Re: [PATCH v2 bpf-next 05/14] bpf: allow to specify user-provided context value for BPF perf links

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jul 26, 2021 at 09:12:02AM -0700, Andrii Nakryiko wrote:
> Add ability for users to specify custom u64 value when creating BPF link for
> perf_event-backed BPF programs (kprobe/uprobe, perf_event, tracepoints).

If I read this right, the value is dependent on the link, not the
program. In which case:

> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 2d510ad750ed..97ab46802800 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -762,6 +762,7 @@ struct perf_event {
>  #ifdef CONFIG_BPF_SYSCALL
>  	perf_overflow_handler_t		orig_overflow_handler;
>  	struct bpf_prog			*prog;
> +	u64				user_ctx;
>  #endif
>  
>  #ifdef CONFIG_EVENT_TRACING
> diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
> index 8ac92560d3a3..4543852f1480 100644
> --- a/include/linux/trace_events.h
> +++ b/include/linux/trace_events.h
> @@ -675,7 +675,7 @@ trace_trigger_soft_disabled(struct trace_event_file *file)
>  
>  #ifdef CONFIG_BPF_EVENTS
>  unsigned int trace_call_bpf(struct trace_event_call *call, void *ctx);
> -int perf_event_attach_bpf_prog(struct perf_event *event, struct bpf_prog *prog);
> +int perf_event_attach_bpf_prog(struct perf_event *event, struct bpf_prog *prog, u64 user_ctx);

This API would be misleading, because it is about setting the program.

>  void perf_event_detach_bpf_prog(struct perf_event *event);
>  int perf_event_query_prog_array(struct perf_event *event, void __user *info);
>  int bpf_probe_register(struct bpf_raw_event_map *btp, struct bpf_prog *prog);

> @@ -9966,6 +9968,7 @@ static int perf_event_set_bpf_handler(struct perf_event *event, struct bpf_prog
>  	}
>  
>  	event->prog = prog;
> +	event->user_ctx = user_ctx;
>  	event->orig_overflow_handler = READ_ONCE(event->overflow_handler);
>  	WRITE_ONCE(event->overflow_handler, bpf_overflow_handler);
>  	return 0;

Also, the name @user_ctx is a bit confusing. Would something like
@bpf_cookie or somesuch not be a better name?

Combined would it not make more sense to add something like:

extern int perf_event_set_bpf_cookie(struct perf_event *event, u64 cookie);