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 <[email protected]>
---
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
