linux-kernel - Re: [PATCH v2 bpf-next] libbpf: Fix signed overflow in ringbuf_process

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Date:   Tue, 4 May 2021 17:37:31 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Brendan Jackman <jackmanb@...gle.com>
Cc:     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>,
        KP Singh <kpsingh@...nel.org>,
        Florent Revest <revest@...omium.org>
Subject: Re: [PATCH v2 bpf-next] libbpf: Fix signed overflow in ringbuf_process_ring

On Tue, May 4, 2021 at 2:01 AM Brendan Jackman <jackmanb@...gle.com> wrote:
>
> On Mon, 3 May 2021 at 19:46, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
> >
> > On Mon, May 3, 2021 at 5:01 AM Brendan Jackman <jackmanb@...gle.com> wrote:
> > >
> > > On Fri, 30 Apr 2021 at 18:31, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
>
> > So while doing that I noticed that you didn't fix ring_buffer__poll(),
> > so I had to fix it up a bit more extensively. Please check the end
> > result in bpf tree and let me know if there are any problems with it:
> >
> > 2a30f9440640 ("libbpf: Fix signed overflow in ringbuf_process_ring")
>
> Ah, thanks for that. Yep, the additional fix looks good to me.
>
> I think it actually fixes another very niche issue:
>
>  int ring_buffer__poll(struct ring_buffer *rb, int timeout_ms)
>  {
> -       int i, cnt, err, res = 0;
> +       int i, cnt;
> +       int64_t err, res = 0;
>
>         cnt = epoll_wait(rb->epoll_fd, rb->events, rb->ring_cnt, timeout_ms);
> +       if (cnt < 0)
> +               return -errno;
> +
>         for (i = 0; i < cnt; i++) {
>                 __u32 ring_id = rb->events[i].data.fd;
>                 struct ring *ring = &rb->rings[ring_id];
> @@ -280,7 +290,9 @@ int ring_buffer__poll(struct ring_buffer *rb, int
> timeout_ms)
>                         return err;
>                 res += err;
>         }
> -       return cnt < 0 ? -errno : res;
>
> If the callback returns an error but errno is 0 this fails to report the error.

Yeah, there was no need to be clever about that. Explicit if (cnt < 0)
check is obvious and correct.

>
> errno(3) says "the value of errno is never set to zero by any system
> call or library function" but then describes a scenario where an
> application might usefully set it to zero itself. Maybe it can also be
> 0 in new threads, depending on your metaphysical interpretation of "by
> a system call or library function".
>
> +       if (res > INT_MAX)
> +               return INT_MAX;
> +       return res;