RE: [Patch bpf] selftests/bpf: Retry for EAGAIN in udp_redir_to_connected()

[John Fastabend <john.fastabend@gmail.com>]

Cong Wang wrote:
> From: Cong Wang <cong.wang@bytedance.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@bytedance.com>
> Cc: John Fastabend <john.fastabend@gmail.com>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: Jakub Sitnicki <jakub@cloudflare.com>
> Cc: Lorenz Bauer <lmb@cloudflare.com>
> Signed-off-by: Cong Wang <cong.wang@bytedance.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.

> +	}
>  	if (n == 0)
>  		FAIL("%s: incomplete read", log_prefix);
>  
> -- 
> 2.25.1
>