Re: [PATCH 1/4] ARM: backtrace-clang: check for NULL lr

If the link register was zeroed out, do not attempt to use it for
address calculations for which there are currently no fixup handlers,
which can lead to a panic during unwind. Since panicking triggers
another unwind, this can lead to an infinite loop.  If this occurs
during start_kernel(), this can prevent a kernel from booting.

commit 59b6359dd92d ("ARM: 8702/1: head-common.S: Clear lr before jumping to start_kernel()")
intentionally zeros out the link register in __mmap_switched which tail
calls into start kernel. Test for this condition so that we can stop
unwinding when initiated within start_kernel() correctly.

Cc: stable@vger.kernel.org
Fixes: commit 6dc5fd93b2f1 ("ARM: 8900/1: UNWINDER_FRAME_POINTER implementation for Clang")
Reported-by: Miles Chen <redacted>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
 arch/arm/lib/backtrace-clang.S | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/arm/lib/backtrace-clang.S b/arch/arm/lib/backtrace-clang.S
index 6174c45f53a5..5388ac664c12 100644
--- a/arch/arm/lib/backtrace-clang.S
+++ b/arch/arm/lib/backtrace-clang.S

@@ -144,6 +144,8 @@ for_each_frame:     tst     frame, mask             @ Check for address exceptions
  */
 1003:          ldr     sv_lr, [sv_fp, #4]      @ get saved lr from next frame

+               tst     sv_lr, #0               @ If there's no previous lr,
+               beq     finished_setup          @ we're done.
                ldr     r0, [sv_lr, #-4]        @ get call instruction
                ldr     r3, .Lopcode+4
                and     r2, r3, r0              @ is this a bl call
--

2.28.0.163.g6104cc2f0b6-goog