RE: [PATCH][gdb/server] Don't overwrite fs/gs_base with -m32

["Metzger, Markus T via Gdb-patches" <gdb-patches@sourceware.org>]

> -----Original Message-----
> From: Simon Marchi <simon.marchi@polymtl.ca>
> On 2021-01-20 7:59 a.m., Tom de Vries wrote:
> > Hi,
> >
> > Consider a minimal test-case test.c:
> > ...
> > int main (void) { return 0; }
> > ...
> > compiled with -m32:
> > ...
> > $ gcc test.c -m32
> > ...
> >
> > When running the exec using gdbserver on openSUSE Factory (currently running a
> > linux kernel version 5.10.5):
> > ...
> > $ gdbserver localhost:12345 a.out
> > ...
> > to which we connect in a gdb session, we run into a segfault in the inferior:
> > ...
> > $ gdb -batch -q -ex "target remote localhost:12345" -ex continue
> > Program received signal SIGSEGV, Segmentation fault.
> > 0xf7dd8bd2 in init_cacheinfo () at ../sysdeps/x86/cacheinfo.c:761
> > ...
> >
> > The segfault is caused by gdbserver overwriting $gs_base with 0 using
> > PTRACE_SETREGS.  After it is overwritten, the next use of $gs in the inferior
> > will trigger the segfault.
> >
> > Before linux kernel version 5.9, the value used by PTRACE_SETREGS for $gs_base
> > was ignored, but starting version 5.9, the linux kernel has support for
> > intel architecture extension FSGSBASE, which allows users to modify $gs_base,
> > and consequently PTRACE_SETREGS can no longer ignore the $gs_base value.
> >
> > The overwrite of $gs_base with 0 is done by a memset in x86_fill_gregset,
> > which was added in commit 9e0aa64f551 "Fix gdbserver qGetTLSAddr for
> > x86_64 -m32".  The memset intends to zero-extend 32-bit registers that are
> > tracked in the regcache to 64-bit when writing them into the PTRACE_SETREGS
> > data argument.  But in addition, it overwrites  other registers that are
> > not tracked in the regcache, such as $gs_base.
> >
> > Fix the segfault by redoing the fix from commit 9e0aa64f551 in minimal form.
> >
> > Tested on x86_64-linux:
> > - openSUSE Leap 15.2 (using kernel version 5.3.18):
> >   - native
> >   - gdbserver -m32
> >   - -m32
> > - openSUSE Factory (using kernel version 5.10.5):
> >   - native
> >   - m32
> >
> > Any comments?
> >
> > Thanks,
> > - Tom
> 
> This looks good as far as I can tell, but please wait for Markus' to chime in as
> well.

Looks good to me, too.

Markus.
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