tcp: populate inet->cork.fl.u.ip6 in tcp_v6_connect()

Instead of using private @FL6 and @Final variables
use respectively inet->cork.fl.u.ip6 and np->final.

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, active 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/20260206173426.1638518-6-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/ipv6/tcp_ipv6.c

@@ -138,15 +138,15 @@ static int tcp_v6_connect(struct sock *sk, struct sockaddr_unsized *uaddr,
 {
 	struct sockaddr_in6 *usin = (struct sockaddr_in6 *) uaddr;
 	struct inet_connection_sock *icsk = inet_csk(sk);
-	struct in6_addr *saddr = NULL, *final_p, final;
 	struct inet_timewait_death_row *tcp_death_row;
 	struct ipv6_pinfo *np = tcp_inet6_sk(sk);
+	struct in6_addr *saddr = NULL, *final_p;
 	struct inet_sock *inet = inet_sk(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct net *net = sock_net(sk);
 	struct ipv6_txoptions *opt;
 	struct dst_entry *dst;
-	struct flowi6 fl6;
+	struct flowi6 *fl6;
 	int addr_type;
 	int err;
 
@@ -156,14 +156,15 @@ static int tcp_v6_connect(struct sock *sk, struct sockaddr_unsized *uaddr,
 	if (usin->sin6_family != AF_INET6)
 		return -EAFNOSUPPORT;
 
-	memset(&fl6, 0, sizeof(fl6));
+	fl6 = &inet_sk(sk)->cork.fl.u.ip6;
+	memset(fl6, 0, sizeof(*fl6));
 
 	if (inet6_test_bit(SNDFLOW, sk)) {
-		fl6.flowlabel = usin->sin6_flowinfo&IPV6_FLOWINFO_MASK;
-		IP6_ECN_flow_init(fl6.flowlabel);
-		if (fl6.flowlabel&IPV6_FLOWLABEL_MASK) {
+		fl6->flowlabel = usin->sin6_flowinfo & IPV6_FLOWINFO_MASK;
+		IP6_ECN_flow_init(fl6->flowlabel);
+		if (fl6->flowlabel & IPV6_FLOWLABEL_MASK) {
 			struct ip6_flowlabel *flowlabel;
-			flowlabel = fl6_sock_lookup(sk, fl6.flowlabel);
+			flowlabel = fl6_sock_lookup(sk, fl6->flowlabel);
 			if (IS_ERR(flowlabel))
 				return -EINVAL;
 			fl6_sock_release(flowlabel);
@@ -212,7 +213,7 @@ static int tcp_v6_connect(struct sock *sk, struct sockaddr_unsized *uaddr,
 	}
 
 	sk->sk_v6_daddr = usin->sin6_addr;
-	np->flow_label = fl6.flowlabel;
+	np->flow_label = fl6->flowlabel;
 
 	/*
 	 *	TCP over IPv4
@@ -260,24 +261,24 @@ static int tcp_v6_connect(struct sock *sk, struct sockaddr_unsized *uaddr,
 	if (!ipv6_addr_any(&sk->sk_v6_rcv_saddr))
 		saddr = &sk->sk_v6_rcv_saddr;
 
-	fl6.flowi6_proto = IPPROTO_TCP;
-	fl6.daddr = sk->sk_v6_daddr;
-	fl6.saddr = saddr ? *saddr : np->saddr;
-	fl6.flowlabel = ip6_make_flowinfo(np->tclass, np->flow_label);
-	fl6.flowi6_oif = sk->sk_bound_dev_if;
-	fl6.flowi6_mark = sk->sk_mark;
-	fl6.fl6_dport = usin->sin6_port;
-	fl6.fl6_sport = inet->inet_sport;
-	if (IS_ENABLED(CONFIG_IP_ROUTE_MULTIPATH) && !fl6.fl6_sport)
-		fl6.flowi6_flags = FLOWI_FLAG_ANY_SPORT;
-	fl6.flowi6_uid = sk_uid(sk);
+	fl6->flowi6_proto = IPPROTO_TCP;
+	fl6->daddr = sk->sk_v6_daddr;
+	fl6->saddr = saddr ? *saddr : np->saddr;
+	fl6->flowlabel = ip6_make_flowinfo(np->tclass, np->flow_label);
+	fl6->flowi6_oif = sk->sk_bound_dev_if;
+	fl6->flowi6_mark = sk->sk_mark;
+	fl6->fl6_dport = usin->sin6_port;
+	fl6->fl6_sport = inet->inet_sport;
+	if (IS_ENABLED(CONFIG_IP_ROUTE_MULTIPATH) && !fl6->fl6_sport)
+		fl6->flowi6_flags = FLOWI_FLAG_ANY_SPORT;
+	fl6->flowi6_uid = sk_uid(sk);
 
 	opt = rcu_dereference_protected(np->opt, lockdep_sock_is_held(sk));
-	final_p = fl6_update_dst(&fl6, opt, &final);
+	final_p = fl6_update_dst(fl6, opt, &np->final);
 
-	security_sk_classify_flow(sk, flowi6_to_flowi_common(&fl6));
+	security_sk_classify_flow(sk, flowi6_to_flowi_common(fl6));
 
-	dst = ip6_dst_lookup_flow(net, sk, &fl6, final_p);
+	dst = ip6_dst_lookup_flow(net, sk, fl6, final_p);
 	if (IS_ERR(dst)) {
 		err = PTR_ERR(dst);
 		goto failure;
@@ -287,7 +288,7 @@ static int tcp_v6_connect(struct sock *sk, struct sockaddr_unsized *uaddr,
 	tcp_death_row = &sock_net(sk)->ipv4.tcp_death_row;
 
 	if (!saddr) {
-		saddr = &fl6.saddr;
+		saddr = &fl6->saddr;
 
 		err = inet_bhash2_update_saddr(sk, saddr, AF_INET6);
 		if (err)