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

[Masahiko Yamada (Fujitsu)]

Itaru san

> -----Original Message-----
> From: Itaru Kitayama <itaru.kitayama@xxxxxxxxxxx>
> Sent: Friday, November 18, 2022 2:12 PM
> To: Yamada, Masahiko/山田 雅彦 <yamada.masahiko@xxxxxxxxxxx>
> Cc: acme@xxxxxxxxxx; alexander.shishkin@xxxxxxxxxxxxxxx; jolsa@xxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; linux-perf-users@xxxxxxxxxxxxxxx;
> mark.rutland@xxxxxxx; mingo@xxxxxxxxxx; namhyung@xxxxxxxxxx;
> peterz@xxxxxxxxxxxxx
> Subject: [PATCH 1/1] perf: fix reset interface potential failure
> 
> 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>

Thank you for your review.

Masahiko Yamada