On Tue, 2016-05-03 at 23:54 +0200, Peter Wu wrote:
> When applications use listen() with a backlog of 0, the kernel would
> set the maximum connection request queue to zero. This causes false
> reports of SYN flooding (if tcp_syncookies is enabled) or packet drops
> otherwise.
>
> Prior kernels enforce a minimum size of 8, so do that now as well.
>
> Fixes: ef547f2ac16b ("tcp: remove max_qlen_log")
> Signed-off-by: Peter Wu <pe...@lekensteyn.nl>
> ---
> Hi,
>
> This patch fixes a regression from Linux 4.4. Use of "qemu-arm -g 1234"
> would trigger the following warning in dmesg:
>
> TCP: request_sock_TCP: Possible SYN flooding on port 1234. Sending
> cookies. Check SNMP counters.
>
> For some users the "tcp: remove max_qlen_log" change already broke
> applications[1]. While listen(3p) says that a backlog argument of 0 sets
> the length to an "implementation-defined minimum value", I doubt that
> "0" should be considered a valid value (as demonstrated in the above two
> real-world applications that worked fine before). It is a hint anyway.
>
> This patch was tested on top of Linux v4.5 and removes the warning which
> would otherwise be present (due to the inet_csk_reqsk_queue_is_full()
> check in tcp_conn_request).
>
> I also looked at modifying the backlog value in inet_listen, but that
> might have other unintended effects:
>
> - If TFO is enabled and tcp_fastopen==0x400, listen(fd, 0) currently
> disables TFO (also possible via setsockopt). Forcing a minimum breaks
> this path (unlikely to be a problem though since TFO users likely set
> a much higher backlog).
> - sk->sk_max_ack_backlog is also reported via tcp statistics and seems
> really to be the hint rather than the actual interpreted value.
>
> Kind regards,
> Peter
>
> [1]:
> https://lkml.kernel.org/r/cann89i+okfw896-n5ksndeikzuidr8yx1jc089hjnggfdq0...@mail.gmail.com
> ---
> include/net/inet_connection_sock.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/net/inet_connection_sock.h
> b/include/net/inet_connection_sock.h
> index 49dcad4..ca0fdbc 100644
> --- a/include/net/inet_connection_sock.h
> +++ b/include/net/inet_connection_sock.h
> @@ -296,7 +296,7 @@ static inline int inet_csk_reqsk_queue_young(const struct
> sock *sk)
>
> static inline int inet_csk_reqsk_queue_is_full(const struct sock *sk)
> {
> - return inet_csk_reqsk_queue_len(sk) >= sk->sk_max_ack_backlog;
> + return inet_csk_reqsk_queue_len(sk) >= max(8U, sk->sk_max_ack_backlog);
> }
>
> void inet_csk_reqsk_queue_drop(struct sock *sk, struct request_sock *req);
Well, I believe I already gave my opinion on this.
listen backlog is not a hint. This is a limit.
It is the limit of outstanding children in accept queue.
If backlog is 0, no child can be put in the accept queue.
It is therefore Working As Intented.
