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Subject[PATCH 1/4] ARM: backtrace-clang: check for NULL lr
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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 <miles.chen@mediatek.com>
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
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