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

On Thu, Sep 17, 2026 at 2:26 AM Shardul Bankar
[off-list ref] wrote:

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]

This link looks random to me.

Fixes: 644f9108f3a5 ("udp: Make rehash4 independent in udp_lib_rehash()")
Assisted-by: LLM
Signed-off-by: Shardul Bankar <redacted>

The change itself looks good.

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>