tcp: add four drop reasons to tcp_check_req()

JIRA: https://issues.redhat.com/browse/RHEL-88890
Upstream Status: linux.git
Conflicts:\
- Removed parts linked to TSEcr as c9s is missing upstream commit
  3ba0752 ("tcp: be less liberal in TSEcr received while in
  SYN_RECV state").
- Context difference as upstream commit 124c4c3 ("tcp: add
  TCP_RFC7323_PAWS_ACK drop reason") was not backported in c9s yet.

commit a11a791
Author: Eric Dumazet <edumazet@google.com>
Date:   Sat Mar 1 20:14:20 2025 +0000

    tcp: add four drop reasons to tcp_check_req()

    Use two existing drop reasons in tcp_check_req():

    - TCP_RFC7323_PAWS

    - TCP_OVERWINDOW

    Add two new ones:

    - TCP_RFC7323_TSECR (corresponds to LINUX_MIB_TSECRREJECTED)

    - TCP_LISTEN_OVERFLOW (when a listener accept queue is full)

    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
    Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
    Link: https://patch.msgid.link/20250301201424.2046477-3-edumazet@google.com
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Antoine Tenart <atenart@redhat.com>

Author: atenart

include/net/dropreason-core.h

@@ -34,6 +34,8 @@
 	FN(TCP_OFOMERGE)		\
 	FN(TCP_RFC7323_PAWS)		\
 	FN(TCP_RFC7323_PAWS_ACK)	\
+	FN(TCP_RFC7323_TSECR)		\
+	FN(TCP_LISTEN_OVERFLOW)		\
 	FN(TCP_OLD_SEQUENCE)		\
 	FN(TCP_INVALID_SEQUENCE)	\
 	FN(TCP_INVALID_ACK_SEQUENCE)	\
@@ -239,6 +241,13 @@ enum skb_drop_reason {
 	 * Corresponds to LINUX_MIB_PAWS_OLD_ACK.
 	 */
 	SKB_DROP_REASON_TCP_RFC7323_PAWS_ACK,
+	/**
+	 * @SKB_DROP_REASON_TCP_RFC7323_TSECR: PAWS check, invalid TSEcr.
+	 * Corresponds to LINUX_MIB_TSECRREJECTED.
+	 */
+	SKB_DROP_REASON_TCP_RFC7323_TSECR,
+	/** @SKB_DROP_REASON_TCP_LISTEN_OVERFLOW: listener queue full. */
+	SKB_DROP_REASON_TCP_LISTEN_OVERFLOW,
 	/** @SKB_DROP_REASON_TCP_OLD_SEQUENCE: Old SEQ field (duplicate packet) */
 	SKB_DROP_REASON_TCP_OLD_SEQUENCE,
 	/** @SKB_DROP_REASON_TCP_INVALID_SEQUENCE: Not acceptable SEQ field */

net/ipv4/tcp_minisocks.c

@@ -721,8 +721,12 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 					  LINUX_MIB_TCPACKSKIPPEDSYNRECV,
 					  &tcp_rsk(req)->last_oow_ack_time))
 			req->rsk_ops->send_ack(sk, skb, req);
-		if (paws_reject)
+		if (paws_reject) {
+			SKB_DR_SET(*drop_reason, TCP_RFC7323_PAWS);
 			NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
+		} else {
+			SKB_DR_SET(*drop_reason, TCP_OVERWINDOW);
+		}
 		return NULL;
 	}
 
@@ -791,6 +795,7 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 	return inet_csk_complete_hashdance(sk, child, req, own_req);
 
 listen_overflow:
+	SKB_DR_SET(*drop_reason, TCP_LISTEN_OVERFLOW);
 	if (sk != req->rsk_listener)
 		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPMIGRATEREQFAILURE);