net: ipv4: report multicast group user count

RTM_GETMULTICAST has been part of the rtnetlink ABI for a long time
and already reports IPv4 multicast group membership through
IFA_MULTICAST and IFA_CACHEINFO. It does not report how many consumers
hold each membership, so userspace still has to parse /proc/net/igmp to
get the Users column.

Add IFA_MC_USERS as a u32 attribute carrying ip_mc_list::users in
RTM_GETMULTICAST replies and entry-lifecycle notifications.

This gives iproute2 enough information to migrate the IPv4 part of
"ip maddr show" from procfs parsing to rtnetlink.

Signed-off-by: Yuyang Huang <sigefriedhyy@gmail.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260630110207.37841-2-sigefriedhyy@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/netlink/specs/rt-addr.yaml

@@ -123,6 +123,9 @@ attribute-sets:
       -
         name: proto
         type: u8
+      -
+        name: mc-users
+        type: u32
 
 
 operations:
@@ -176,6 +179,7 @@ operations:
           value: 58
           attributes: &mcaddr-attrs
             - multicast
+            - mc-users
             - cacheinfo
       dump:
         request:

include/uapi/linux/if_addr.h

@@ -36,6 +36,7 @@ enum {
 	IFA_RT_PRIORITY,	/* u32, priority/metric for prefix route */
 	IFA_TARGET_NETNSID,
 	IFA_PROTO,		/* u8, address protocol */
+	IFA_MC_USERS,		/* u32, multicast group users */
 	__IFA_MAX,
 };
 

net/ipv4/igmp.c

@@ -1473,6 +1473,7 @@ int inet_fill_ifmcaddr(struct sk_buff *skb, struct net_device *dev,
 	ci.ifa_valid = INFINITY_LIFE_TIME;
 
 	if (nla_put_in_addr(skb, IFA_MULTICAST, im->multiaddr) < 0 ||
+	    nla_put_u32(skb, IFA_MC_USERS, READ_ONCE(im->users)) < 0 ||
 	    nla_put(skb, IFA_CACHEINFO, sizeof(ci), &ci) < 0) {
 		nlmsg_cancel(skb, nlh);
 		return -EMSGSIZE;
@@ -1494,6 +1495,7 @@ static void inet_ifmcaddr_notify(struct net_device *dev,
 
 	skb = nlmsg_new(NLMSG_ALIGN(sizeof(struct ifaddrmsg)) +
 			nla_total_size(sizeof(__be32)) +
+			nla_total_size(sizeof(u32)) +
 			nla_total_size(sizeof(struct ifa_cacheinfo)),
 			GFP_KERNEL);
 	if (!skb)