Re: [PATCH v2 bpf-next 1/3] bpf: Add bpf_perf_prog_read_branches() helper

[Daniel Borkmann]

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