ARM: backtrace-clang: avoid crash on bogus frame pointer

The Clang backtrace code dereferences the link register value pulled
from the stack to decide whether the caller was a branch-and-link
instruction, in order to subsequently decode the offset to find the
start of the calling function. Unlike other loads in this routine, this
one is not protected by a fixup, and may therefore cause a crash if the
address in question is bogus.

So let's fix this, by treating the fault as a failure to decode the 'bl'
instruction. To avoid a label renum, reuse a fixup label that guards an
instruction that cannot fault to begin with.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Marc Zyngier <maz@kernel.org>
Tested-by: Vladimir Murzin <vladimir.murzin@arm.com> # ARMv7M

arch/arm/lib/backtrace-clang.S

@@ -144,7 +144,7 @@ for_each_frame:	tst	frame, mask		@ Check for address exceptions
  */
 1003:		ldr	sv_lr, [sv_fp, #4]	@ get saved lr from next frame
 
-		ldr	r0, [sv_lr, #-4]	@ get call instruction
+1004:		ldr	r0, [sv_lr, #-4]	@ get call instruction
 		ldr	r3, .Lopcode+4
 		and	r2, r3, r0		@ is this a bl call
 		teq	r2, r3
@@ -164,7 +164,7 @@ finished_setup:
 /*
  * Print the function (sv_pc) and where it was called from (sv_lr).
  */
-1004:		mov	r0, sv_pc
+		mov	r0, sv_pc
 
 		mov	r1, sv_lr
 		mov	r2, frame
@@ -210,7 +210,7 @@ ENDPROC(c_backtrace)
 		.long	1001b, 1006b
 		.long	1002b, 1006b
 		.long	1003b, 1006b
-		.long	1004b, 1006b
+		.long	1004b, finished_setup
 		.long   1005b, 1006b
 		.popsection