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From:   Martin Lau <kafai@fb.com>
To:     Daniel Borkmann <daniel@iogearbox.net>
CC:     Daniel Xu <dxu@dxuuu.xyz>,
        John Fastabend <john.fastabend@gmail.com>,
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        "ast@kernel.org" <ast@kernel.org>,
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        Andrii Nakryiko <andriin@fb.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Kernel Team <Kernel-team@fb.com>,
        "peterz@infradead.org" <peterz@infradead.org>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "acme@kernel.org" <acme@kernel.org>
Subject: Re: [PATCH v2 bpf-next 1/3] bpf: Add bpf_perf_prog_read_branches()
 helper
Thread-Topic: [PATCH v2 bpf-next 1/3] bpf: Add bpf_perf_prog_read_branches()
 helper
Thread-Index: AQHV0WHOsrtDC0PlC0ePBDNERArXBKf3vCiAgADzN4CAACVegIAAAcsAgAAEKICAAAZwgA==
Date:   Thu, 23 Jan 2020 23:07:59 +0000
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On Thu, Jan 23, 2020 at 11:44:53PM +0100, Daniel Borkmann wrote:
> On 1/23/20 11:30 PM, Daniel Xu wrote:
> > On Thu Jan 23, 2020 at 11:23 PM, Daniel Borkmann wrote:
> > [...]
> > >=20
> > > 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 rea=
d
> > > 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.
> >=20
> > Right, that makes sense. Do you want me to leave it in for this patchse=
t
> > and then remove all of them in a followup patchset?
>=20
> Lets leave it in and in a different set, we can clean this up for all tra=
cing
> related helpers at once.
>=20
> > > Different question related to your set. It looks like br_stack is onl=
y
> > > 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?
> >=20
> > I think perf_event_open() should fail on !x86 if a user tries to config=
ure
> > 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'l=
l
> > look through the code / install a VM and test it.
>=20
> 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 sh=
ould be
> changed into -ENOENT to avoid confusion wrt whether user provided some in=
valid
+1 on -ENOENT.

> input args. Should this actually bail out with -EINVAL if size is not a m=
ultiple
> of sizeof(struct perf_branch_entry) as otherwise we'd end up copying half=
 broken
> branch entry information?