Re: [PATCH v3 1/4] perf/bpf: Call bpf handler directly, not through overflow machinery

[Marco Elver]

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.

>  /*
>   * 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
>