netdev - Re: [bpf-next] selftests/bpf: fix race in test_tcp

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Date:   Fri, 16 Aug 2019 09:37:46 -0700
From:   Petar Penkov <ppenkov.kernel@...il.com>
To:     Stanislav Fomichev <sdf@...ichev.me>
Cc:     Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        davem@...emloft.net, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Stanislav Fomichev <sdf@...gle.com>,
        Petar Penkov <ppenkov@...gle.com>
Subject: Re: [bpf-next] selftests/bpf: fix race in test_tcp_rtt test

On Fri, Aug 16, 2019 at 9:13 AM Stanislav Fomichev <sdf@...ichev.me> wrote:
>
> On 08/16, Petar Penkov wrote:
> > From: Petar Penkov <ppenkov@...gle.com>
> >
> > There is a race in this test between receiving the ACK for the
> > single-byte packet sent in the test, and reading the values from the
> > map.
> >
> > This patch fixes this by having the client wait until there are no more
> > unacknowledged packets.
> >
> > Before:
> > for i in {1..1000}; do ../net/in_netns.sh ./test_tcp_rtt; \
> > done | grep -c PASSED
> > < trimmed error messages >
> > 993
> >
> > After:
> > for i in {1..10000}; do ../net/in_netns.sh ./test_tcp_rtt; \
> > done | grep -c PASSED
> > 10000
> >
> > Fixes: b55873984dab ("selftests/bpf: test BPF_SOCK_OPS_RTT_CB")
> > Signed-off-by: Petar Penkov <ppenkov@...gle.com>
> > ---
> >  tools/testing/selftests/bpf/test_tcp_rtt.c | 31 ++++++++++++++++++++++
> >  1 file changed, 31 insertions(+)
> >
> > diff --git a/tools/testing/selftests/bpf/test_tcp_rtt.c b/tools/testing/selftests/bpf/test_tcp_rtt.c
> > index 90c3862f74a8..2b4754473956 100644
> > --- a/tools/testing/selftests/bpf/test_tcp_rtt.c
> > +++ b/tools/testing/selftests/bpf/test_tcp_rtt.c
> > @@ -6,6 +6,7 @@
> >  #include <sys/types.h>
> >  #include <sys/socket.h>
> >  #include <netinet/in.h>
> > +#include <netinet/tcp.h>
> >  #include <pthread.h>
> >
> >  #include <linux/filter.h>
> > @@ -34,6 +35,30 @@ static void send_byte(int fd)
> >               error(1, errno, "Failed to send single byte");
> >  }
> >
> > +static int wait_for_ack(int fd, int retries)
> > +{
> > +     struct tcp_info info;
> > +     socklen_t optlen;
> > +     int i, err;
> > +
> > +     for (i = 0; i < retries; i++) {
> > +             optlen = sizeof(info);
> > +             err = getsockopt(fd, SOL_TCP, TCP_INFO, &info, &optlen);
> > +             if (err < 0) {
> > +                     log_err("Failed to lookup TCP stats");
> > +                     return err;
> > +             }
> > +
> > +             if (info.tcpi_unacked == 0)
> > +                     return 0;
> > +
> > +             sleep(1);
> Isn't it too big of a hammer? Maybe usleep(10) here and do x100 retries
> instead?
>
I guess this is more consistent with the time we expect to wait for an
ACK in the test, will change and send out a v2 shortly. Thank you for
your quick review!
> > +     }
> > +
> > +     log_err("Did not receive ACK");
> > +     return -1;
> > +}
> > +
> >  static int verify_sk(int map_fd, int client_fd, const char *msg, __u32 invoked,
> >                    __u32 dsack_dups, __u32 delivered, __u32 delivered_ce,
> >                    __u32 icsk_retransmits)
> > @@ -149,6 +174,11 @@ static int run_test(int cgroup_fd, int server_fd)
> >                        /*icsk_retransmits=*/0);
> >
> >       send_byte(client_fd);
> > +     if (wait_for_ack(client_fd, 5) < 0) {
> > +             err = -1;
> > +             goto close_client_fd;
> > +     }
> > +
> >
> >       err += verify_sk(map_fd, client_fd, "first payload byte",
> >                        /*invoked=*/2,
> > @@ -157,6 +187,7 @@ static int run_test(int cgroup_fd, int server_fd)
> >                        /*delivered_ce=*/0,
> >                        /*icsk_retransmits=*/0);
> >
> > +close_client_fd:
> >       close(client_fd);
> >
> >  close_bpf_object:
> > --
> > 2.23.0.rc1.153.gdeed80330f-goog
> >