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

[Mark Wielaard <mark@klomp.org>]

Hi Sergio,

On Sat, 2020-11-14 at 18:48 -0500, Sergio Durigan Junior wrote:
> Separate debug info file: /usr/lib/debug/.dwz/x86_64-linux-gnu/coreutils.debug
> [...]
> 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 double checked what elfutils libdw (the dwfl interface) does for alt
files. In general it tries to find them similar to how .debug files are
found described in libdwfl.h under *** Standard callbacks ***:
https://sourceware.org/git/?p=elfutils.git;a=blob;f=libdwfl/libdwfl.h;hb=HEAD#l242
Then for alt files it also looks for a .dwz subdir to find the basename
of the alt file. Which is close to what you are proposing. But your
version would also find alt files in (arch) subdirs of the .dwz dir.
Which seems to be an extension of Debian/Ubuntu. In Fedora/CentOS the
arch is part of the basename (e.g. the elfutils supplemental file is
called elfutils-0.179-1.fc32.i386 or elfutils-0.182-1.fc33.x86_64). It
might make sense to match what you are doing if the alt file path
contains /.dwz/ and has any directories after it (presumably
representing the arch).

Cheers,

Mark