[PATCH] ARM: perf/oprofile: fix off-by-one in stack check

[will.deacon@arm.com (Will Deacon)]

Hi Rabin,

> Since it's fp - 1 that gets passed back in as tail in the next iteration, we
> need to ensure that fp - 1 is not the same as tail in order to avoid a
> potential infinite loop in the perf interrupt handler (which has been observed
> to occur).  A similar fix seems to be needed in the OProfile code.

Hehe, that's a nasty loop to hit!
 
> Do we need to  explicitly check for overflow (buftail.fp - 1 > buftail.fp)
> also?  Though this should be already caught by the access check in the next
> iteration of the loop.

I don't think we need to worry about overflow for user backtracing
because the permissions should fail before we get that far.
 
> diff --git a/arch/arm/kernel/perf_event.c b/arch/arm/kernel/perf_event.c
> index 5efa264..dc885f0 100644
> --- a/arch/arm/kernel/perf_event.c
> +++ b/arch/arm/kernel/perf_event.c
> @@ -700,7 +700,7 @@ user_backtrace(struct frame_tail __user *tail,
>  	 * Frame pointers should strictly progress back up the stack
>  	 * (towards higher addresses).
>  	 */
> -	if (tail >= buftail.fp)
> +	if (tail >= buftail.fp - 1)
>  		return NULL;

For a well formed fp chain, the terminating frame should have a saved
NULL frame pointer so it might be more obvious to do tail + 1 >= buftail.fp
(although I think it will work either way).
 
>  	return buftail.fp - 1;
> diff --git a/arch/arm/oprofile/common.c b/arch/arm/oprofile/common.c
> index 8aa9744..67b6b87 100644
> --- a/arch/arm/oprofile/common.c
> +++ b/arch/arm/oprofile/common.c
> @@ -85,7 +85,7 @@ static struct frame_tail* user_backtrace(struct frame_tail *tail)
> 
>  	/* frame pointers should strictly progress back up the stack
>  	 * (towards higher addresses) */
> -	if (tail >= buftail[0].fp)
> +	if (tail >= buftail[0].fp - 1)
>  		return NULL;
> 
>  	return buftail[0].fp-1;

Same here.

Thanks,

Will