linux-kernel - Re: [RESEND PATCH v5 3/4] perf/bpf: Allow a bpf program to suppress all sample side effects

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Date: Thu, 15 Feb 2024 17:59:41 -0800
From: Kyle Huey <me@...ehuey.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Kyle Huey <khuey@...ehuey.com>, linux-kernel@...r.kernel.org, 
	Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>, Marco Elver <elver@...gle.com>, 
	Yonghong Song <yonghong.song@...ux.dev>, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>, 
	"Robert O'Callahan" <robert@...llahan.org>, Song Liu <song@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Ian Rogers <irogers@...gle.com>, 
	Adrian Hunter <adrian.hunter@...el.com>, linux-perf-users@...r.kernel.org, 
	bpf@...r.kernel.org
Subject: Re: [RESEND PATCH v5 3/4] perf/bpf: Allow a bpf program to suppress
 all sample side effects

On Thu, Feb 15, 2024 at 4:14 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Wed, Feb 14, 2024 at 9:40 AM Kyle Huey <me@...ehuey.com> wrote:
> >
> > Returning zero from a bpf program attached to a perf event already
> > suppresses any data output. Return early from __perf_event_overflow() in
> > this case so it will also suppress event_limit accounting, SIGTRAP
> > generation, and F_ASYNC signalling.
> >
> > Signed-off-by: Kyle Huey <khuey@...ehuey.com>
> > Acked-by: Song Liu <song@...nel.org>
> > Acked-by: Jiri Olsa <jolsa@...nel.org>
> > Acked-by: Namhyung Kim <namhyung@...nel.org>
> > ---
> >  kernel/events/core.c | 10 ++++++----
> >  1 file changed, 6 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 24a718e7eb98..a329bec42c4d 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -9574,6 +9574,11 @@ static int __perf_event_overflow(struct perf_event *event,
> >
> >         ret = __perf_event_account_interrupt(event, throttle);
> >
> > +#ifdef CONFIG_BPF_SYSCALL
> > +       if (event->prog && !bpf_overflow_handler(event, data, regs))
> > +               return ret;
> > +#endif
> > +
> >         /*
> >          * XXX event_limit might not quite work as expected on inherited
> >          * events
> > @@ -9623,10 +9628,7 @@ static int __perf_event_overflow(struct perf_event *event,
> >                 irq_work_queue(&event->pending_irq);
> >         }
> >
> > -#ifdef CONFIG_BPF_SYSCALL
> > -       if (!(event->prog && !bpf_overflow_handler(event, data, regs)))
> > -#endif
> > -               READ_ONCE(event->overflow_handler)(event, data, regs);
> > +       READ_ONCE(event->overflow_handler)(event, data, regs);
> >
>
> Sorry, I haven't followed previous discussions, but why can't this
> change be done as part of patch 1?

The idea was to refactor the code without making any behavior changes
(patches 1 and 2) and then to change the behavior (patch 3).

- Kyle

> >         if (*perf_event_fasync(event) && event->pending_kill) {
> >                 event->pending_wakeup = 1;
> > --
> > 2.34.1
> >