commit 518120f41aec30bdde39caf58818e1e5d2d6a4a4
Author: Marc Dionne <marc.dionne@auristor.com>
Date:   Fri Jan 15 16:02:32 2016 -0400

    soreuseport: Allow bind before setting SO_REUSEPORT
    
    If a socket is bound before the SO_REUSEPORT option is
    set, it will have no sk_reuseport_cb structure allocated
    and attached to it.  If there is then an attempt to bind
    a second socket to the same port, the first socket will
    be found as matching and reuseport_add_sock will attempt
    to use the NULL sk_reuseport_cb pointer, resulting in an
    oops.
    
    Allocate sk_reuseport_cb in setsockopt if not already set.
    Also, don't preclude reusing a port because the sk has
    sk_reuseport_cb set.
    
    Signed-off-by: Marc Dionne <marc.dionne@auristor.com>

diff --git a/net/core/sock.c b/net/core/sock.c
index 5127023..587de5b 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -724,6 +724,8 @@ int sock_setsockopt(struct socket *sock, int level, int optname,
 		break;
 	case SO_REUSEPORT:
 		sk->sk_reuseport = valbool;
+		if (!rcu_access_pointer(sk->sk_reuseport_cb))
+		    reuseport_alloc(sk);
 		break;
 	case SO_TYPE:
 	case SO_PROTOCOL:
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index dc45b53..13884b6 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -154,7 +154,6 @@ 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) &&
 		    (!sk2->sk_reuseport || !sk->sk_reuseport ||
-		     rcu_access_pointer(sk->sk_reuseport_cb) ||
 		     !uid_eq(uid, sock_i_uid(sk2))) &&
 		    saddr_comp(sk, sk2, true)) {
 			if (!bitmap)
@@ -190,7 +189,6 @@ static int udp_lib_lport_inuse2(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) &&
 		    (!sk2->sk_reuseport || !sk->sk_reuseport ||
-		     rcu_access_pointer(sk->sk_reuseport_cb) ||
 		     !uid_eq(uid, sock_i_uid(sk2))) &&
 		    saddr_comp(sk, sk2, true)) {
 			res = 1;