Re: [PATCH] gdb: defer warnings when loading separate debug files

[Simon Marchi <simark@simark.ca>]

On 12/12/22 07:15, AlexandraH á jkov á ahajkova--- via Gdb-patches wrote:
> From: Alexandra Hájková <ahajkova@redhat.com>
> 
> Currently, when GDB loads debug information from a separate debug file,
> there are a couple of warnings that could be produced if things go
> wrong.
> 
> In find_separate_debug_file_by_buildid (build-id.c) GDB can give a
> warning if the separate debug file doesn't include any actual debug
> information, and in separate_debug_file_exists (symfile.c) we can warn
> if the CRC checksum in the separate debug file doesn't match the
> checksum in the original executable.
> 
> The problem here is that, when looking up debug information, GDB will
> try several different approaches, lookup by build-id, lookup by
> debug-link, and then a lookup from debuginfod.  GDB can potentially give
> a warning from an earlier attempt, and then succeed with a later
> attempt.  In the cases I have run into this is primarily a warning about
> some out of date debug information on my machine, but then GDB finds the
> correct information using debuginfod.  This can be confusing to a user,
> they will see warnings from GDB when really everything is working just
> fine.
> 
> For example:
> warning: the debug information found in "/usr/lib/debug//lib64/ld-2.32.so.debug"
> does not match "/lib64/ld-linux-x86-64.so.2" (CRC mismatch).
> 
> This diagnostic was printed on Fedora 33 even when the correct debuginfo
> was downloaded.
> 
> In this patch I propose that we defer any warnings related to looking up
> debug information from a separate debug file.  If any of the approaches
> are successful then GDB will not print any of the warnings.  As far as
> the user is concerned, everything "just worked".  Only if GDB completely
> fails to find any suitable debug information will the warnings be
> printed.

I don't really like this behavior change.  The situations you described:

 - a separate debug file without any actual debug information
 - CRC checksum that doesn't match

... are abnormal situations which still deserve being warned about, I
think.  If the files are there, it's because they are meant to be used,
so if something prevents GDB from using them, I want to know.  Silencing
the warning just makes investigating "why doesn't GDB read my separate
debug file" harder.

I can understand why this can be a bit confusing to the user, but
the warning is still factually correct.  For instance, the one you
quoted:

    warning: the debug information found in "/usr/lib/debug//lib64/ld-2.32.so.debug"
    does not match "/lib64/ld-linux-x86-64.so.2" (CRC mismatch).

This doesn't say that GDB didn't end up finding *some* debug info for
the shared object, just that this particular one is broken.  Maybe it's
an old one I installed by hand, that I should delete, maybe the package
from the distro is broken.  In any case it's good for the user to know
so they can fix the situation.

Simon