[Bug gdb/26804] gdbserver crash at <init_cacheinfo+22> when program has canary and is 32 bit.

["vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=26804

--- Comment #7 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Tom de Vries from comment #6)
> I found the place in gdbserver where gs_base is overwritten. It's in
> x86_fill_gregset:
> ...
>   /* 32-bit inferior registers need to be zero-extended.                    
> 
>      Callers would read uninitialized memory otherwise.  */
>   memset (buf, 0x00, X86_64_USER_REGS * 8);                                 
> 
> ...
> 
> Commenting out the memset fixes the segmentation fault.

Hmm, but that causes regressions on Leap 15.2 (with 5.3.18 kernel).

This doesn't cause any regressions on Leap 15.2, and still fixes the problem on
Tumbleweed (with 5.10.5 kernel):
...
diff --git a/gdbserver/linux-x86-low.cc b/gdbserver/linux-x86-low.cc
index 35dfdd7e0f3..d2abf3c2712 100644
--- a/gdbserver/linux-x86-low.cc
+++ b/gdbserver/linux-x86-low.cc
@@ -414,7 +414,11 @@ x86_fill_gregset (struct regcache *regcache, void *buf)

   /* 32-bit inferior registers need to be zero-extended.
      Callers would read uninitialized memory otherwise.  */
-  memset (buf, 0x00, X86_64_USER_REGS * 8);
+  {
+    unsigned long long *ptr = (unsigned long long *)buf;
+    for (i = 0; i < X86_64_USER_REGS; ++i)
+      ptr[i] = ptr [i] & 0xffffffffULL;
+  }
 #endif

   for (i = 0; i < I386_NUM_REGS; i++)
...

Effectively, the patch fixes the code to match the comment.

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