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tcp/dccp: allow a connection when sk_max_ack_backlog is zero

If the backlog of listen() is set to zero, sk_acceptq_is_full() allows
one connection to be made, but inet_csk_reqsk_queue_is_full() does not.
When the net.ipv4.tcp_syncookies is zero, inet_csk_reqsk_queue_is_full()
will cause an immediate drop before the sk_acceptq_is_full() check in
tcp_conn_request(), resulting in no connection can be made.

This patch tries to keep consistent with 64a146513f8f ("[NET]: Revert
incorrect accept queue backlog changes.").

Link: https://lore.kernel.org/netdev/20250102080258.53858-1-kuniyu@amazon.com/
Fixes: ef547f2ac16b ("tcp: remove max_qlen_log")
Signed-off-by: Zhongqiu Duan <dzq.aishenghu0@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250102171426.915276-1-dzq.aishenghu0@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

authored by

Zhongqiu Duan and committed by
Jakub Kicinski 3479c754 1e9b0e1c

+1 -1
+1 -1
include/net/inet_connection_sock.h
··· 282 282 283 283 static inline int inet_csk_reqsk_queue_is_full(const struct sock *sk) 284 284 { 285 - return inet_csk_reqsk_queue_len(sk) >= READ_ONCE(sk->sk_max_ack_backlog); 285 + return inet_csk_reqsk_queue_len(sk) > READ_ONCE(sk->sk_max_ack_backlog); 286 286 } 287 287 288 288 bool inet_csk_reqsk_queue_drop(struct sock *sk, struct request_sock *req);