On Wed, May 19, 2021 at 2:56 PM John Fastabend <john.fastabend@xxxxxxxxx> wrote:
>
> Cong Wang wrote:
> > From: Cong Wang <cong.wang@xxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxx>
> > Cc: John Fastabend <john.fastabend@xxxxxxxxx>
> > Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> > Cc: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>
> > Cc: Lorenz Bauer <lmb@xxxxxxxxxxxxxx>
> > Signed-off-by: Cong Wang <cong.wang@xxxxxxxxxxxxx>
> > ---
> > 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.
