linux-kernel - Re: [PATCH bpf-next 1/2] bpf/perf: Call perf_prepare_sample() before bpf_prog

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Message-ID: <2d164a5f-2885-2a6e-581a-2673ca0b1b81@iogearbox.net>
Date:   Thu, 22 Dec 2022 14:17:31 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Peter Zijlstra <peterz@...radead.org>,
        Namhyung Kim <namhyung@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Song Liu <songliubraving@...com>, Jiri Olsa <jolsa@...nel.org>,
        Martin KaFai Lau <kafai@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Hao Luo <haoluo@...gle.com>,
        Stanislav Fomichev <sdf@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>, bpf@...r.kernel.org,
        Ingo Molnar <mingo@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [PATCH bpf-next 1/2] bpf/perf: Call perf_prepare_sample() before
 bpf_prog_run()

On 12/22/22 1:55 PM, Peter Zijlstra wrote:
> On Tue, Dec 20, 2022 at 02:01:43PM -0800, Namhyung Kim wrote:
>> When the BPF program calls bpf_cast_to_kern_ctx(), it assumes the program will
>> access perf sample data directly and call perf_prepare_sample() to make sure
>> the sample data is populated.
> 
> I don't understand a word of this :/ What are you doing and why?

Yeah, above commit message is too terse and unclear. Also, not following where
this assumption comes from; bpf_cast_to_kern_ctx() can be used elsewhere, too,
not just tracing. Recent example from CI side can be found [0].

Thanks,
Daniel

   [0] bpf tree, 70a00e2f1dba ("selftests/bpf: Test bpf_skb_adjust_room on CHECKSUM_PARTIAL")