[PATCH net 1/2] udp: relocate a connected socket in the 4-tuple hash table on re-connect

A connected UDP socket that connects again to a different peer is not
re-filed in the 4-tuple hash table:

    sk binds to 127.0.0.1:21001
    sk connects to 127.0.0.2:20001	// filed under hash(sk, peer1)
    sk connects to 127.0.0.3:20002	// still filed under hash(sk, peer1)
    packet from 127.0.0.3:20002		// hash(sk, peer2) misses, so the
					// lookup falls back to scoring the
					// hash2 chain for this address
					// and port

udp_lib_hash4() returns early when the socket is already hashed, assuming
->rehash() relocates it. ->rehash() runs from __ip{4,6}_datagram_connect()
only while the receive address is unset, which a second connect never is:
the first connect assigns it, whether the socket was bound to a specific
address or to the wildcard. commit 644f9108f3a5 ("udp: Make rehash4
independent in udp_lib_rehash()") added that early return and named
connect(AF_UNSPEC) as the way around it. That workaround does not help a
socket with both SOCK_BINDADDR_LOCK and SOCK_BINDPORT_LOCK set, because
__udp_disconnect() skips ->rehash() for the first and ->unhash() for the
second.

Delivery is correct either way.

Relocate the socket when the hash it is filed under differs from the one
requested, which is what commit 78c91ae2c6de ("ipv4/udp: Add 4-tuple hash
for connected socket") did before the early return became unconditional. It
is done here under hslot->lock, which that version did not take, to match
udp_lib_rehash() and udp_lib_unhash(). hslot2 is unchanged, so hash4_cnt
needs no adjustment, as in udp_lib_rehash(). A first connect is unaffected,
and IPv6 shares the code.

With 500 sockets on the port, a re-connected socket measured 522,553 pps
without this change and 2,055,078 with it. The UDP side was noted as
remaining work in [1].

Link: https://lore.kernel.org/netdev/apnHqmYZQ4yzOP4N@v4bel/ (local) [1]
Fixes: 644f9108f3a5 ("udp: Make rehash4 independent in udp_lib_rehash()")
Assisted-by: LLM
Signed-off-by: Shardul Bankar <redacted>
---
 net/ipv4/udp.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index bb8cfc62cb00..0fa3cdbdcc21 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c

@@ -617,14 +617,23 @@ void udp_lib_hash4(struct sock *sk, u16 hash)
 	struct net *net = sock_net(sk);
 	struct udp_table *udptable;
 
-	/* Connected udp socket can re-connect to another remote address, which
-	 * will be handled by rehash. Thus no need to redo hash4 here.
+	udptable = net->ipv4.udp_table;
+	hslot = udp_hashslot(udptable, net, udp_sk(sk)->udp_port_hash);
+
+	/* A connected socket can re-connect to another address. rehash()
+	 * relocates it, but only runs when the local address changes, so a
+	 * socket bound to a specific address would stay filed under the
+	 * previous peer's hash. Move it here.
 	 */
-	if (udp_hashed4(sk))
+	if (udp_hashed4(sk)) {
+		if (udp_sk(sk)->udp_lrpa_hash != hash) {
+			spin_lock_bh(&hslot->lock);
+			udp_rehash4(udptable, sk, hash);
+			spin_unlock_bh(&hslot->lock);
+		}
 		return;
+	}
 
-	udptable = net->ipv4.udp_table;
-	hslot = udp_hashslot(udptable, net, udp_sk(sk)->udp_port_hash);
 	hslot2 = udp_hashslot2(udptable, udp_sk(sk)->udp_portaddr_hash);
 	hslot4 = udp_hashslot4(udptable, hash);
 	udp_sk(sk)->udp_lrpa_hash = hash;

-- 
2.34.1