[PATCH v4 2/3] arm64: stacktrace: Relax frame record alignment requirement to 8 bytes

[Mark Rutland]

Hi Peter,

On Wed, May 12, 2021 at 07:27:08PM -0700, Peter Collingbourne wrote:
> The AAPCS places no requirements on the alignment of the frame
> record. In theory it could be placed anywhere, although it seems
> sensible to require it to be aligned to 8 bytes. With an upcoming
> enhancement to tag-based KASAN Clang will begin creating frame records
> located at an address that is only aligned to 8 bytes. Accommodate
> such frame records in the stack unwinding code.
> 
> As pointed out by Mark Rutland, the userspace stack unwinding code
> has the same problem, so fix it there as well.
> 
> Signed-off-by: Peter Collingbourne <pcc at google.com>
> Link: https://linux-review.googlesource.com/id/Ia22c375230e67ca055e9e4bb639383567f7ad268
> Acked-by: Andrey Konovalov <andreyknvl at gmail.com>
> Reviewed-by: Mark Rutland <mark.rutland at arm.com>
> Tested-by: Mark Rutland <mark.rutland at arm.com>
> ---
> v4:
> - rebase to 5.13rc1

I think the rebase went slightly wrong; note below.

> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
> index 2fecbf152e80..020e575e5cd3 100644
> --- a/arch/arm64/kernel/stacktrace.c
> +++ b/arch/arm64/kernel/stacktrace.c
> @@ -68,7 +68,11 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
>  	unsigned long fp = frame->fp;
>  	struct stack_info info;
>  
> -	if (fp & 0xf)
> +	/* Terminal record; nothing to unwind */
> +	if (!fp)
> +		return -ENOENT;
> +
> +	if (fp & 0x7)
>  		return -EINVAL;

The terminal record check got moved later in the function by commit:

  8533d5bfad41e74b ("arm64: stacktrace: restore terminal records")

... so this patch shouldn't re-add it. With that gone (and this just
changing the alignemnt check), this looks good to me.

Thanks,
Mark.