tcp/dccp: warn user for preferred ip_local_port_range

After commit 07f4c900 ("tcp/dccp: try to not exhaust
ip_local_port_range in connect()") it is advised to have an even number
of ports described in /proc/sys/net/ipv4/ip_local_port_range

This means start/end values should have a different parity.

Let's warn sysadmins of this, so that they can update their settings
if they want to.
Suggested-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netns/ipv4.h

@@ -19,6 +19,7 @@ struct sock;
 struct local_ports {
 	seqlock_t	lock;
 	int		range[2];
+	bool		warned;
 };
 
 struct ping_group_range {

net/ipv4/sysctl_net_ipv4.c

@@ -45,7 +45,13 @@ static int ip_ping_group_range_max[] = { GID_T_MAX, GID_T_MAX };
 /* Update system visible IP port range */
 static void set_local_port_range(struct net *net, int range[2])
 {
+	bool same_parity = !((range[0] ^ range[1]) & 1);
+
 	write_seqlock(&net->ipv4.ip_local_ports.lock);
+	if (same_parity && !net->ipv4.ip_local_ports.warned) {
+		net->ipv4.ip_local_ports.warned = true;
+		pr_err_ratelimited("ip_local_port_range: prefer different parity for start/end values.\n");
+	}
 	net->ipv4.ip_local_ports.range[0] = range[0];
 	net->ipv4.ip_local_ports.range[1] = range[1];
 	write_sequnlock(&net->ipv4.ip_local_ports.lock);