Re: [PATCH bpf-next] libbpf: Fix signed overflow in ringbuf_process_ring

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, Apr 28, 2021 at 1:18 AM Brendan Jackman <jackmanb@google.com> wrote:
>
> On Wed, 28 Apr 2021 at 01:19, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> >
> > On Tue, Apr 27, 2021 at 4:05 PM KP Singh <kpsingh@kernel.org> wrote:
> > >
> > > On Tue, Apr 27, 2021 at 11:34 PM Andrii Nakryiko
> > > <andrii.nakryiko@gmail.com> wrote:
> > > >
> > > > On Tue, Apr 27, 2021 at 10:09 AM Brendan Jackman <jackmanb@google.com> wrote:
> > > > >
> > > > > One of our benchmarks running in (Google-internal) CI pushes data
> > > > > through the ringbuf faster than userspace is able to consume
> > > > > it. In this case it seems we're actually able to get >INT_MAX entries
> > > > > in a single ringbuf_buffer__consume call. ASAN detected that cnt
> > > > > overflows in this case.
> > > > >
> > > > > Fix by just setting a limit on the number of entries that can be
> > > > > consumed.
> > > > >
> > > > > Fixes: bf99c936f947 (libbpf: Add BPF ring buffer support)
> > > > > Signed-off-by: Brendan Jackman <jackmanb@google.com>
> > > > > ---
> > > > >  tools/lib/bpf/ringbuf.c | 3 ++-
> > > > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/tools/lib/bpf/ringbuf.c b/tools/lib/bpf/ringbuf.c
> > > > > index e7a8d847161f..445a21df0934 100644
> > > > > --- a/tools/lib/bpf/ringbuf.c
> > > > > +++ b/tools/lib/bpf/ringbuf.c
> > > > > @@ -213,8 +213,8 @@ static int ringbuf_process_ring(struct ring* r)
> > > > >         do {
> > > > >                 got_new_data = false;
> > > > >                 prod_pos = smp_load_acquire(r->producer_pos);
> > > > > -               while (cons_pos < prod_pos) {
> > > > > +               /* Don't read more than INT_MAX, or the return vale won't make sense. */
> > > > > +               while (cons_pos < prod_pos && cnt < INT_MAX) {
> > > >
> > > > ring_buffer__pool() is assumed to not return until all the enqueued
> > > > messages are consumed. That's the requirement for the "adaptive"
> > > > notification scheme to work properly. So this will break that and
> > > > cause the next ring_buffer__pool() to never wake up.
>
> Ah yes, good point, thanks.
>
> > > > We could use __u64 internally and then cap it to INT_MAX on return
> > > > maybe? But honestly, this sounds like an artificial corner case, if
> > > > you are producing data faster than you can consume it and it goes
> > > > beyond INT_MAX, something is seriously broken in your application and
>
> Yes it's certainly artificial but IMO it's still highly desirable for
> libbpf to hold up its side of the bargain even when the application is
> behaving very strangely like this.

One can also argue that if application consumed more than 2 billion
messages in one go, that's an error. ;-P But of course that is not
great.

>
> [...]
>
> > I think we have two alternatives here:
> > 1) consume all but cap return to INT_MAX
> > 2) consume all but return long long as return result
> >
> > Third alternative is to have another API with maximum number of
> > samples to consume. But then user needs to know what they are doing
> > (e.g., they do FORCE on BPF side, or they do their own epoll_wait, or
> > they do ring_buffer__poll with timeout = 0, etc).
> >
> > I'm just not sure anyone would want to understand all the
> > implications. And it's easy to miss those implications. So maybe let's
> > do long long (or __s64) return type instead?
>
> Wouldn't changing the API to 64 bit return type break existing users
> on some ABIs?
>

Yes, it might, not perfect.

> I think capping the return value to INT_MAX and adding a note to the
> function definition comment would also be fine, it doesn't feel like a
> very complex thing for the user to understand: "Returns number of
> records consumed (or INT_MAX, whichever is less)".

Yep, let's cap. But to not penalize a hot loop with extra checks.
Let's use int64_t internally for counting and only cap it before the
return.