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

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Message-ID: <9341443f-b29a-e92e-0e12-7990927b4e33@iogearbox.net>
Date:   Thu, 23 Jan 2020 23:44:53 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Daniel Xu <dxu@...uu.xyz>,
        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 1/23/20 11:30 PM, Daniel Xu wrote:
> 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?

Lets leave it in and in a different set, we can clean this up for all tracing
related helpers at once.

>> 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.

As far as I can see the prog would still be attachable and runnable, just that
the helper always will return -EINVAL on these archs. Maybe error code should be
changed into -ENOENT to avoid confusion wrt whether user provided some invalid
input args. Should this actually bail out with -EINVAL if size is not a multiple
of sizeof(struct perf_branch_entry) as otherwise we'd end up copying half broken
branch entry information?

Thanks,
Daniel