[PATCH 2/4] ARM: backtrace-clang: add fixup for lr dereference

[Nick Desaulniers]

If the value of the link register is not correct (tail call from asm
that didn't set it, stack corruption, memory no longer mapped), then
using it for an address calculation may trigger an exception.  Without a
fixup handler, this will lead to a panic, which will unwind, which will
trigger the fault repeatedly in an infinite loop.

We don't observe such failures currently, but we have. Just to be safe,
add a fixup handler here so that at least we don't have an infinite
loop.

Cc: stable at vger.kernel.org
Fixes: commit 6dc5fd93b2f1 ("ARM: 8900/1: UNWINDER_FRAME_POINTER implementation for Clang")
Reported-by: Miles Chen <miles.chen at mediatek.com>
Signed-off-by: Nick Desaulniers <ndesaulniers at google.com>
---
 arch/arm/lib/backtrace-clang.S | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/arm/lib/backtrace-clang.S b/arch/arm/lib/backtrace-clang.S
index 5388ac664c12..40eb2215eaf4 100644
--- a/arch/arm/lib/backtrace-clang.S
+++ b/arch/arm/lib/backtrace-clang.S
@@ -146,7 +146,7 @@ for_each_frame:	tst	frame, mask		@ Check for address exceptions
 
 		tst	sv_lr, #0		@ If there's no previous lr,
 		beq	finished_setup		@ we're done.
-		ldr	r0, [sv_lr, #-4]	@ get call instruction
+prev_call:	ldr	r0, [sv_lr, #-4]	@ get call instruction
 		ldr	r3, .Lopcode+4
 		and	r2, r3, r0		@ is this a bl call
 		teq	r2, r3
@@ -206,6 +206,13 @@ finished_setup:
 		mov	r2, frame
 		bl	printk
 no_frame:	ldmfd	sp!, {r4 - r9, fp, pc}
+/*
+ * Accessing the address pointed to by the link register triggered an
+ * exception, don't try to unwind through it.
+ */
+bad_lr:		mov	sv_fp, #0
+		mov	sv_lr, #0
+		b	finished_setup
 ENDPROC(c_backtrace)
 		.pushsection __ex_table,"a"
 		.align	3
@@ -214,6 +221,7 @@ ENDPROC(c_backtrace)
 		.long	1003b, 1006b
 		.long	1004b, 1006b
 		.long   1005b, 1006b
+		.long	prev_call, bad_lr
 		.popsection
 
 .Lbad:		.asciz	"%sBacktrace aborted due to bad frame pointer <%p>\n"
-- 
2.28.0.163.g6104cc2f0b6-goog