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

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