92 lines
4.3 KiB
Markdown
92 lines
4.3 KiB
Markdown
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title = "Ubuntu debuginfod and source code indexing"
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author = ["Sergio Durigan Junior"]
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date = 2023-05-13T16:43:00-04:00
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tags = ["english", "ubuntu", "debuginfod", "debian", "free-software", "gdb"]
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draft = false
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You might remember that in my [last post](/posts/debuginfod-is-coming-to-ubuntu/) about the [Ubuntu debuginfod
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service](https://debuginfod.ubuntu.com) I talked about wanting to extend it and make it index and
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serve source code from packages. I'm excited to announce that this is
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now a reality since the Ubuntu Lunar (23.04) release.
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The feature should work for a lot of packages from the archive, but
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not all of them. Keep reading to better understand why.
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## The problem {#the-problem}
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While debugging a package in Ubuntu, one of the first steps you need
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to take is to install its source code. There are some problems with
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this:
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- `apt-get source` required `dpkg-dev` to be installed, which ends up
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pulling in a lot of other dependencies.
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- GDB needs to be taught how to find the source code for the package
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being debugged. This can usually be done by using the `dir`
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command, but finding the proper path to be is usually not trivial,
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and you find yourself having to use more "complex" commands like
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`set substitute-path`, for example.
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- You have to make sure that the version of the source package is the
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same as the version of the binary package(s) you want to debug.
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- If you want to debug the libraries that the package links against,
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you will face the same problems described above for each library.
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So yeah, not a trivial/pleasant task after all.
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## The solution... {#the-solution-dot-dot-dot}
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Debuginfod can index source code as well as debug symbols. It is
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smart enough to keep a relationship between the source package and the
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corresponding binary's Build-ID, which is what GDB will use when
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making a request for a specific source file. This means that, just
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like what happens for debug symbol files, the user does not need to
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keep track of the source package version.
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While indexing source code, debuginfod will also maintain a record of
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the relative pathname of each source file. No more fiddling with
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paths inside the debugger to get things working properly.
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Last, but not least, if there's a need for a library source file and
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if it's indexed by debuginfod, then it will get downloaded
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automatically as well.
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## ... but not a perfect one {#dot-dot-dot-but-not-a-perfect-one}
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In order to make debuginfod happy when indexing source files, I had to
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patch `dpkg` and make it always use `-fdebug-prefix-map` when
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compiling stuff. This GCC option is used to remap pathnames inside
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the DWARF, which is needed because in Debian/Ubuntu we build our
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packages inside chroots and the build directories end up containing a
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bunch of random cruft (like `/build/ayusd-ASDSEA/something/here`). So
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we need to make sure the path prefix (the `/build/ayusd-ASDSEA` part)
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is uniform across all packages, and that's where `-fdebug-prefix-map`
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helps.
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This means that the package **must** honour `dpkg-buildflags` during its
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build process, otherwise the magic flag won't be passed and your DWARF
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will end up with bogus paths. This should not be a big problem,
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because most of our packages do honour `dpkg-buildflags`, and those
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who don't should be fixed anyway.
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## ... especially if you're using LTO {#dot-dot-dot-especially-if-you-re-using-lto}
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Ubuntu enables [LTO](https://gcc.gnu.org/onlinedocs/gccint/LTO-Overview.html) by default, and unfortunately we are affected by an
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[annoying (and complex) bug](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109805) that results in those bogus pathnames not
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being properly remapped. The bug doesn't affect all packages, but if
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you see GDB having trouble finding a source file whose full path
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starts without `/usr/src/...`, that is a good indication that you're
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being affected by this bug. Hopefully we should see some progress in
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the following weeks.
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## Your feedback is important to us {#your-feedback-is-important-to-us}
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If you have any comments, or if you found something strange that looks
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like a bug in the service, please reach out. You can either send an
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email to my [public inbox](https://lists.sr.ht/~sergiodj/public-inbox) (see below) or file a bug against the
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[ubuntu-debuginfod project on Launchpad](https://bugs.launchpad.net/ubuntu-debuginfod).
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