Re: ARM Ftrace Function Graph Fails With UNWINDER_FRAME_POINTER

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Fri, Dec 01, 2023 at 10:12:48AM +0100, Ard Biesheuvel wrote:
> It appears the sub instruction at 0x6dd0 correctly accounts for the
> extra 8 bytes, so the frame pointer is valid. So it is our assumption
> that there are no gaps between the stack frames is invalid.
> 
> Could you try the following change please?
> 
> --- a/arch/arm/kernel/ftrace.c
> +++ b/arch/arm/kernel/ftrace.c
> @@ -235,8 +235,12 @@
>                 return;
> 
>         if (IS_ENABLED(CONFIG_UNWINDER_FRAME_POINTER)) {
> -               /* FP points one word below parent's top of stack */
> -               frame_pointer += 4;
> +               /*
> +                * The top of stack of the parent is recorded in the stack
> +                * frame at offset [fp, #-8].
> +                */
> +               get_kernel_nofault(frame_pointer,
> +                                  (unsigned long *)(frame_pointer - 8));

Yes, this will get the value of the stack pointer when the function
was entered - which may be the bottom of the parent function's stack
_or_ the start of non-register arguments to this function. So your
replacement has always been more correct.

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