ipv6: Remove IPV6_ADDRFORM.

Recently, IPV6_ADDRFORM has received many AI-driven bug reports.
Fixing them properly would needlessly churn the fast paths in TCP
and UDP.

IPV6_ADDRFORM was initially introduced in RFC 2133 in 1997,
but only two years later, it was removed from RFC 2553 in 1999.

In 2026, modern applications natively support dual-stack sockets;
notably, systemd's socket activation does not use IPV6_ADDRFORM.

Also, getsockopt(IPV6_ADDRFORM) can be replaced with SO_DOMAIN.

Let's remove IPV6_ADDRFORM.

Later, we can remove sk->sk_prot_creator and revert commit
c26c192c3d ("udp: properly deal with xfrm encap and ADDRFORM").

Reported-by: Daehyeon Ko <4ncienth@gmail.com>
Closes: https://lore.kernel.org/netdev/20260902010408.1057857-1-4ncienth@gmail.com/
Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
Closes: https://lore.kernel.org/netdev/20260824033331.1084971-1-imv4bel@gmail.com/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260904033543.2635540-3-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/net.h

@@ -166,7 +166,7 @@ struct socket {
 
 	struct file		*file;
 	struct sock		*sk;
-	const struct proto_ops	*ops; /* Might change with IPV6_ADDRFORM or MPTCP. */
+	const struct proto_ops	*ops; /* Might change with MPTCP. */
 
 	struct socket_wq	wq;
 };

net/core/sock.c

@@ -770,7 +770,7 @@ bool sk_mc_loop(const struct sock *sk)
 		return false;
 	if (!sk)
 		return true;
-	/* IPV6_ADDRFORM can change sk->sk_family under us. */
+
 	switch (READ_ONCE(sk->sk_family)) {
 	case AF_INET:
 		return inet_test_bit(MC_LOOP, sk);
@@ -4010,7 +4010,6 @@ int sock_common_getsockopt(struct socket *sock, int level, int optname,
 {
 	struct sock *sk = sock->sk;
 
-	/* IPV6_ADDRFORM can change sk->sk_prot under us. */
 	return READ_ONCE(sk->sk_prot)->getsockopt(sk, level, optname, optval, optlen);
 }
 EXPORT_SYMBOL(sock_common_getsockopt);
@@ -4032,7 +4031,6 @@ int sock_common_setsockopt(struct socket *sock, int level, int optname,
 {
 	struct sock *sk = sock->sk;
 
-	/* IPV6_ADDRFORM can change sk->sk_prot under us. */
 	return READ_ONCE(sk->sk_prot)->setsockopt(sk, level, optname, optval, optlen);
 }
 EXPORT_SYMBOL(sock_common_setsockopt);

net/ipv4/af_inet.c

@@ -582,7 +582,6 @@ int inet_dgram_connect(struct socket *sock, struct sockaddr_unsized *uaddr,
 	if (addr_len < sizeof(uaddr->sa_family))
 		return -EINVAL;
 
-	/* IPV6_ADDRFORM can change sk->sk_prot under us. */
 	prot = READ_ONCE(sk->sk_prot);
 
 	if (uaddr->sa_family == AF_UNSPEC)
@@ -789,7 +788,6 @@ int inet_accept(struct socket *sock, struct socket *newsock,
 {
 	struct sock *sk1 = sock->sk, *sk2;
 
-	/* IPV6_ADDRFORM can change sk->sk_prot under us. */
 	arg->err = -EINVAL;
 	sk2 = READ_ONCE(sk1->sk_prot)->accept(sk1, arg);
 	if (!sk2)
@@ -875,7 +873,6 @@ void inet_splice_eof(struct socket *sock)
 	if (unlikely(inet_send_prepare(sk)))
 		return;
 
-	/* IPV6_ADDRFORM can change sk->sk_prot under us. */
 	prot = READ_ONCE(sk->sk_prot);
 	if (prot->splice_eof)
 		prot->splice_eof(sock);

net/ipv6/af_inet6.c

@@ -429,7 +429,6 @@ int inet6_bind_sk(struct sock *sk, struct sockaddr_unsized *uaddr, int addr_len)
 	const struct proto *prot;
 	int err = 0;
 
-	/* IPV6_ADDRFORM can change sk->sk_prot under us. */
 	prot = READ_ONCE(sk->sk_prot);
 	/* If the socket has its own bind function then use it. */
 	if (prot->bind)
@@ -567,7 +566,6 @@ int inet6_ioctl(struct socket *sock, unsigned int cmd, unsigned long arg)
 	case SIOCSIFDSTADDR:
 		return addrconf_set_dstaddr(net, argp);
 	default:
-		/* IPV6_ADDRFORM can change sk->sk_prot under us. */
 		prot = READ_ONCE(sk->sk_prot);
 		if (!prot->ioctl)
 			return -ENOIOCTLCMD;
@@ -636,7 +634,6 @@ int inet6_sendmsg(struct socket *sock, struct msghdr *msg, size_t size)
 	if (unlikely(inet_send_prepare(sk)))
 		return -EAGAIN;
 
