Linux x86 unwinder: Handle __NR_sigreturn for __kernel_sigreturn support

[Florian Weimer]

I believe this may address recent unwinder failures in Fedora if the
vDSO unwinder does not contain unwinding data:

  <https://bugzilla.redhat.com/show_bug.cgi?id=1659295>

The question is: Do we want to move in that direction?  Or should we
make clear that the userspace ABI *requires* unwinding information?

Thanks,
Florian

2018-12-14  Florian Weimer  <fweimer@redhat.com>

	* config/i386/linux-unwind.h (x86_frob_update_context): Also check
	for __NR_sigreturn.

diff --git a/libgcc/config/i386/linux-unwind.h b/libgcc/config/i386/linux-unwind.h
index ea838e4e47b..502a87a2cb0 100644
--- a/libgcc/config/i386/linux-unwind.h
+++ b/libgcc/config/i386/linux-unwind.h
@@ -190,9 +190,10 @@ x86_frob_update_context (struct _Unwind_Context *context,
 {
   unsigned char *pc = context->ra;
 
-  /* movl $__NR_rt_sigreturn,%eax ; {int $0x80 | syscall}  */
+  /* movl $__NR_{rt_|}sigreturn,%eax ; {int $0x80 | syscall}  */
   if (*(unsigned char *)(pc+0) == 0xb8
-      && *(unsigned int *)(pc+1) == 173
+      && (*(unsigned int *)(pc+1) == 119
+	  || *(unsigned int *)(pc+1) == 173)
       && (*(unsigned short *)(pc+5) == 0x80cd
 	  || *(unsigned short *)(pc+5) == 0x050f))
     _Unwind_SetSignalFrame (context, 1);