[PATCH net-next 0/3] net: report multicast group user count

[PATCH net-next 0/3] net: report multicast group user count

[Yuyang Huang]

RTM_GETMULTICAST reports IPv4 and IPv6 multicast group membership, but
does not include the per-group user count. Userspace therefore still has
to parse /proc/net/igmp and /proc/net/igmp6 to obtain the Users column.
In particular, this prevents iproute2 from moving "ip maddr show"
entirely from procfs to rtnetlink.

Add IFA_MC_USERS to carry the user count in RTM_GETMULTICAST dumps and
RTM_NEWMULTICAST / RTM_DELMULTICAST notifications for both address
families. Update the rt-addr YNL specification and extend the rtnetlink
selftest to verify that two joins increase the reported count by two.

Yuyang Huang (3):
  net: ipv4: report multicast group user count
  net: ipv6: report multicast group user count
  selftests: net: check multicast group user count

 Documentation/netlink/specs/rt-addr.yaml |   4 +
 include/uapi/linux/if_addr.h             |   1 +
 net/ipv4/igmp.c                          |   2 +
 net/ipv6/addrconf.c                      |   1 +
 net/ipv6/mcast.c                         |   1 +
 tools/testing/selftests/net/rtnetlink.py | 101 ++++++++++++++++++++---
 6 files changed, 99 insertions(+), 11 deletions(-)

-- 
2.43.0

[PATCH net-next 1/3] net: ipv4: report multicast group user count

[Yuyang Huang]

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>
---
 Documentation/netlink/specs/rt-addr.yaml | 4 ++++
 include/uapi/linux/if_addr.h             | 1 +
 net/ipv4/igmp.c                          | 2 ++
 3 files changed, 7 insertions(+)

diff --git a/Documentation/netlink/specs/rt-addr.yaml b/Documentation/netlink/specs/rt-addr.yaml
index 163a106c41bb..0ecbd24c890c 100644
--- a/Documentation/netlink/specs/rt-addr.yaml
+++ b/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:
diff --git a/include/uapi/linux/if_addr.h b/include/uapi/linux/if_addr.h
index aa7958b4e41d..7fb630b7fe31 100644
--- a/include/uapi/linux/if_addr.h
+++ b/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,
 };
 
diff --git a/net/ipv4/igmp.c b/net/ipv4/igmp.c
index b6337a47c141..116ce7cec80e 100644
--- a/net/ipv4/igmp.c
+++ b/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)
-- 
2.43.0

[PATCH net-next 2/3] net: ipv6: report multicast group user count

[Yuyang Huang]

The previous patch added IFA_MC_USERS and emits it for IPv4 multicast
groups. Add the same snapshot attribute to IPv6 RTM_GETMULTICAST
replies and entry-lifecycle notifications, carrying
ifmcaddr6::mca_users.

This makes the multicast rtnetlink ABI symmetric across IPv4 and IPv6
and gives userspace the same user count that /proc/net/igmp6 exposes.

Signed-off-by: Yuyang Huang <sigefriedhyy@gmail.com>
---
 net/ipv6/addrconf.c | 1 +
 net/ipv6/mcast.c    | 1 +
 2 files changed, 2 insertions(+)

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index cbe681de3818..f1fe9ede1edb 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -5264,6 +5264,7 @@ int inet6_fill_ifmcaddr(struct sk_buff *skb,
 
 	put_ifaddrmsg(nlh, 128, IFA_F_PERMANENT, scope, ifindex);
 	if (nla_put_in6_addr(skb, IFA_MULTICAST, &ifmca->mca_addr) < 0 ||
+	    nla_put_u32(skb, IFA_MC_USERS, READ_ONCE(ifmca->mca_users)) < 0 ||
 	    put_cacheinfo(skb, ifmca->mca_cstamp, READ_ONCE(ifmca->mca_tstamp),
 			  INFINITY_LIFE_TIME, INFINITY_LIFE_TIME) < 0) {
 		nlmsg_cancel(skb, nlh);
diff --git a/net/ipv6/mcast.c b/net/ipv6/mcast.c
index 04b811b3be97..774f4c72a6fa 100644
--- a/net/ipv6/mcast.c
+++ b/net/ipv6/mcast.c
@@ -908,6 +908,7 @@ static void inet6_ifmcaddr_notify(struct net_device *dev,
 
 	skb = nlmsg_new(NLMSG_ALIGN(sizeof(struct ifaddrmsg)) +
 			nla_total_size(sizeof(struct in6_addr)) +
+			nla_total_size(sizeof(u32)) +
 			nla_total_size(sizeof(struct ifa_cacheinfo)),
 			GFP_KERNEL);
 	if (!skb)
-- 
2.43.0

[PATCH net-next 3/3] selftests: net: check multicast group user count

[Yuyang Huang]

Extend the RTM_GETMULTICAST dump test to verify IFA_MC_USERS for both
IPv4 and IPv6 multicast groups.

