Mostly looks good to me. Just a minor nit.
On Thu, Jul 30, 2020 at 3:51 PM Nick Desaulniers
<ndesaulniers@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxxxx
> Fixes: commit 6dc5fd93b2f1 ("ARM: 8900/1: UNWINDER_FRAME_POINTER implementation for Clang")
> Reported-by: Miles Chen <miles.chen@xxxxxxxxxxxx>
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> ---
> 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
It might be nice to emit a warning here since we'll
only hit this case if something fishy is going on
with the saved lr.
> + 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
>
Thanks,
Huck
