tcp: populate inet->cork.fl.u.ip6 in tcp_v6_syn_recv_sock()

As explained in commit 85d05e2 ("ipv6: change inet6_sk_rebuild_header()
to use inet->cork.fl.u.ip6"):

TCP v6 spends a good amount of time rebuilding a fresh fl6 at each
transmit in inet6_csk_xmit()/inet6_csk_route_socket().

TCP v4 caches the information in inet->cork.fl.u.ip4 instead.

After this patch, passive TCP ipv6 flows have correctly initialized
inet->cork.fl.u.ip6 structure.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260206173426.1638518-7-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

include/net/inet6_connection_sock.h

@@ -18,7 +18,9 @@ struct sk_buff;
 struct sock;
 struct sockaddr;
 
-struct dst_entry *inet6_csk_route_req(const struct sock *sk, struct flowi6 *fl6,
+struct dst_entry *inet6_csk_route_req(const struct sock *sk,
+				      struct dst_entry *dst,
+				      struct flowi6 *fl6,
 				      const struct request_sock *req, u8 proto);
 
 int inet6_csk_xmit(struct sock *sk, struct sk_buff *skb, struct flowi *fl);

net/ipv6/inet6_connection_sock.c

@@ -25,14 +25,14 @@
 #include <net/sock_reuseport.h>
 
 struct dst_entry *inet6_csk_route_req(const struct sock *sk,
+				      struct dst_entry *dst,
 				      struct flowi6 *fl6,
 				      const struct request_sock *req,
 				      u8 proto)
 {
 	const struct inet_request_sock *ireq = inet_rsk(req);
 	const struct ipv6_pinfo *np = inet6_sk(sk);
 	struct in6_addr *final_p, final;
-	struct dst_entry *dst;
 
 	memset(fl6, 0, sizeof(*fl6));
 	fl6->flowi6_proto = proto;
@@ -48,10 +48,11 @@ struct dst_entry *inet6_csk_route_req(const struct sock *sk,
 	fl6->flowi6_uid = sk_uid(sk);
 	security_req_classify_flow(req, flowi6_to_flowi_common(fl6));
 
-	dst = ip6_dst_lookup_flow(sock_net(sk), sk, fl6, final_p);
-	if (IS_ERR(dst))
-		return NULL;
-
+	if (!dst) {
+		dst = ip6_dst_lookup_flow(sock_net(sk), sk, fl6, final_p);
+		if (IS_ERR(dst))
+			return NULL;
+	}
 	return dst;
 }
 

net/ipv6/tcp_ipv6.c

@@ -539,7 +539,7 @@ static int tcp_v6_send_synack(const struct sock *sk, struct dst_entry *dst,
 	u8 tclass;
 
 	/* First, grab a route. */
-	if (!dst && (dst = inet6_csk_route_req(sk, fl6, req,
+	if (!dst && (dst = inet6_csk_route_req(sk, NULL, fl6, req,
 					       IPPROTO_TCP)) == NULL)
 		goto done;
 
@@ -789,7 +789,7 @@ static struct dst_entry *tcp_v6_route_req(const struct sock *sk,
 	if (security_inet_conn_request(sk, skb, req))
 		return NULL;
 
-	return inet6_csk_route_req(sk, &fl->u.ip6, req, IPPROTO_TCP);
+	return inet6_csk_route_req(sk, NULL, &fl->u.ip6, req, IPPROTO_TCP);
 }
 
 struct request_sock_ops tcp6_request_sock_ops __read_mostly = {
@@ -1318,12 +1318,12 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
 					 struct request_sock *req_unhash,
 					 bool *own_req)
 {
-	struct inet_request_sock *ireq;
-	struct ipv6_pinfo *newnp;
 	const struct ipv6_pinfo *np = tcp_inet6_sk(sk);
+	struct inet_request_sock *ireq;
 	struct ipv6_txoptions *opt;
 	struct inet_sock *newinet;
 	bool found_dup_sk = false;
+	struct ipv6_pinfo *newnp;
 	struct tcp_sock *newtp;
 	struct sock *newsk;
 #ifdef CONFIG_TCP_MD5SIG
@@ -1392,11 +1392,9 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
 	if (sk_acceptq_is_full(sk))
 		goto exit_overflow;
 
-	if (!dst) {
-		dst = inet6_csk_route_req(sk, &fl6, req, IPPROTO_TCP);
-		if (!dst)
-			goto exit;
-	}
+	dst = inet6_csk_route_req(sk, dst, &fl6, req, IPPROTO_TCP);
+	if (!dst)
+		goto exit;
 
 	newsk = tcp_create_openreq_child(sk, req, skb);
 	if (!newsk)
@@ -1412,6 +1410,7 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
 	inet6_sk_rx_dst_set(newsk, skb);
 
 	newinet = inet_sk(newsk);
+	newinet->cork.fl.u.ip6 = fl6;
 	newinet->pinet6 = tcp_inet6_sk(newsk);
 	newinet->ipv6_fl_list = NULL;
 	newinet->inet_opt = NULL;