Re: [PATCH bpf-next 2/3] bpf: Add bpf_perf_event_read_sample() helper

From: Namhyung Kim <namhyung@kernel.org>
To: Song Liu <song@kernel.org>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Jiri Olsa <olsajiri@gmail.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Song Liu <songliubraving@fb.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Martin KaFai Lau <kafai@fb.com>, Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>, Hao Luo <haoluo@google.com>,
	Stanislav Fomichev <sdf@google.com>,
	LKML <linux-kernel@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ingo Molnar <mingo@kernel.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>
Subject: Re: [PATCH bpf-next 2/3] bpf: Add bpf_perf_event_read_sample() helper
Date: Wed, 2 Nov 2022 15:18:12 -0700	[thread overview]
Message-ID: <CAM9d7chq+Y5M-4S1HWwxBkL+aRysGt8griGbo_jXG4g+EQK_gg@mail.gmail.com> (raw)
In-Reply-To: <CAPhsuW70GMFfzvgp__GOhebPu9bXnG7PzEby6xEExFgg+JmeTQ@mail.gmail.com>

On Tue, Nov 1, 2022 at 5:13 PM Song Liu <song@kernel.org> wrote:
>
> On Tue, Nov 1, 2022 at 3:17 PM Namhyung Kim <namhyung@kernel.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