[PATCH] Fix infinite loop in ARM user perf_event backtrace code

[Will Deacon]

Hi Sonny,

On Sat, 2011-04-16 at 04:27 +0100, Sonny Rao wrote:
> The ARM user backtrace code can get into an infinite loop if it
> runs into an invalid stack frame which points back to itself.
> This situation has been observed in practice.  Fix it by capping
> the number of entries in the backtrace.  This is also what other
> architectures do in their backtrace code.
> 
> Signed-off-by: Sonny Rao <sonnyrao at chromium.org>
> ---
>  arch/arm/kernel/perf_event.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/arm/kernel/perf_event.c b/arch/arm/kernel/perf_event.c
> index 69cfee0..1e61d60 100644
> --- a/arch/arm/kernel/perf_event.c
> +++ b/arch/arm/kernel/perf_event.c
> @@ -746,7 +746,8 @@ perf_callchain_user(struct perf_callchain_entry *entry, struct pt_regs *regs)
> 
>         tail = (struct frame_tail __user *)regs->ARM_fp - 1;
> 
> -       while (tail && !((unsigned long)tail & 0x3))
> +       while ((entry->nr < PERF_MAX_STACK_DEPTH) &&
> +              tail && !((unsigned long)tail & 0x3))
>                 tail = user_backtrace(tail, entry);
>  }

Ok. Please can you put this into Russell's patch system?

Will