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);
--
1.9.1
