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    SubjectRe: [PATCH 1/4] ARM: backtrace-clang: check for NULL lr
    On Thu, Jul 30, 2020 at 3:51 PM Nick Desaulniers
    <ndesaulniers@google.com> wrote:
    >
    > 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
    >

    Reviewed-by: Nathan Huckleberry <nhuck15@gmail.com>

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