Run each protocol test in a fresh network namespace to avoid changing
host-network state or racing with unrelated multicast users. Join a
fixed multicast group twice using separate sockets and check that the
reported user count increases by two.

Signed-off-by: Yuyang Huang <sigefriedhyy@gmail.com>
---
 tools/testing/selftests/net/rtnetlink.py | 101 ++++++++++++++++++++---
 1 file changed, 90 insertions(+), 11 deletions(-)

diff --git a/tools/testing/selftests/net/rtnetlink.py b/tools/testing/selftests/net/rtnetlink.py
index 3622413d793d..0c67c7c00d84 100755
--- a/tools/testing/selftests/net/rtnetlink.py
+++ b/tools/testing/selftests/net/rtnetlink.py
@@ -2,27 +2,106 @@
 # SPDX-License-Identifier: GPL-2.0
 
 import socket
+import struct
 import time
-from lib.py import bkg, ip, ksft_exit, ksft_run, ksft_ge, ksft_true, KsftSkipEx
+from lib.py import bkg, ip, ksft_exit, ksft_run, ksft_eq, ksft_ge, ksft_true, KsftSkipEx
 from lib.py import CmdExitFailure, NetNS, NetNSEnter, RtnlAddrFamily
 
 IPV4_ALL_HOSTS_MULTICAST = b'\xe0\x00\x00\x01'
+IPV4_TEST_MULTICAST = b'\xef\x01\x01\x01'
+IPV6_TEST_MULTICAST = bytes.fromhex('ff020000000000000000000000000123')
+
+
+def _users_for(rtnl: RtnlAddrFamily, family: int, grp: bytes, ifindex: int):
+    """Return mc-users for grp on ifindex, or 0 if absent."""
+
+    addrs = rtnl.getmulticast({"ifa-family": family}, dump=True)
+    matches = [addr for addr in addrs
+               if addr['multicast'] == grp and addr['ifa-index'] == ifindex]
+    if not matches:
+        return 0
+    if 'mc-users' not in matches[0]:
+        return None
+
+    return matches[0]['mc-users']
+
 
 def dump_mcaddr_check() -> None:
     """
-    Verify that at least one interface has the IPv4 all-hosts multicast address.
-    At least the loopback interface should have this address.
+    Verify IPv4 multicast addresses and their user counts in RTM_GETMULTICAST.
+    """
+
+    with NetNS() as ns:
+        with NetNSEnter(str(ns)):
+            ip("link set lo up")
+            rtnl = RtnlAddrFamily()
+            lo_idx = socket.if_nametoindex('lo')
+            addresses = rtnl.getmulticast({"ifa-family": socket.AF_INET}, dump=True)
+
+            all_host_multicasts = [
+                addr for addr in addresses
+                if addr['multicast'] == IPV4_ALL_HOSTS_MULTICAST
+            ]
+
+            ksft_ge(len(all_host_multicasts), 1,
+                    "No interface found with the IPv4 all-hosts multicast address")
+
+            mreq = IPV4_TEST_MULTICAST + socket.inet_aton('127.0.0.1')
+            before = _users_for(rtnl, socket.AF_INET, IPV4_TEST_MULTICAST, lo_idx)
+            if before is None:
+                raise KsftSkipEx("kernel does not expose IFA_MC_USERS")
+
+            s1 = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+            s2 = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+            try:
+                s1.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, mreq)
+                s2.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, mreq)
+
+                after_join = _users_for(rtnl, socket.AF_INET,
+                                        IPV4_TEST_MULTICAST, lo_idx)
+                if after_join is None:
+                    raise KsftSkipEx("kernel does not expose IFA_MC_USERS")
+                ksft_eq(after_join - before, 2,
+                        f"users delta != 2 after two joins "
+                        f"(before={before}, after={after_join})")
+            finally:
+                s1.close()
+                s2.close()
+
+
+def dump_mcaddr6_check() -> None:
+    """
+    Verify IPv6 multicast addresses and their user counts in RTM_GETMULTICAST.
     """
 
