netdev - Re: [PATCH v7 tip 2/8] tracing: attach BPF programs to kprobes

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Date:	Thu, 19 Mar 2015 08:33:24 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Ingo Molnar <mingo@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <daniel@...earbox.net>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-api@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 tip 2/8] tracing: attach BPF programs to kprobes

On 3/19/15 8:07 AM, Steven Rostedt wrote:
>>   struct trace_print_flags {
>>   	unsigned long		mask;
>> @@ -252,6 +253,7 @@ enum {
>>   	TRACE_EVENT_FL_WAS_ENABLED_BIT,
>>   	TRACE_EVENT_FL_USE_CALL_FILTER_BIT,
>>   	TRACE_EVENT_FL_TRACEPOINT_BIT,
>> +	TRACE_EVENT_FL_KPROBE_BIT,
>
> I think this should be broken up into two patches. One that adds the
> KPROBE_BIT flag to kprobe events, and the other that adds the bpf
> program.

sure. will do.

>
> There should be kerneldoc comments above this function.
>
>> +unsigned int trace_call_bpf(struct bpf_prog *prog, void *ctx)

ok.

>> +	per_cpu(bpf_prog_active, cpu)--;
>
> Hmm, as cpu == smp_processor_id(), you should be using
> __this_cpu_inc_return(), and __this_cpu_dec().

yeah. picked a wrong place to copy paste from.
will do. good point.

>
> Why not just make kprobe_prog_funcs[] = {
> 	[BPF_FUNC_map_lookup_elem] = &bpf_map_lookup_elem_proto,
> 	[BPF_FUNC_map_update_elem] = &bpf_map_update_elem_proto,
> 	[BPF_FUNC_map_delete_elem] = &bpf_map_delete_elem_proto,
> 	[BPF_FUNC_probe_read] = &kprobe_prog_proto,
>
> And define kprobe_prog_proto separately.
>
> And then you don't need the switch statement, you could just use the
> if (func_id < 0 || func_id >= ARRAY_SIZE(kprobe_prog_funcs))
> 	return kprobe_prog_funcs[func_id];
>
> I think there's several advantages to my approach. One, is that you are
> not wasting memory with empty objects in the array. Also, if the array
> gets out of sync with the enum, it would be possible to return an empty
> object. That is, &kprobe_prog_funcs[out_of_sync_func_id], would not be
> NULL if in the future someone added an enum before BPF_FUNC_probe_read,
> and the func_id passed in is that enum.

yes! There will be gaps in func_ids, so that would have
been a bug. Thanks for catching it!

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