linux-kernel - Re: [PATCH bpf-next] bpf: Add bpf_read_raw

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Message-ID: <FCC75F8E-4C2F-42A4-B582-9BE3BB87E15A@fb.com>
Date:   Fri, 26 Aug 2022 18:44:58 +0000
From:   Song Liu <songliubraving@...com>
To:     Namhyung Kim <namhyung@...nel.org>
CC:     Song Liu <song@...nel.org>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next] bpf: Add bpf_read_raw_record() helper



> On Aug 26, 2022, at 11:09 AM, Song Liu <songliubraving@...com> wrote:
> 
> 
> 
>> On Aug 26, 2022, at 9:33 AM, Namhyung Kim <namhyung@...nel.org> wrote:
>> 
>> On Thu, Aug 25, 2022 at 10:53 PM Song Liu <song@...nel.org> wrote:
>>> 
>>> On Thu, Aug 25, 2022 at 10:22 PM Namhyung Kim <namhyung@...nel.org> wrote:
>>>> 
>>>> On Thu, Aug 25, 2022 at 7:35 PM Song Liu <songliubraving@...com> wrote:
>>>>> Actually, since we are on this, can we make it more generic, and handle
>>>>> all possible PERF_SAMPLE_* (in enum perf_event_sample_format)? Something
>>>>> like:
>>>>> 
>>>>> long bpf_perf_event_read_sample(void *ctx, void *buf, u64 size, u64 flags);
>>>>> 
>>>>> WDYT Namhyung?
>>>> 
>>>> Do you mean reading the whole sample data at once?
>>>> Then it needs to parse the sample data format properly
>>>> which is non trivial due to a number of variable length
>>>> fields like callchains and branch stack, etc.
>>>> 
>>>> Also I'm afraid I might need event configuration info
>>>> other than sample data like attr.type, attr.config,
>>>> attr.sample_type and so on.
>>>> 
>>>> Hmm.. maybe we can add it to the ctx directly like ctx.attr_type?
>>> 
>>> The user should have access to the perf_event_attr used to
>>> create the event. This is also available in ctx->event->attr.
>> 
>> Do you mean from BPF?  I'd like to have a generic BPF program
>> that can handle various filtering according to the command line
>> arguments.  I'm not sure but it might do something differently
>> for each event based on the attr settings.
> 
> Yeah, we can access perf_event_attr from BPF program. Note that
> the ctx for perf_event bpf program is struct bpf_perf_event_data_kern:
> 
> SEC("perf_event")
> int perf_e(struct bpf_perf_event_data_kern *ctx)
> {	
> 	...
> }
> 
> struct bpf_perf_event_data_kern {
>        bpf_user_pt_regs_t *regs;
>        struct perf_sample_data *data;
>        struct perf_event *event;
> };
> 
> Alternatively, we can also have bpf user space configure the BPF 
> program via a few knobs. 
> 
> And actually, we can just read ctx->data and get the raw record, 
> right..?

Played with this for a little bit. ctx->data appears to be not 
reliable sometimes. I guess (not 100% sure) this is because we 
call bpf program before event->orig_overflow_handler. We can 
probably add a flag to specify we want to call orig_overflow_handler
first. 

Thanks,
Song