[PATCH 1/1] perf: fix reset interface potential failure

[Itaru Kitayama]

On Fri, Nov 18, 2022 at 11:00:16AM +0900, Masahiko wrote:
> There is a potential bug where PERF_EVENT_IOC_RESET
> does not work when accessing PMU registers directly
> from userspace in the perf_event interface.
> we have created a patch on the kernel
> that fixes a potential perf_event reset failure.
> 
> The motivation is to initialize pc->offset.
> The perf_mmap__read_self function in tools/lib/perf/mmap.c is set by:.
> cnt = READ_ONCE(pc->offset);
> The pc->offset value is set in the following process
> in the perf_event_update_userpage function:.
> userpg->offset -= local64_read(&event->hw.prev_count);
> hw->prev_count is set in the armpmu_event_set_period function
> in drivers/perf/arm_pmu.c and in the x86_perf_event_set_period function
> in arch/x86/events/core.c as follows:.
> local64_set(&hwc->prev_count, (u64)-left);
> 
> Therefore, this patch was created to initialize hwc->prev_count
> during reset processing.
> 
> Signed-off-by: Masahiko, Yamada <yamada.masahiko@xxxxxxxxxxx>
> ---
>  kernel/events/core.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 4ec3717003d5..296549755a9c 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -5468,8 +5468,13 @@ static __poll_t perf_poll(struct file *file, poll_table *wait)
>  
>  static void _perf_event_reset(struct perf_event *event)
>  {
> +	struct hw_perf_event *hwc = &event->hw;
> +	s64 left;
> +
>  	(void)perf_event_read(event, false);
>  	local64_set(&event->count, 0);
> +	left = local64_read(&hwc->period_left);
> +	local64_set(&hwc->prev_count, (u64)-left);
>  	perf_event_update_userpage(event);
>  }
>  
> -- 
> 2.27.0

Reviewed-by: Itaru Kitayama <itaru.kitayama@xxxxxxxxxxx>