ipv6: icmp: remove obsolete code in icmpv6_xrlim_allow()

Following part was needed before the blamed commit, because
inet_getpeer_v6() second argument was the prefix.

	/* Give more bandwidth to wider prefixes. */
	if (rt->rt6i_dst.plen < 128)
		tmo >>= ((128 - rt->rt6i_dst.plen)>>5);

Now inet_getpeer_v6() retrieves hosts, we need to remove
@tmo adjustement or wider prefixes likes /24 allow 8x
more ICMP to be sent for a given ratelimit.

As we had this issue for a while, this patch changes net.ipv6.icmp.ratelimit
default value from 1000ms to 100ms to avoid potential regressions.

Also add a READ_ONCE() when reading net->ipv6.sysctl.icmpv6_time.

Fixes: fd0273d ("ipv6: Remove external dependency on rt6i_dst and rt6i_src")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Cc: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20260216142832.3834174-4-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

Documentation/networking/ip-sysctl.rst

@@ -3234,12 +3234,13 @@ enhanced_dad - BOOLEAN
 ===========
 
 ratelimit - INTEGER
-	Limit the maximal rates for sending ICMPv6 messages.
+	Limit the maximal rates for sending ICMPv6 messages to a particular
+	peer.
 
 	0 to disable any limiting,
-	otherwise the minimal space between responses in milliseconds.
+	otherwise the space between responses in milliseconds.
 
-	Default: 1000
+	Default: 100
 
 ratemask - list of comma separated ranges
 	For ICMPv6 message types matching the ranges in the ratemask, limit

net/ipv6/af_inet6.c

@@ -952,7 +952,7 @@ static int __net_init inet6_net_init(struct net *net)
 	int err = 0;
 
 	net->ipv6.sysctl.bindv6only = 0;
-	net->ipv6.sysctl.icmpv6_time = 1*HZ;
+	net->ipv6.sysctl.icmpv6_time = HZ / 10;
 	net->ipv6.sysctl.icmpv6_echo_ignore_all = 0;
 	net->ipv6.sysctl.icmpv6_echo_ignore_multicast = 0;
 	net->ipv6.sysctl.icmpv6_echo_ignore_anycast = 0;

net/ipv6/icmp.c

@@ -217,14 +217,9 @@ static bool icmpv6_xrlim_allow(struct sock *sk, u8 type,
 	} else if (dev && (dev->flags & IFF_LOOPBACK)) {
 		res = true;
 	} else {
-		struct rt6_info *rt = dst_rt6_info(dst);
-		int tmo = net->ipv6.sysctl.icmpv6_time;
+		int tmo = READ_ONCE(net->ipv6.sysctl.icmpv6_time);
 		struct inet_peer *peer;
 
-		/* Give more bandwidth to wider prefixes. */
-		if (rt->rt6i_dst.plen < 128)
-			tmo >>= ((128 - rt->rt6i_dst.plen)>>5);
-
 		peer = inet_getpeer_v6(net->ipv6.peers, &fl6->daddr);
 		res = inet_peer_xrlim_allow(peer, tmo);
 	}