ipv4: icmp: icmpv4_xrlim_allow() optimization if net.ipv4.icmp_ratelimit is zero

If net.ipv4.icmp_ratelimit is zero, we do not have to call
inet_getpeer_v4() and inet_peer_xrlim_allow().

Both can be very expensive under DDOS.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260216142832.3834174-5-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/ipv4/icmp.c

@@ -316,23 +316,29 @@ static bool icmpv4_xrlim_allow(struct net *net, struct rtable *rt,
 	struct dst_entry *dst = &rt->dst;
 	struct inet_peer *peer;
 	struct net_device *dev;
+	int peer_timeout;
 	bool rc = true;
 
 	if (!apply_ratelimit)
 		return true;
 
+	peer_timeout = READ_ONCE(net->ipv4.sysctl_icmp_ratelimit);
+	if (!peer_timeout)
+		goto out;
+
 	/* No rate limit on loopback */
 	rcu_read_lock();
 	dev = dst_dev_rcu(dst);
 	if (dev && (dev->flags & IFF_LOOPBACK))
-		goto out;
+		goto out_unlock;
 
 	peer = inet_getpeer_v4(net->ipv4.peers, fl4->daddr,
 			       l3mdev_master_ifindex_rcu(dev));
-	rc = inet_peer_xrlim_allow(peer,
-				   READ_ONCE(net->ipv4.sysctl_icmp_ratelimit));
-out:
+	rc = inet_peer_xrlim_allow(peer, peer_timeout);
+
+out_unlock:
 	rcu_read_unlock();
+out:
 	if (!rc)
 		__ICMP_INC_STATS(net, ICMP_MIB_RATELIMITHOST);
 	else