linux-kernel - Re: [PATCH bpf-next] libbpf: Fix signed overflow in ringbuf_process

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Date:   Wed, 28 Apr 2021 01:05:45 +0200
From:   KP Singh <kpsingh@...nel.org>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Brendan Jackman <jackmanb@...gle.com>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next] libbpf: Fix signed overflow in ringbuf_process_ring

On Tue, Apr 27, 2021 at 11:34 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Tue, Apr 27, 2021 at 10:09 AM Brendan Jackman <jackmanb@...gle.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@...gle.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.
>
> 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

Disclaimer: I don't know what Brendan's benchmark is actually doing

That said, I have seen similar boundaries being reached when
doing process monitoring and then a kernel gets compiled (esp. with ccache)
and generates a large amount of process events in a very short span of time.
Another example is when someone runs a short process in a tight while loop.

I agree it's a matter of tuning, but since these corner cases can be
easily triggered
even on real (non CI) systems no matter how much one tunes, I wouldn't
really call it artificial :)

- KP

> you have more important things to handle :)
>
> >                         len_ptr = r->data + (cons_pos & r->mask);
> >                         len = smp_load_acquire(len_ptr);
> >
> > --
> > 2.31.1.498.g6c1eba8ee3d-goog
> >