Re: [PATCH v3 1/4] perf/bpf: Call bpf handler directly, not through overflow machinery
From: Kyle Huey
Date: Mon Dec 11 2023 - 10:20:45 EST
On Mon, Dec 11, 2023 at 6:20 AM Marco Elver <elver@xxxxxxxxxx> wrote:
>
> On Mon, 11 Dec 2023 at 05:55, Kyle Huey <me@xxxxxxxxxxxx> wrote:
> >
> > To ultimately allow bpf programs attached to perf events to completely
> > suppress all of the effects of a perf event overflow (rather than just the
> > sample output, as they do today), call bpf_overflow_handler() from
> > __perf_event_overflow() directly rather than modifying struct perf_event's
> > overflow_handler. Return the bpf program's return value from
> > bpf_overflow_handler() so that __perf_event_overflow() knows how to
> > proceed. Remove the now unnecessary orig_overflow_handler from struct
> > perf_event.
> >
> > This patch is solely a refactoring and results in no behavior change.
> >
> > Signed-off-by: Kyle Huey <khuey@xxxxxxxxxxxx>
> > Suggested-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> > ---
> > include/linux/perf_event.h | 6 +-----
> > kernel/events/core.c | 28 +++++++++++++++-------------
> > 2 files changed, 16 insertions(+), 18 deletions(-)
> >
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index 5547ba68e6e4..312b9f31442c 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -810,7 +810,6 @@ struct perf_event {
> > perf_overflow_handler_t overflow_handler;
> > void *overflow_handler_context;
> > #ifdef CONFIG_BPF_SYSCALL
> > - perf_overflow_handler_t orig_overflow_handler;
> > struct bpf_prog *prog;
> > u64 bpf_cookie;
> > #endif
> > @@ -1337,10 +1336,7 @@ __is_default_overflow_handler(perf_overflow_handler_t overflow_handler)
> > #ifdef CONFIG_BPF_SYSCALL
> > static inline bool uses_default_overflow_handler(struct perf_event *event)
> > {
> > - if (likely(is_default_overflow_handler(event)))
> > - return true;
> > -
> > - return __is_default_overflow_handler(event->orig_overflow_handler);
> > + return is_default_overflow_handler(event);
> > }
> > #else
> > #define uses_default_overflow_handler(event) \
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index b704d83a28b2..54f6372d2634 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -9515,6 +9515,12 @@ static inline bool sample_is_allowed(struct perf_event *event, struct pt_regs *r
> > return true;
> > }
> >
> > +#ifdef CONFIG_BPF_SYSCALL
> > +static int bpf_overflow_handler(struct perf_event *event,
> > + struct perf_sample_data *data,
> > + struct pt_regs *regs);
> > +#endif
>
> To avoid more #ifdefs we usually add a stub, something like:
>
> #ifdef ...
> static int bpf_overflow_handler(...);
> #else
> static inline int bpf_overflow_handler(...) { return 0; }
> #endif
>
> Then you can avoid more #ifdefs below, esp. when it surrounds an
> if-statement it easily leads to confusion or subtle bugs in future
> changes. The compiler will optimize out the constants and the
> generated code will be the same.
This would not allow removing any #ifdefs because event->prog is only
present if CONFIG_BPF_SYSCALL is defined.
- Kyle
> > /*
> > * Generic event overflow handling, sampling.
> > */
> > @@ -9584,7 +9590,10 @@ static int __perf_event_overflow(struct perf_event *event,
> > irq_work_queue(&event->pending_irq);
> > }
> >
> > - READ_ONCE(event->overflow_handler)(event, data, regs);
> > +#ifdef CONFIG_BPF_SYSCALL
> > + if (!(event->prog && !bpf_overflow_handler(event, data, regs)))
> > +#endif
> > + READ_ONCE(event->overflow_handler)(event, data, regs);
> >
> > if (*perf_event_fasync(event) && event->pending_kill) {
> > event->pending_wakeup = 1;
> > @@ -10394,9 +10403,9 @@ static void perf_event_free_filter(struct perf_event *event)
> > }
> >
> > #ifdef CONFIG_BPF_SYSCALL
> > -static void bpf_overflow_handler(struct perf_event *event,
> > - struct perf_sample_data *data,
> > - struct pt_regs *regs)
> > +static int bpf_overflow_handler(struct perf_event *event,
> > + struct perf_sample_data *data,
> > + struct pt_regs *regs)
> > {
> > struct bpf_perf_event_data_kern ctx = {
> > .data = data,
> > @@ -10417,10 +10426,8 @@ static void bpf_overflow_handler(struct perf_event *event,
> > rcu_read_unlock();
> > out:
> > __this_cpu_dec(bpf_prog_active);
> > - if (!ret)
> > - return;
> >
> > - event->orig_overflow_handler(event, data, regs);
> > + return ret;
> > }
> >
> > static int perf_event_set_bpf_handler(struct perf_event *event,
> > @@ -10456,8 +10463,6 @@ static int perf_event_set_bpf_handler(struct perf_event *event,
> >
> > event->prog = prog;
> > event->bpf_cookie = bpf_cookie;
> > - event->orig_overflow_handler = READ_ONCE(event->overflow_handler);
> > - WRITE_ONCE(event->overflow_handler, bpf_overflow_handler);
> > return 0;
> > }
> >
> > @@ -10468,7 +10473,6 @@ static void perf_event_free_bpf_handler(struct perf_event *event)
> > if (!prog)
> > return;
> >
> > - WRITE_ONCE(event->overflow_handler, event->orig_overflow_handler);
> > event->prog = NULL;
> > bpf_prog_put(prog);
> > }
> > @@ -11928,13 +11932,11 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu,
> > overflow_handler = parent_event->overflow_handler;
> > context = parent_event->overflow_handler_context;
> > #if defined(CONFIG_BPF_SYSCALL) && defined(CONFIG_EVENT_TRACING)
> > - if (overflow_handler == bpf_overflow_handler) {
> > + if (parent_event->prog) {
> > struct bpf_prog *prog = parent_event->prog;
> >
> > bpf_prog_inc(prog);
> > event->prog = prog;
> > - event->orig_overflow_handler =
> > - parent_event->orig_overflow_handler;
> > }
> > #endif
> > }
> > --
> > 2.34.1
> >