From 5fd0783b99d4af98f65cd58b56ec203d1d426104 Mon Sep 17 00:00:00 2001 From: Shardul Bankar Date: Thu, 17 Sep 2026 14:55:32 +0530 Subject: [PATCH] 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/ [1] Fixes: 644f9108f3a5 ("udp: Make rehash4 independent in udp_lib_rehash()") Assisted-by: LLM Signed-off-by: Shardul Bankar Reviewed-by: Kuniyuki Iwashima Link: https://patch.msgid.link/20260917-udp_hash4_fix_v1-v1-1-718891af0d7a@mpiricsoftware.com Signed-off-by: Paolo Abeni --- net/ipv4/udp.c | 21 +++++++++++++++------ 1 file changed, 15 insertions(+), 6 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. - */ - if (udp_hashed4(sk)) - return; - 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_sk(sk)->udp_lrpa_hash != hash) { + spin_lock_bh(&hslot->lock); + udp_rehash4(udptable, sk, hash); + spin_unlock_bh(&hslot->lock); + } + return; + } + hslot2 = udp_hashslot2(udptable, udp_sk(sk)->udp_portaddr_hash); hslot4 = udp_hashslot4(udptable, hash); udp_sk(sk)->udp_lrpa_hash = hash;