Re: Automatically fetching Build ID from remote libraries and resole them locally?

[Norbert Lange <nolange79@gmail.com>]

Am Mo., 30. März 2020 um 10:45 Uhr schrieb Jan Kratochvil
<jan.kratochvil@redhat.com>:
>
> On Mon, 30 Mar 2020 10:35:30 +0200, Norbert Lange via Gdb wrote:
> > Currently its not possible to detect the Build ID without transferring the
> > whole library from Remote target to the local gdb. I suppose with coredumps
> > its a similar problem. What I would like is using build IDs wherever
> > possible. That means retrieve them from the remote, and maybe define some
> > fileformat to augment coredumps with the information.
>
> Coredumps already contain build-ids of executables + shared libraries as long
> as it is enabled (it is by default):
>         https://www.kernel.org/doc/Documentation/filesystems/proc.txt
>   - (bit 4) ELF header pages in file-backed private memory areas (it is
>             effective only if the bit 2 is cleared)
>
> build-id loading from remote files should be also somehow supported by recent
> GDBs (done by Gary Benson).

Thats great then.

>
> Fedora+RHEL GDB has patchset
>         https://src.fedoraproject.org/rpms/gdb/blob/master/f/gdb-6.6-buildid-locate.patch
>         https://src.fedoraproject.org/rpms/gdb/blob/master/f/gdb-6.6-buildid-locate-solib-missing-ids.patch
>         https://src.fedoraproject.org/rpms/gdb/blob/master/f/gdb-6.6-buildid-locate-core-as-arg.patch
> to locate appropriate executable + shared libraries from such core file by
> build-id - it locates then through symlinks in:
>         /usr/lib/debug/.build-id/6b/6cd649300ec28b47638e766e9cbb58648cbdac{,.debug}
>
> IIUC that patchset should be now being replaced by the elfutils debuginfod.

Still thinking you are misunderstanding me, debuginfod aint helping here.

I am running gdbserver on 'remote' and gdb on 'localpc',
then attaching to the remote target.

I have a version of the libraries on 'localpc' (almost mirrored for
compiling), but they might
not be the exact ones used on the 'remote' (rebuilds, additional libs, etc..)
I would want to use 'set sysroot /path/to/staging/dir', but the downside is that
mismatches will not be detected and can badly mess up debugging.
GDB 9.0 certainly does not use *Build-Ids from the 'remote'*,
neither for files existing on the 'localpc' or not (used strace to
monitor the file lookups).

I could only set 'set sysroot target:/',
at which point all DSOs will be transmitted from 'remote' to
'localpc'. Not exactly what I had in mind.

So, instead I would want GDB to just inspect the Build-ID's from 'remote',
and look them up from 'localpc' (whether its using debuginfod or not,
this is irrelevant here).

Norbert