linux-kernel - Re: [PATCH v2 bpf-next 1/3] bpf: Add bpf_perf_prog_read

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Date:   Thu, 23 Jan 2020 14:30:00 -0800
From:   "Daniel Xu" <dxu@...uu.xyz>
To:     "Daniel Borkmann" <daniel@...earbox.net>,
        "John Fastabend" <john.fastabend@...il.com>, <bpf@...r.kernel.org>,
        <ast@...nel.org>, <songliubraving@...com>, <yhs@...com>,
        <andriin@...com>
Cc:     <linux-kernel@...r.kernel.org>, <kernel-team@...com>,
        <peterz@...radead.org>, <mingo@...hat.com>, <acme@...nel.org>
Subject: Re: [PATCH v2 bpf-next 1/3] bpf: Add bpf_perf_prog_read_branches()
 helper

On Thu Jan 23, 2020 at 11:23 PM, Daniel Borkmann wrote:
[...]
> 
> Yes, so we've been following this practice for all the BPF helpers no
> matter
> which program type. Though for tracing it may be up to debate whether it
> makes
> still sense given there's nothing to be leaked here since you can read
> this data
> anyway via probe read if you'd wanted to. So we might as well get rid of
> the
> clearing for all tracing helpers.

Right, that makes sense. Do you want me to leave it in for this patchset
and then remove all of them in a followup patchset?

> 
> Different question related to your set. It looks like br_stack is only
> available
> on x86, is that correct? For other archs this will always bail out on
> !br_stack
> test. Perhaps we should document this fact so users are not surprised
> why their
> prog using this helper is not working on !x86. Wdyt?

I think perf_event_open() should fail on !x86 if a user tries to configure
it with branch stack collection. So there would not be the opportunity for
the bpf prog to be attached and run. I haven't tested this, though. I'll
look through the code / install a VM and test it.

[...]

Thanks,
Daniel