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

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Date:   Fri, 26 Aug 2022 20:58:48 +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 12:30 PM, Namhyung Kim <namhyung@...nel.org> wrote:
> 
> On Fri, Aug 26, 2022 at 11:45 AM Song Liu <songliubraving@...com> wrote:
> 
>>> 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.
> 
> I'm not sure.  The sample_data should be provided by the caller
> of perf_event_overflow.  So I guess the bpf program should see
> a valid ctx->data.

Let's dig into this. Maybe we need some small changes in 
pe_prog_convert_ctx_access. 

> Also I want to control calling the orig_overflow_handler based
> on the return value of the BPF program.  So calling the orig
> handler before BPF won't work for me. :)

Interesting. Could you share more information about the use case?

Thanks,
Song