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

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Message-ID: <Y6S7BcblAHO4nQTf@hirez.programming.kicks-ass.net>
Date:   Thu, 22 Dec 2022 21:16:05 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        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 Thu, Dec 22, 2022 at 09:34:42AM -0800, Namhyung Kim wrote:

> Sorry about that.  Let me rephrase it like below:
> 
> With bpf_cast_to_kern_ctx(), BPF programs attached to a perf event
> can access perf sample data directly from the ctx. 

This is the bpf_prog_run() in bpf_overflow_handler(), right?

> But the perf sample
> data is not fully prepared at this point, and some fields can have invalid
> uninitialized values.  So it needs to call perf_prepare_sample() before
> calling the BPF overflow handler.

It never was, why is it a problem now?

> But just calling perf_prepare_sample() can be costly when the BPF

So you potentially call it twice now, how's that useful?