-    rtnl = RtnlAddrFamily()
-    addresses = rtnl.getmulticast({"ifa-family": socket.AF_INET}, dump=True)
+    with NetNS() as ns:
+        with NetNSEnter(str(ns)):
+            ip("link set lo up")
+            rtnl = RtnlAddrFamily()
+            lo_idx = socket.if_nametoindex('lo')
+            before = _users_for(rtnl, socket.AF_INET6,
+                                IPV6_TEST_MULTICAST, lo_idx)
+            if before is None:
+                raise KsftSkipEx("kernel does not expose IFA_MC_USERS for IPv6")
+
+            mreq = IPV6_TEST_MULTICAST + struct.pack('=I', lo_idx)
+            s1 = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM)
+            s2 = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM)
+            try:
+                s1.setsockopt(socket.IPPROTO_IPV6, socket.IPV6_JOIN_GROUP, mreq)
+                s2.setsockopt(socket.IPPROTO_IPV6, socket.IPV6_JOIN_GROUP, mreq)
 
-    all_host_multicasts = [
-        addr for addr in addresses if addr['multicast'] == IPV4_ALL_HOSTS_MULTICAST
-    ]
+                after_join = _users_for(rtnl, socket.AF_INET6,
+                                        IPV6_TEST_MULTICAST, lo_idx)
+                if after_join is None:
+                    raise KsftSkipEx("kernel does not expose IFA_MC_USERS for IPv6")
+                ksft_eq(after_join - before, 2,
+                        f"IPv6 users delta != 2 after two joins "
+                        f"(before={before}, after={after_join})")
+            finally:
+                s1.close()
+                s2.close()
 
-    ksft_ge(len(all_host_multicasts), 1,
-            "No interface found with the IPv4 all-hosts multicast address")
 
 def ipv4_devconf_notify() -> None:
     """
@@ -56,7 +135,7 @@ def ipv4_devconf_notify() -> None:
               f"No 'forwarding on' notificiation found for interface {ifname}")
 
 def main() -> None:
-    ksft_run([dump_mcaddr_check, ipv4_devconf_notify])
+    ksft_run([dump_mcaddr_check, dump_mcaddr6_check, ipv4_devconf_notify])
     ksft_exit()
 
 if __name__ == "__main__":
-- 
2.43.0

Re: [PATCH net-next 1/3] net: ipv4: report multicast group user count

[Vadim Fedorenko]

On 30/06/2026 12:02, Yuyang Huang wrote:
> 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>
> ---
>   Documentation/netlink/specs/rt-addr.yaml | 4 ++++
>   include/uapi/linux/if_addr.h             | 1 +
>   net/ipv4/igmp.c                          | 2 ++
>   3 files changed, 7 insertions(+)
> 

Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>

Re: [PATCH net-next 2/3] net: ipv6: report multicast group user count

[Vadim Fedorenko]

On 30/06/2026 12:02, Yuyang Huang wrote:
> The previous patch added IFA_MC_USERS and emits it for IPv4 multicast
> groups. Add the same snapshot attribute to IPv6 RTM_GETMULTICAST
> replies and entry-lifecycle notifications, carrying
> ifmcaddr6::mca_users.
> 
> This makes the multicast rtnetlink ABI symmetric across IPv4 and IPv6
> and gives userspace the same user count that /proc/net/igmp6 exposes.
> 
> Signed-off-by: Yuyang Huang <sigefriedhyy@gmail.com>

Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>

Re: [PATCH net-next 3/3] selftests: net: check multicast group user count

[Vadim Fedorenko]

On 30/06/2026 12:02, Yuyang Huang wrote:
> Extend the RTM_GETMULTICAST dump test to verify IFA_MC_USERS for both
> IPv4 and IPv6 multicast groups.
> 
> Run each protocol test in a fresh network namespace to avoid changing
> host-network state or racing with unrelated multicast users. Join a
> fixed multicast group twice using separate sockets and check that the
> reported user count increases by two.
> 
> Signed-off-by: Yuyang Huang <sigefriedhyy@gmail.com>
> ---
>   tools/testing/selftests/net/rtnetlink.py | 101 ++++++++++++++++++++---
>   1 file changed, 90 insertions(+), 11 deletions(-)
> 

Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>