symbols.

Unfortunately, obtaining debug symbols when you do need them is unreasonably hard.

We begin by configuring an additional apt repository which contains automatically generated debug packages:

raspi# cat >>/etc/apt/sources.list.d/debug.list <<'EOT'
deb http://deb.debian.org/debian-debug buster-debug main contrib non-free
EOT
raspi# apt update

Notably, not all Debian packages have debug packages. As the DebugPackage Debian Wiki page explains, debhelper/9.20151219 started generating debug packages (ending in -dbgsym) automatically. Packages which have not been updated might come with their own debug packages (ending in -dbg) or might not preserve debug symbols at all!

Now that we can install debug packages, how do we know which ones we need?

Finding debug symbol packages in Debian

For debugging i3, we obviously need at least the i3-dbgsym package, but i3 uses a number of other libraries through whose code we may need to step.

The debian-goodies package ships a tool called find-dbgsym-packages which prints the required packages to debug an executable, core dump or running process:

raspi# apt install debian-goodies
raspi# apt install $(find-dbgsym-packages $(which i3))

Now we should have symbol names and line number information available in gdb. But for effectively stepping through the program, access to the source code is required.

Obtaining source code in Debian

Naively, one would assume that apt source should be sufficient for obtaining the source code of any Debian package. However, apt source defaults to the package candidate version, not the version you have installed on your system.

I have addressed this issue with the pk4 tool, which defaults to the installed version.

Before we can extract any sources, we need to configure yet another apt repository:

raspi# cat >>/etc/apt/sources.list.d/source.list <<'EOT'
deb-src http://deb.debian.org/debian buster main contrib non-free
EOT
raspi# apt update

Regardless of whether you use apt source or pk4, one remaining problem is the directory mismatch: the debug symbols contain a certain path, and that path is typically not where you extracted your sources to. While debugging, you will need to tell gdb about the location of the sources. This is tricky when you debug a call across different source packages:

(gdb) pwd
Working directory /usr/src/i3.
(gdb) list main
229     * the main loop. */
230     ev_unref(main_loop);
231   }
232 }
233
234 int main(int argc, char *argv[]) {
235  /* Keep a symbol pointing to the I3_VERSION string constant so that
236   * we have it in gdb backtraces. */
237  static const char *_i3_version __attribute__((used)) = I3_VERSION;
238  char *override_configpath = NULL;
(gdb) list xcb_connect
484	../../src/xcb_util.c: No such file or directory.

See Specifying Source Directories in the gdb manual for the dir command which allows you to add multiple directories to the source path. This is pretty tedious, though, and does not work for all programs.

Positive example: Fedora

While Fedora conceptually shares all the same steps, the experience on Fedora is so much better: when you run gdb /usr/bin/i3, it will tell you what the next step is:

# gdb /usr/bin/i3
[…]
Reading symbols from /usr/bin/i3...(no debugging symbols found)...done.
Missing separate debuginfos, use: dnf debuginfo-install i3-4.16-1.fc28.x86_64

Watch what happens when we run the suggested command:

# dnf debuginfo-install i3-4.16-1.fc28.x86_64
enabling updates-debuginfo repository
enabling fedora-debuginfo repository
[…]
Installed:
  i3-debuginfo.x86_64 4.16-1.fc28
  i3-debugsource.x86_64 4.16-1.fc28
Complete!

A single command understood our intent, enabled the required repositories and installed the required packages, both for debug symbols and source code (stored in e.g. /usr/src/debug/i3-4.16-1.fc28.x86_64). Unfortunately, gdb doesn’t seem to locate the sources, which seems like a bug to me.

One downside of Fedora’s approach is that gdb will only print all required dependencies once you actually run the program, so you may need to run multiple dnf commands.

In an ideal world

Ideally, none of the manual steps described above would be necessary. It seems absurd to me that so much knowledge is required to efficiently debug programs in Debian. Case in point: I only learnt about find-dbgsym-packages a few days ago when talking to one of its contributors.

Installing gdb should be all that a user needs to do. Debug symbols and sources can be transparently provided through a lazy-loading FUSE file system. If our build/packaging infrastructure assured predictable paths and automated debug symbol extraction, we could have transparent, quick and reliable debugging of all programs within Debian.

NixOS’s dwarffs is an implementation of this idea: https://github.com/edolstra/dwarffs

Conclusion

While I agree with the removal of debug symbols as a general optimization, I think every Linux distribution should strive to provide an entirely transparent debugging experience: you should not even have to know that debug symbols are not present by default. Debian really falls short in this regard.

Getting Debian to a fully transparent debugging experience requires a lot of technical work and a lot of social convincing. In my experience, programmatically working with the Debian archive and packages is tricky, and ensuring that all packages in a Debian release have debug packages (let alone predictable paths) seems entirely unachievable due to the fragmentation of packaging infrastructure and holdouts blocking any progress.

My go-to example is rsync’s debian/rules, which intentionally (!) still has not adopted debhelper. It is not a surprise that there are no debug symbols for rsync in Debian.