Re: [PATCH v3] Search for DWZ files in debug-file-directories as well

[Simon Marchi <simark@simark.ca>]

On 2020-11-28 8:08 p.m., Sergio Durigan Junior via Gdb-patches wrote:
> Changes from v2:
>
> - Put the new code into a new function.
>
> - The code now makes sure that the debug-file-directory being
>   processed ends with a directory separator, which makes things more
>   robust.
>
> - Minimized indentation level; fixed typos and improved code
>   readability by adding more comments.
>
>
> When Debian (and Ubuntu) builds its binaries, it (still) doesn't use
> dwz's "--relative" option.  This causes their debuginfo files to
> carry a .gnu_debugaltlink section containing a full pathname to the
> DWZ alt debug file, like this:
>
>   $ readelf -wk /usr/bin/cat
>   Contents of the .gnu_debugaltlink section:
>
>     Separate debug info file: /usr/lib/debug/.dwz/x86_64-linux-gnu/coreutils.debug
>     Build-ID (0x14 bytes):
>    ee 76 5d 71 97 37 ce 46 99 44 32 bb e8 a9 1a ef 99 96 88 db
>
>   Contents of the .gnu_debuglink section:
>
>     Separate debug info file: 06d3bee37b8c7e67b31cb2689cb351102ae73b.debug
>     CRC value: 0x53267655
>
> This usually works OK, because most of the debuginfo files installed
> via apt will be present in /usr/lib/debug anyway.  However, imagine
> the following scenario:
>
> - You are using /usr/bin/cat, it crashes on you and generates a
>   corefile.
>
> - You don't want/need to "apt install" the debuginfo file for
>   coreutils from the repositories.  Instead, you already have the
>   debuginfo files in a separate directory (e.g., $HOME/dbgsym).
>
> - You start GDB and "set debug-file-directory $HOME/dbgsym/usr/lib/debug".
>   You then get the following message:
>
>   $ gdb -ex 'set debug-file-directory ./dbgsym/usr/lib/debug' -ex 'file /bin/cat' -ex 'core-file ./cat.core'
>   GNU gdb (Ubuntu 10.1-0ubuntu1) 10.1
>   ...
>   Reading symbols from /bin/cat...
>   Reading symbols from /home/sergio/gdb/dbgsym/usr/lib/debug/.build-id/bc/06d3bee37b8c7e67b31cb2689cb351102ae73b.debug...
>   could not find '.gnu_debugaltlink' file for /home/sergio/gdb/dbgsym/usr/lib/debug/.build-id/bc/06d3bee37b8c7e67b31cb2689cb351102ae73b.debug
>
> This error happens because GDB is trying to locate the build-id
> link (inside /home/sergio/gdb/dbgsym/usr/lib/debug/.build-id) for the
> DWZ alt debug file, which doesn't exist.  Arguably, this is a problem
> with how dh_dwz works in Debian, and it's something I'm also planning
> to tackle.  But, back at the problem at hand.
>
> Besides not being able to find the build-id link in the directory
> mentioned above, GDB also tried to open the DWZ alt file using its
> filename.  The problem here is that, since we don't have the distro's
> debuginfo installed, it can't find anything under /usr/lib/debug that
> satisfies it.
>
> It occurred to me that a good way to workaround this problem is to
> actually try to locate the DWZ alt debug file inside the
> debug-file-directories (that were likely provided by the user).  So
> this is what the proposed patch does.
>
> The idea here is simple: get the filename extracted from the
> .gnu_debugaltlink section, and manipulate it in order to replace the
> initial part of the path (everything before "/.dwz/") by whatever
> debug-file-directories the user might have provided.
>
> I talked with Mark Wielaard and he agrees this is a sensible approach.
> In fact, apparently this is something that eu-readelf also does.
>
> I regtested this code, and no regressions were found.
>
> 2020-11-28  Sergio Durigan Junior  <sergiodj@sergiodj.net>
>
> 	* dwarf2/read.c (dwz_search_other_debugdirs): New function.
> 	(dwarf2_get_dwz_file): Convert 'filename' to a
> 	std::string.  Use dwz_search_other_debugdirs to search for DWZ
> 	files in the debug-file-directories provided by the user as well.

Thanks this is OK.

I forgot to mention that before, but a test would be nice to make sure
this feature doesn't break when doing changes to the DWARF reader.

Simon