linux-kernel - Re: [PATCH bpf-next 2/3] bpf: Add bpf_perf_event_read

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Date:   Wed, 2 Nov 2022 15:18:12 -0700
From:   Namhyung Kim <namhyung@...nel.org>
To:     Song Liu <song@...nel.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Jiri Olsa <olsajiri@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Song Liu <songliubraving@...com>,
        Peter Zijlstra <peterz@...radead.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 <bpf@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [PATCH bpf-next 2/3] bpf: Add bpf_perf_event_read_sample() helper

On Tue, Nov 1, 2022 at 5:13 PM Song Liu <song@...nel.org> wrote:
>
> On Tue, Nov 1, 2022 at 3:17 PM Namhyung Kim <namhyung@...nel.org> wrote:
> > IIUC we want something like below to access sample data directly,
> > right?
> >
> >   BPF_CORE_READ(ctx, data, ip);
> >
>
> I haven't tried this, but I guess we may need something like
>
> data = ctx->data;
> BPF_CORE_READ(data, ip);

Ok, will try.

>
> > Some fields like raw and callchains will have variable length data
> > so it'd be hard to check the boundary at load time.
>
> I think we are fine as long as we can check boundaries at run time.

Sure, that means it's the responsibility of BPF writers, right?

>
> > Also it's possible
> > that some fields are not set (according to sample type), and it'd be
> > the user's (or programmer's) responsibility to check if the data is
> > valid.  If these are not the concerns, I think I'm good.
>
> So we still need 1/3 of the set to make sure the data is valid?

Of course, I'll keep it in the v2.

Thanks,
Namhyung