-	/* IPV6_ADDRFORM can change sk->sk_prot under us. */
 	prot = READ_ONCE(sk->sk_prot);
 	return INDIRECT_CALL_2(prot->sendmsg, tcp_sendmsg, udpv6_sendmsg,
 			       sk, msg, size);
@@ -651,7 +648,6 @@ int inet6_recvmsg(struct socket *sock, struct msghdr *msg, size_t size,
 	if (likely(!(flags & MSG_ERRQUEUE)))
 		sock_rps_record_flow(sk);
 
-	/* IPV6_ADDRFORM can change sk->sk_prot under us. */
 	prot = READ_ONCE(sk->sk_prot);
 	return INDIRECT_CALL_2(prot->recvmsg, tcp_recvmsg, udpv6_recvmsg,
 			       sk, msg, size, flags);

net/ipv6/ipv6_sockglue.c

@@ -547,86 +547,7 @@ int do_ipv6_setsockopt(struct sock *sk, int level, int optname,
 
 	sockopt_lock_sock(sk);
 
-	/* Another thread has converted the socket into IPv4 with
-	 * IPV6_ADDRFORM concurrently.
-	 */
-	if (unlikely(sk->sk_family != AF_INET6))
-		goto unlock;
-
 	switch (optname) {
-
-	case IPV6_ADDRFORM:
-		if (optlen < sizeof(int))
-			goto e_inval;
-		if (val == PF_INET) {
-			if (sk->sk_type == SOCK_RAW)
-				break;
-
-			if (sk->sk_protocol == IPPROTO_UDP) {
-				if (udp_sk(sk)->pending == AF_INET6) {
-					retv = -EBUSY;
-					break;
-				}
-			} else if (sk->sk_protocol == IPPROTO_TCP) {
-				if (sk->sk_prot != &tcpv6_prot) {
-					retv = -EBUSY;
-					break;
-				}
-			} else {
-				break;
-			}
-
-			if (sk->sk_state != TCP_ESTABLISHED) {
-				retv = -ENOTCONN;
-				break;
-			}
-
-			if (ipv6_only_sock(sk) ||
-			    !ipv6_addr_v4mapped(&sk->sk_v6_daddr)) {
-				retv = -EADDRNOTAVAIL;
-				break;
-			}
-
-			__ipv6_sock_mc_close(sk);
-			__ipv6_sock_ac_close(sk);
-
-			if (sk->sk_protocol == IPPROTO_TCP) {
-				struct inet_connection_sock *icsk = inet_csk(sk);
-
-				sock_prot_inuse_add(net, sk->sk_prot, -1);
-				sock_prot_inuse_add(net, &tcp_prot, 1);
-
-				/* Paired with READ_ONCE(sk->sk_prot) in inet6_stream_ops */
-				WRITE_ONCE(sk->sk_prot, &tcp_prot);
-				/* Paired with READ_ONCE() in tcp_(get|set)sockopt() */
-				WRITE_ONCE(icsk->icsk_af_ops, &ipv4_specific);
-				WRITE_ONCE(sk->sk_socket->ops, &inet_stream_ops);
-				WRITE_ONCE(sk->sk_family, PF_INET);
-				tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
-			} else {
-				sock_prot_inuse_add(net, sk->sk_prot, -1);
-				sock_prot_inuse_add(net, &udp_prot, 1);
-
-				/* Paired with READ_ONCE(sk->sk_prot) in inet6_dgram_ops */
-				WRITE_ONCE(sk->sk_prot, &udp_prot);
-				WRITE_ONCE(sk->sk_socket->ops, &inet_dgram_ops);
-				WRITE_ONCE(sk->sk_family, PF_INET);
-			}
-
-			/* Disable all options not to allocate memory anymore,
-			 * but there is still a race.  See the lockless path
-			 * in udpv6_sendmsg() and ipv6_local_rxpmtu().
-			 */
-			np->rxopt.all = 0;
-
-			inet6_cleanup_sock(sk);
-
-			module_put(THIS_MODULE);
-			retv = 0;
-			break;
-		}
-		goto e_inval;
-
 	case IPV6_V6ONLY:
 		if (optlen < sizeof(int) ||
 		    inet_sk(sk)->inet_num)
@@ -1088,14 +1009,6 @@ int do_ipv6_getsockopt(struct sock *sk, int level, int optname,
 	if (copy_from_sockptr(&len, optlen, sizeof(int)))
 		return -EFAULT;
 	switch (optname) {
-	case IPV6_ADDRFORM:
-		if (sk->sk_protocol != IPPROTO_UDP &&
-		    sk->sk_protocol != IPPROTO_TCP)
-			return -ENOPROTOOPT;
-		if (sk->sk_state != TCP_ESTABLISHED)
-			return -ENOTCONN;
-		val = sk->sk_family;
-		break;
 	case MCAST_MSFILTER:
 		if (in_compat_syscall())
 			return compat_ipv6_get_msfilter(sk, optval, optlen, len);