Re: [PATCH] perf/bpf: Don't call bpf_overflow_handler() for tracing events

[Jiri Olsa <olsajiri@gmail.com>]

On Fri, Jul 12, 2024 at 09:46:45PM -0700, Kyle Huey wrote:
> The regressing commit is new in 6.10. It assumed that anytime event->prog
> is set bpf_overflow_handler() should be invoked to execute the attached bpf
> program. This assumption is false for tracing events, and as a result the
> regressing commit broke bpftrace by invoking the bpf handler with garbage
> inputs on overflow.
> 
> Prior to the regression the overflow handlers formed a chain (of length 0,
> 1, or 2) and perf_event_set_bpf_handler() (the !tracing case) added
> bpf_overflow_handler() to that chain, while perf_event_attach_bpf_prog()
> (the tracing case) did not. Both set event->prog. The chain of overflow
> handlers was replaced by a single overflow handler slot and a fixed call to
> bpf_overflow_handler() when appropriate. This modifies the condition there
> to include !perf_event_is_tracing(), restoring the previous behavior and
> fixing bpftrace.
> 
> Signed-off-by: Kyle Huey <khuey@kylehuey.com>
> Reported-by: Joe Damato <jdamato@fastly.com>
> Fixes: f11f10bfa1ca ("perf/bpf: Call BPF handler directly, not through overflow machinery")
> Tested-by: Joe Damato <jdamato@fastly.com> # bpftrace
> Tested-by: Kyle Huey <khuey@kylehuey.com> # bpf overflow handlers
> ---
>  kernel/events/core.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 8f908f077935..f0d7119585dc 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -9666,6 +9666,8 @@ static inline void perf_event_free_bpf_handler(struct perf_event *event)
>   * Generic event overflow handling, sampling.
>   */
>  
> +static bool perf_event_is_tracing(struct perf_event *event);
> +
>  static int __perf_event_overflow(struct perf_event *event,
>  				 int throttle, struct perf_sample_data *data,
>  				 struct pt_regs *regs)
> @@ -9682,7 +9684,9 @@ static int __perf_event_overflow(struct perf_event *event,
>  
>  	ret = __perf_event_account_interrupt(event, throttle);
>  
> -	if (event->prog && !bpf_overflow_handler(event, data, regs))
> +	if (event->prog &&
> +	    !perf_event_is_tracing(event) &&
> +	    !bpf_overflow_handler(event, data, regs))
>  		return ret;

ok makes sense, it's better to follow the perf_event_set_bpf_prog condition

Reviewed-by: Jiri Olsa <jolsa@kernel.org>

jirka

>  
>  	/*
> @@ -10612,6 +10616,11 @@ void perf_event_free_bpf_prog(struct perf_event *event)
>  
>  #else
>  
> +static inline bool perf_event_is_tracing(struct perf_event *event)
> +{
> +	return false;
> +}
> +
>  static inline void perf_tp_register(void)
>  {
>  }
> -- 
> 2.34.1
>