Re: [PATCH bpf-next v2] bpf: check flags in 'bpf_ringbuf_discard()' and 'bpf_ringbuf_submit()'

[Andrii Nakryiko]

On Tue, Mar 30, 2021 at 4:16 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Tue, Mar 30, 2021 at 3:54 PM Pedro Tammela <pctammela@xxxxxxxxx> wrote:
> >
> >  BPF_CALL_2(bpf_ringbuf_submit, void *, sample, u64, flags)
> >  {
> > +       if (unlikely(flags & ~(BPF_RB_NO_WAKEUP | BPF_RB_FORCE_WAKEUP)))
> > +               return -EINVAL;
> > +
> >         bpf_ringbuf_commit(sample, flags, false /* discard */);
> > +
> >         return 0;
>
> I think ringbuf design was meant for bpf_ringbuf_submit to never fail.
> If we do flag validation it probably should be done at the verifier time.

Oops, replied on another version already. But yes, BPF verifier relies
on it succeeding. I don't think we can do flags validation at BPF
verification time, though, because it is defined as non-const integer
and we do have valid cases where we dynamically determine whether to
FORCE_WAKEUP or NO_WAKEUP, based on application-driven criteria (e.g.,
amount of enqueued data).