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

[Nathan Huckleberry]

On Thu, Jul 30, 2020 at 3:51 PM Nick Desaulniers
<ndesaulniers at 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 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 | 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 at gmail.com>