ipv6: add icmpv6_error_anycast_as_unicast for ICMPv6

ICMPv6 error packets are not sent to the anycast destinations and this
prevents things like traceroute from working. So create a setting similar
to ECHO when dealing with Anycast sources (icmpv6_echo_ignore_anycast).

Signed-off-by: Mahesh Bandewar <[e-mail]>
Reviewed-by: David Ahern <[e-mail]>
Reviewed-by: Maciej Żenczykowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/networking/ip-sysctl.rst

···
 
 	Default: 0
 
+error_anycast_as_unicast - BOOLEAN
+	If set to 1, then the kernel will respond with ICMP Errors
+	resulting from requests sent to it over the IPv6 protocol destined
+	to anycast address essentially treating anycast as unicast.
+
+	Default: 0
+
 xfrm6_gc_thresh - INTEGER
 	(Obsolete since linux-4.14)
 	The threshold at which we will start garbage collecting for IPv6

include/net/netns/ipv6.h

···
 	u64 ioam6_id_wide;
 	bool skip_notify_on_dev_down;
 	u8 fib_notify_on_flag_change;
+	u8 icmpv6_error_anycast_as_unicast;
 };
 
 struct netns_ipv6 {

net/ipv6/af_inet6.c

···
 	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.sysctl.icmpv6_error_anycast_as_unicast = 0;
 
 	/* By default, rate limit error messages.
 	 * Except for pmtu discovery, it would break it.

net/ipv6/icmp.c

···
 
 	/*
 	 * We won't send icmp if the destination is known
-	 * anycast.
+	 * anycast unless we need to treat anycast as unicast.
 	 */
-	if (ipv6_anycast_destination(dst, &fl6->daddr)) {
+	if (!READ_ONCE(net->ipv6.sysctl.icmpv6_error_anycast_as_unicast) &&
+	    ipv6_anycast_destination(dst, &fl6->daddr)) {
 		net_dbg_ratelimited("icmp6_send: acast source\n");
 		dst_release(dst);
 		return ERR_PTR(-EINVAL);
···
 		.mode		= 0644,
 		.proc_handler = proc_do_large_bitmap,
 	},
+	{
+		.procname	= "error_anycast_as_unicast",
+		.data		= &init_net.ipv6.sysctl.icmpv6_error_anycast_as_unicast,
+		.maxlen		= sizeof(u8),
+		.mode		= 0644,
+		.proc_handler	= proc_dou8vec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_ONE,
+	},
 	{ },
 };
 
···
 		table[2].data = &net->ipv6.sysctl.icmpv6_echo_ignore_multicast;
 		table[3].data = &net->ipv6.sysctl.icmpv6_echo_ignore_anycast;
 		table[4].data = &net->ipv6.sysctl.icmpv6_ratemask_ptr;
+		table[5].data = &net->ipv6.sysctl.icmpv6_error_anycast_as_unicast;
 	}
 	return table;
 }