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

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Date:   Thu, 23 Jan 2020 15:27:07 -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 11:44 PM, Daniel Borkmann wrote:
[...]
> >> 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. 

Ok, will add.

> 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?

Sure, makes sense.
>
> 
> Thanks,
> Daniel
>
> 
>
>