tcp: add a drop_reason pointer to tcp_check_req()

We want to add new drop reasons for packets dropped in 3WHS in the
following patches.

tcp_rcv_state_process() has to set reason to TCP_FASTOPEN,
because tcp_check_req() will conditionally overwrite the drop_reason.

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/20250301201424.2046477-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

include/net/tcp.h

@@ -392,7 +392,7 @@ enum tcp_tw_status tcp_timewait_state_process(struct inet_timewait_sock *tw,
 					      u32 *tw_isn);
 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 			   struct request_sock *req, bool fastopen,
-			   bool *lost_race);
+			   bool *lost_race, enum skb_drop_reason *drop_reason);
 enum skb_drop_reason tcp_child_process(struct sock *parent, struct sock *child,
 				       struct sk_buff *skb);
 void tcp_enter_loss(struct sock *sk);

net/ipv4/tcp_input.c

@@ -6812,10 +6812,9 @@ tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
 		WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&
 		    sk->sk_state != TCP_FIN_WAIT1);
 
-		if (!tcp_check_req(sk, skb, req, true, &req_stolen)) {
-			SKB_DR_SET(reason, TCP_FASTOPEN);
+		SKB_DR_SET(reason, TCP_FASTOPEN);
+		if (!tcp_check_req(sk, skb, req, true, &req_stolen, &reason))
 			goto discard;
-		}
 	}
 
 	if (!th->ack && !th->rst && !th->syn) {

net/ipv4/tcp_ipv4.c

@@ -2265,7 +2265,8 @@ int tcp_v4_rcv(struct sk_buff *skb)
 			th = (const struct tcphdr *)skb->data;
 			iph = ip_hdr(skb);
 			tcp_v4_fill_cb(skb, iph, th);
-			nsk = tcp_check_req(sk, skb, req, false, &req_stolen);
+			nsk = tcp_check_req(sk, skb, req, false, &req_stolen,
+					    &drop_reason);
 		} else {
 			drop_reason = SKB_DROP_REASON_SOCKET_FILTER;
 		}

net/ipv4/tcp_minisocks.c

@@ -657,7 +657,8 @@ EXPORT_SYMBOL(tcp_create_openreq_child);
 
 struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 			   struct request_sock *req,
-			   bool fastopen, bool *req_stolen)
+			   bool fastopen, bool *req_stolen,
+			   enum skb_drop_reason *drop_reason)
 {
 	struct tcp_options_received tmp_opt;
 	struct sock *child;

net/ipv6/tcp_ipv6.c

@@ -1828,7 +1828,8 @@ INDIRECT_CALLABLE_SCOPE int tcp_v6_rcv(struct sk_buff *skb)
 			th = (const struct tcphdr *)skb->data;
 			hdr = ipv6_hdr(skb);
 			tcp_v6_fill_cb(skb, hdr, th);
-			nsk = tcp_check_req(sk, skb, req, false, &req_stolen);
+			nsk = tcp_check_req(sk, skb, req, false, &req_stolen,
+					    &drop_reason);
 		} else {
 			drop_reason = SKB_DROP_REASON_SOCKET_FILTER;
 		}