Observed with two independent servers in the same process: fd1 = socket(AF_INET, SOCK_DGRAM, 0); setsockopt(fd1, SOL_SOCKET, SO_REUSEPORT, ...); bind(fd1, port 0); /* got 40000 */ fd2 = socket(AF_INET, SOCK_DGRAM, 0); setsockopt(fd2, SOL_SOCKET, SO_REUSEPORT, ...); bind(fd2, port 0); /* got 40000 as well */ Both sockets end up on the same port and join the same reuseport group, so each of them takes part of the other's datagrams. TCP does not do this. udp_lib_lport_inuse() keeps the reuseport rule when it scans for a free port: a socket with the same uid and SO_REUSEPORT set is not a conflict, so its port is never marked in the bitmap and the scan can hand it out again. That rule only makes sense when the user asks for a specific port. TCP fixed the same thing in commit 0643ee4fd1b7 ("inet: Fix get port to handle zero port number with soreuseport set"): reuseport is only honoured for an explicit port, not during a port scan. Do the same for UDP. Mark compatible reuseport sockets in the bitmap during a scan and only skip them when checking a specific port. Using bitmap to tell the two modes apart is not explicit, but udp_lib_lport_inuse() already does that for the port match itself. Note that this still collides when both sockets also have SO_REUSEADDR set: udp_lib_lport_inuse() skips such a pair before it reaches the reuseport check, so the port is never marked either. That is not a common setup and is left as is here. Fixes: ba418fa357a7 ("soreuseport: UDP/IPv4 implementation") Signed-off-by: Jiayuan Chen --- net/ipv4/udp.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-) diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c index b3887c42adfd..61cb1e3f5d93 100644 --- a/net/ipv4/udp.c +++ b/net/ipv4/udp.c @@ -147,11 +147,10 @@ static int udp_lib_lport_inuse(struct net *net, __u16 num, (!sk2->sk_bound_dev_if || !sk->sk_bound_dev_if || sk2->sk_bound_dev_if == sk->sk_bound_dev_if) && inet_rcv_saddr_equal(sk, sk2, true)) { - if (sk2->sk_reuseport && sk->sk_reuseport && + if (!bitmap && sk2->sk_reuseport && sk->sk_reuseport && !rcu_access_pointer(sk->sk_reuseport_cb) && uid_eq(uid, sk_uid(sk2))) { - if (!bitmap) - return 0; + return 0; } else { if (!bitmap) return 1; -- 2.43.0