[PATCH v2] ARM: stacktrace: Fix unwind_frame for clang-built kernels

[Nick Desaulniers]

Hi Nathan,
Would you please submit this to Russell's patch tracking system:
https://www.arm.linux.org.uk/developer/patches/
login -> add new patch.

On Mon, Jul 6, 2020 at 11:27 AM Nathan Huckleberry <nhuck at google.com> wrote:
>
> Since clang does not push pc and sp in function prologues, the current
> implementation of unwind_frame does not work. By using the previous
> frame's lr/fp instead of saved pc/sp we get valid unwinds on clang-built
> kernels.
>
> The bounds check on next frame pointer must be changed as well since
> there are 8 less bytes between frames.
>
> This fixes /proc/<pid>/stack.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/912
> Reported-by: Miles Chen <miles.chen at mediatek.com>
> Tested-by: Miles Chen <miles.chen at mediatek.com>
> Cc: stable at vger.kernel.org
> Reviewed-by: Nick Desaulniers <ndesaulniers at google.com>
> Signed-off-by: Nathan Huckleberry <nhuck at google.com>
> ---
>  arch/arm/kernel/stacktrace.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
>
> diff --git a/arch/arm/kernel/stacktrace.c b/arch/arm/kernel/stacktrace.c
> index cc726afea023..76ea4178a55c 100644
> --- a/arch/arm/kernel/stacktrace.c
> +++ b/arch/arm/kernel/stacktrace.c
> @@ -22,6 +22,19 @@
>   * A simple function epilogue looks like this:
>   *     ldm     sp, {fp, sp, pc}
>   *
> + * When compiled with clang, pc and sp are not pushed. A simple function
> + * prologue looks like this when built with clang:
> + *
> + *     stmdb   {..., fp, lr}
> + *     add     fp, sp, #x
> + *     sub     sp, sp, #y
> + *
> + * A simple function epilogue looks like this when built with clang:
> + *
> + *     sub     sp, fp, #x
> + *     ldm     {..., fp, pc}
> + *
> + *
>   * Note that with framepointer enabled, even the leaf functions have the same
>   * prologue and epilogue, therefore we can ignore the LR value in this case.
>   */
> @@ -34,6 +47,16 @@ int notrace unwind_frame(struct stackframe *frame)
>         low = frame->sp;
>         high = ALIGN(low, THREAD_SIZE);
>
> +#ifdef CONFIG_CC_IS_CLANG
> +       /* check current frame pointer is within bounds */
> +       if (fp < low + 4 || fp > high - 4)
> +               return -EINVAL;
> +
> +       frame->sp = frame->fp;
> +       frame->fp = *(unsigned long *)(fp);
> +       frame->pc = frame->lr;
> +       frame->lr = *(unsigned long *)(fp + 4);
> +#else
>         /* check current frame pointer is within bounds */
>         if (fp < low + 12 || fp > high - 4)
>                 return -EINVAL;
> @@ -42,6 +65,7 @@ int notrace unwind_frame(struct stackframe *frame)
>         frame->fp = *(unsigned long *)(fp - 12);
>         frame->sp = *(unsigned long *)(fp - 8);
>         frame->pc = *(unsigned long *)(fp - 4);
> +#endif
>
>         return 0;
>  }
> --
> 2.27.0.212.ge8ba1cc988-goog
>


-- 
Thanks,
~Nick Desaulniers