netdev - Re: [PATCH net-next v3 1/2] bpf: add support for sys_enter_* and sys_exit

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Date:   Thu, 3 Aug 2017 16:08:07 -1000
From:   Alexei Starovoitov <ast@...com>
To:     Yonghong Song <yhs@...com>, <peterz@...radead.org>,
        <rostedt@...dmis.org>, <daniel@...earbox.net>,
        <netdev@...r.kernel.org>
CC:     <kernel-team@...com>
Subject: Re: [PATCH net-next v3 1/2] bpf: add support for sys_enter_* and
 sys_exit_* tracepoints

On 8/3/17 6:29 AM, Yonghong Song wrote:
> @@ -578,8 +596,9 @@ static void perf_syscall_enter(void *ignore, struct pt_regs *regs, long id)
>  	if (!sys_data)
>  		return;
>
> +	prog = READ_ONCE(sys_data->enter_event->prog);
>  	head = this_cpu_ptr(sys_data->enter_event->perf_events);
> -	if (hlist_empty(head))
> +	if (!prog && hlist_empty(head))
>  		return;
>
>  	/* get the size after alignment with the u32 buffer size field */
> @@ -594,6 +613,13 @@ static void perf_syscall_enter(void *ignore, struct pt_regs *regs, long id)
>  	rec->nr = syscall_nr;
>  	syscall_get_arguments(current, regs, 0, sys_data->nb_args,
>  			       (unsigned long *)&rec->args);
> +
> +	if ((prog && !perf_call_bpf_enter(prog, regs, sys_data, rec)) ||
> +	    hlist_empty(head)) {
> +		perf_swevent_put_recursion_context(rctx);
> +		return;
> +	}

hmm. if I read the patch correctly that makes it different from
kprobe/uprobe/tracepoints+bpf behavior. Why make it different and
force user space to perf_event_open() on every cpu?
In other cases it's the job of the bpf program to filter by cpu
if necessary and that is well understood by bcc scripts.