netlink: add IPv6 anycast join/leave notifications

This change introduces a mechanism for notifying userspace
applications about changes to IPv6 anycast addresses via netlink. It
includes:

* Addition and deletion of IPv6 anycast addresses are reported using
  RTM_NEWANYCAST and RTM_DELANYCAST.
* A new netlink group (RTNLGRP_IPV6_ACADDR) for subscribing to these
  notifications.

This enables user space applications(e.g. ip monitor) to efficiently
track anycast addresses through netlink messages, improving metrics
collection and system monitoring. It also unlocks the potential for
advanced anycast management in user space, such as hardware offload
control and fine grained network control.

Cc: Maciej Żenczykowski <maze@google.com>
Cc: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Yuyang Huang <yuyanghuang@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20250107114355.1766086-1-yuyanghuang@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

2 files changed, 38 insertions, 3 deletions

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 4da409bc4577..c3729382be3b 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -5240,9 +5240,9 @@ int inet6_fill_ifmcaddr(struct sk_buff *skb,
 	return 0;
 }
 
-static int inet6_fill_ifacaddr(struct sk_buff *skb,
-			       const struct ifacaddr6 *ifaca,
-			       struct inet6_fill_args *args)
+int inet6_fill_ifacaddr(struct sk_buff *skb,
+			const struct ifacaddr6 *ifaca,
+			struct inet6_fill_args *args)
 {
 	struct net_device *dev = fib6_info_nh_dev(ifaca->aca_rt);
 	int ifindex = dev ? dev->ifindex : 1;
diff --git a/net/ipv6/anycast.c b/net/ipv6/anycast.c
index 562cace50ca9..21e01695b48c 100644
--- a/net/ipv6/anycast.c
+++ b/net/ipv6/anycast.c
@@ -278,6 +278,37 @@ static struct ifacaddr6 *aca_alloc(struct fib6_info *f6i,
 	return aca;
 }
 
+static void inet6_ifacaddr_notify(struct net_device *dev,
+				  const struct ifacaddr6 *ifaca, int event)
+{
+	struct inet6_fill_args fillargs = {
+		.event = event,
+		.netnsid = -1,
+	};
+	struct net *net = dev_net(dev);
+	struct sk_buff *skb;
+	int err = -ENOMEM;
+
+	skb = nlmsg_new(NLMSG_ALIGN(sizeof(struct ifaddrmsg)) +
+			nla_total_size(sizeof(struct in6_addr)) +
+			nla_total_size(sizeof(struct ifa_cacheinfo)),
+			GFP_KERNEL);
+	if (!skb)
+		goto error;
+
+	err = inet6_fill_ifacaddr(skb, ifaca, &fillargs);
+	if (err < 0) {
+		pr_err("Failed to fill in anycast addresses (err %d)\n", err);
+		nlmsg_free(skb);
+		goto error;
+	}
+
+	rtnl_notify(skb, net, 0, RTNLGRP_IPV6_ACADDR, NULL, GFP_KERNEL);
+	return;
+error:
+	rtnl_set_sk_err(net, RTNLGRP_IPV6_ACADDR, err);
+}
+
 /*
  *	device anycast group inc (add if not found)
  */
@@ -333,6 +364,8 @@ int __ipv6_dev_ac_inc(struct inet6_dev *idev, const struct in6_addr *addr)
 
 	addrconf_join_solict(idev->dev, &aca->aca_addr);
 
+	inet6_ifacaddr_notify(idev->dev, aca, RTM_NEWANYCAST);
+
 	aca_put(aca);
 	return 0;
 out:
@@ -375,6 +408,8 @@ int __ipv6_dev_ac_dec(struct inet6_dev *idev, const struct in6_addr *addr)
 
 	ip6_del_rt(dev_net(idev->dev), aca->aca_rt, false);
 
+	inet6_ifacaddr_notify(idev->dev, aca, RTM_DELANYCAST);
+
 	aca_put(aca);
 	return 0;
 }