udp: Fix wildcard bind conflict check when using hash2

When binding a udp_sock to a local address and port, UDP uses
two hashes (udptable->hash and udptable->hash2) for collision
detection. The current code switches to "hash2" when
hslot->count > 10.

"hash2" is keyed by local address and local port.
"hash" is keyed by local port only.

The issue can be shown in the following bind sequence (pseudo code):

bind(fd1,  "[fd00::1]:8888")
bind(fd2,  "[fd00::2]:8888")
bind(fd3,  "[fd00::3]:8888")
bind(fd4,  "[fd00::4]:8888")
bind(fd5,  "[fd00::5]:8888")
bind(fd6,  "[fd00::6]:8888")
bind(fd7,  "[fd00::7]:8888")
bind(fd8,  "[fd00::8]:8888")
bind(fd9,  "[fd00::9]:8888")
bind(fd10, "[fd00::10]:8888")

/* Correctly return -EADDRINUSE because "hash" is used
 * instead of "hash2". udp_lib_lport_inuse() detects the
 * conflict.
 */
bind(fail_fd, "[::]:8888")

/* After one more socket is bound to "[fd00::11]:8888",
 * hslot->count exceeds 10 and "hash2" is used instead.
 */
bind(fd11, "[fd00::11]:8888")
bind(fail_fd, "[::]:8888")      /* succeeds unexpectedly */

The same issue applies to the IPv4 wildcard address "0.0.0.0"
and the IPv4-mapped wildcard address "::ffff:0.0.0.0". For
example, if there are existing sockets bound to
"192.168.1.[1-11]:8888", then binding "0.0.0.0:8888" or
"[::ffff:0.0.0.0]:8888" can also miss the conflict when
hslot->count > 10.

TCP inet_csk_get_port() already has the correct check in
inet_use_bhash2_on_bind(). Rename it to
inet_use_hash2_on_bind() and move it to inet_hashtables.h
so udp.c can reuse it in this fix.

Fixes: 30fff9231f ("udp: bind() optimisation")
Reported-by: Andrew Onyshchuk <oandrew@meta.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260319181817.1901357-1-martin.lau@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/inet_hashtables.h

@@ -264,6 +264,20 @@ inet_bhashfn_portaddr(const struct inet_hashinfo *hinfo, const struct sock *sk,
 	return &hinfo->bhash2[hash & (hinfo->bhash_size - 1)];
 }
 
+static inline bool inet_use_hash2_on_bind(const struct sock *sk)
+{
+#if IS_ENABLED(CONFIG_IPV6)
+	if (sk->sk_family == AF_INET6) {
+		if (ipv6_addr_any(&sk->sk_v6_rcv_saddr))
+			return false;
+
+		if (!ipv6_addr_v4mapped(&sk->sk_v6_rcv_saddr))
+			return true;
+	}
+#endif
+	return sk->sk_rcv_saddr != htonl(INADDR_ANY);
+}
+
 struct inet_bind_hashbucket *
 inet_bhash2_addr_any_hashbucket(const struct sock *sk, const struct net *net, int port);
 

net/ipv4/inet_connection_sock.c

@@ -154,20 +154,6 @@ bool inet_sk_get_local_port_range(const struct sock *sk, int *low, int *high)
 }
 EXPORT_SYMBOL(inet_sk_get_local_port_range);
 
-static bool inet_use_bhash2_on_bind(const struct sock *sk)
-{
-#if IS_ENABLED(CONFIG_IPV6)
-	if (sk->sk_family == AF_INET6) {
-		if (ipv6_addr_any(&sk->sk_v6_rcv_saddr))
-			return false;
-
-		if (!ipv6_addr_v4mapped(&sk->sk_v6_rcv_saddr))
-			return true;
-	}
-#endif
-	return sk->sk_rcv_saddr != htonl(INADDR_ANY);
-}
-
 static bool inet_bind_conflict(const struct sock *sk, struct sock *sk2,
 			       kuid_t uid, bool relax,
 			       bool reuseport_cb_ok, bool reuseport_ok)
@@ -259,7 +245,7 @@ static int inet_csk_bind_conflict(const struct sock *sk,
 	 * checks separately because their spinlocks have to be acquired/released
 	 * independently of each other, to prevent possible deadlocks
 	 */
-	if (inet_use_bhash2_on_bind(sk))
+	if (inet_use_hash2_on_bind(sk))
 		return tb2 && inet_bhash2_conflict(sk, tb2, uid, relax,
 						   reuseport_cb_ok, reuseport_ok);
 
@@ -376,7 +362,7 @@ other_parity_scan:
 		head = &hinfo->bhash[inet_bhashfn(net, port,
 						  hinfo->bhash_size)];
 		spin_lock_bh(&head->lock);
-		if (inet_use_bhash2_on_bind(sk)) {
+		if (inet_use_hash2_on_bind(sk)) {
 			if (inet_bhash2_addr_any_conflict(sk, port, l3mdev, relax, false))
 				goto next_port;
 		}
@@ -562,7 +548,7 @@ int inet_csk_get_port(struct sock *sk, unsigned short snum)
 				check_bind_conflict = false;
 		}
 
-		if (check_bind_conflict && inet_use_bhash2_on_bind(sk)) {
+		if (check_bind_conflict && inet_use_hash2_on_bind(sk)) {
 			if (inet_bhash2_addr_any_conflict(sk, port, l3mdev, true, true))
 				goto fail_unlock;
 		}

net/ipv4/udp.c

@@ -287,7 +287,7 @@ int udp_lib_get_port(struct sock *sk, unsigned short snum,
 	} else {
 		hslot = udp_hashslot(udptable, net, snum);
 		spin_lock_bh(&hslot->lock);
-		if (hslot->count > 10) {
+		if (inet_use_hash2_on_bind(sk) && hslot->count > 10) {
 			int exist;
 			unsigned int slot2 = udp_sk(sk)->udp_portaddr_hash ^ snum;