netlink: Use random autobind rover

Currently we use a global rover to select a port ID that is unique.
This used to work consistently when it was protected with a global
lock.  However as we're now lockless, the global rover can exhibit
pathological behaviour should multiple threads all stomp on it at
the same time.

Granted this will eventually resolve itself but the process is
suboptimal.

This patch replaces the global rover with a pseudorandom starting
point to avoid this issue.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netlink/af_netlink.c

@@ -1299,20 +1299,24 @@ static int netlink_autobind(struct socket *sock)
 	struct netlink_table *table = &nl_table[sk->sk_protocol];
 	s32 portid = task_tgid_vnr(current);
 	int err;
-	static s32 rover = -4097;
+	s32 rover = -4096;
+	bool ok;
 
 retry:
 	cond_resched();
 	rcu_read_lock();
-	if (__netlink_lookup(table, portid, net)) {
-		/* Bind collision, search negative portid values. */
-		portid = rover--;
-		if (rover > -4097)
-			rover = -4097;
+	ok = !__netlink_lookup(table, portid, net);
 	rcu_read_unlock();
+	if (!ok) {
+		/* Bind collision, search negative portid values. */
+		if (rover == -4096)
+			/* rover will be in range [S32_MIN, -4097] */
+			rover = S32_MIN + prandom_u32_max(-4096 - S32_MIN);
+		else if (rover >= -4096)
+			rover = -4097;
+		portid = rover--;
 		goto retry;
 	}
-	rcu_read_unlock();
 
 	err = netlink_insert(sk, portid);
 	if (err == -EADDRINUSE)