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

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 9e0aa64f55 "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 9e0aa64f55 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

gdbserver/ChangeLog:

2021-01-20  Tom de Vries  <tdevries@suse.de>

	* linux-x86-low.cc (collect_register_i386): New function.
	(x86_fill_gregset):  Remove memset.  Use collect_register_i386.

gdbserver/linux-x86-low.cc

@@ -397,6 +397,35 @@ x86_target::low_cannot_fetch_register (int regno)
   return regno >= I386_NUM_REGS;
 }
 
+static void
+collect_register_i386 (struct regcache *regcache, int regno, void *buf)
+{
+  collect_register (regcache, regno, buf);
+
+#ifdef __x86_64__
+  /* In case of x86_64 -m32, collect_register only writes 4 bytes, but the
+     space reserved in buf for the register is 8 bytes.  Make sure the entire
+     reserved space is initialized.  */
+
+  gdb_assert (register_size (regcache->tdesc, regno) == 4);
+
+  if (regno == RAX)
+    {
+      /* Sign extend EAX value to avoid potential syscall restart
+	 problems.
+
+	 See amd64_linux_collect_native_gregset() in
+	 gdb/amd64-linux-nat.c for a detailed explanation.  */
+      *(int64_t *) buf = *(int32_t *) buf;
+    }
+  else
+    {
+      /* Zero-extend.  */
+      *(uint64_t *) buf = *(uint32_t *) buf;
+    }
+#endif
+}
+
 static void
 x86_fill_gregset (struct regcache *regcache, void *buf)
 {
@@ -411,32 +440,14 @@ x86_fill_gregset (struct regcache *regcache, void *buf)
 
       return;
     }
-
-  /* 32-bit inferior registers need to be zero-extended.
-     Callers would read uninitialized memory otherwise.  */
-  memset (buf, 0x00, X86_64_USER_REGS * 8);
 #endif
 
   for (i = 0; i < I386_NUM_REGS; i++)
-    collect_register (regcache, i, ((char *) buf) + i386_regmap[i]);
+    collect_register_i386 (regcache, i, ((char *) buf) + i386_regmap[i]);
 
-  collect_register_by_name (regcache, "orig_eax",
-			    ((char *) buf) + ORIG_EAX * REGSIZE);
-
-#ifdef __x86_64__
-  /* Sign extend EAX value to avoid potential syscall restart
-     problems. 
-
-     See amd64_linux_collect_native_gregset() in gdb/amd64-linux-nat.c
-     for a detailed explanation.  */
-  if (register_size (regcache->tdesc, 0) == 4)
-    {
-      void *ptr = ((gdb_byte *) buf
-		   + i386_regmap[find_regno (regcache->tdesc, "eax")]);
-
-      *(int64_t *) ptr = *(int32_t *) ptr;
-    }
-#endif
+  /* Handle ORIG_EAX, which is not in i386_regmap.  */
+  collect_register_i386 (regcache, find_regno (regcache->tdesc, "orig_eax"),
+			 ((char *) buf) + ORIG_EAX * REGSIZE);
 }
 
 static void