netdev - Re: [Patch bpf] selftests/bpf: Retry for EAGAIN in udp_redir_to

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Date:   Wed, 19 May 2021 16:36:11 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Cong Wang <cong.wang@...edance.com>,
        Jiang Wang <jiang.wang@...edance.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Sitnicki <jakub@...udflare.com>,
        Lorenz Bauer <lmb@...udflare.com>
Subject: Re: [Patch bpf] selftests/bpf: Retry for EAGAIN in udp_redir_to_connected()

On Wed, May 19, 2021 at 2:56 PM John Fastabend <john.fastabend@...il.com> wrote:
>
> Cong Wang wrote:
> > From: Cong Wang <cong.wang@...edance.com>
> >
> > We use non-blocking sockets for testing sockmap redirections,
> > and got some random EAGAIN errors from UDP tests.
> >
> > There is no guarantee the packet would be immediately available
> > to receive as soon as it is sent out, even on the local host.
> > For UDP, this is especially true because it does not lock the
> > sock during BH (unlike the TCP path). This is probably why we
> > only saw this error in UDP cases.
> >
> > No matter how hard we try to make the queue empty check accurate,
> > it is always possible for recvmsg() to beat ->sk_data_ready().
> > Therefore, we should just retry in case of EAGAIN.
> >
> > Fixes: d6378af615275 ("selftests/bpf: Add a test case for udp sockmap")
> > Reported-by: Jiang Wang <jiang.wang@...edance.com>
> > Cc: John Fastabend <john.fastabend@...il.com>
> > Cc: Daniel Borkmann <daniel@...earbox.net>
> > Cc: Jakub Sitnicki <jakub@...udflare.com>
> > Cc: Lorenz Bauer <lmb@...udflare.com>
> > Signed-off-by: Cong Wang <cong.wang@...edance.com>
> > ---
> >  tools/testing/selftests/bpf/prog_tests/sockmap_listen.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/tools/testing/selftests/bpf/prog_tests/sockmap_listen.c b/tools/testing/selftests/bpf/prog_tests/sockmap_listen.c
> > index 648d9ae898d2..b1ed182c4720 100644
> > --- a/tools/testing/selftests/bpf/prog_tests/sockmap_listen.c
> > +++ b/tools/testing/selftests/bpf/prog_tests/sockmap_listen.c
> > @@ -1686,9 +1686,13 @@ static void udp_redir_to_connected(int family, int sotype, int sock_mapfd,
> >       if (pass != 1)
> >               FAIL("%s: want pass count 1, have %d", log_prefix, pass);
> >
> > +again:
> >       n = read(mode == REDIR_INGRESS ? p0 : c0, &b, 1);
> > -     if (n < 0)
> > +     if (n < 0) {
> > +             if (errno == EAGAIN)
> > +                     goto again;
> >               FAIL_ERRNO("%s: read", log_prefix);
>
> Needs a counter and abort logic we don't want to loop forever in the
> case the packet is lost.

It should not be lost because selftests must be self-contained,
if the selftests could not even predict whether its own packet is
lost or not, we would have a much bigger trouble than just this
infinite loop.

Thanks.