[PATCH net-next v2] ipv4: igmp: report IFA_TARGET_NETNSID in multicast address dumps

[Wenbo Sun]

Commit eb4e17a1d915 ("netlink: support dumping IPv4 multicast addresses")
extended RTM_GETMULTICAST to dump IPv4 multicast addresses by reusing
inet_valid_dump_ifaddr_req(), which already accepts IFA_TARGET_NETNSID
and switches to the target network namespace. However,
inet_fill_ifmcaddr() does not include IFA_TARGET_NETNSID in the dump
replies yet.

In contrast, IPv4 unicast dumps (inet_fill_ifaddr()), IPv6 multicast
dumps (inet6_fill_ifmcaddr()), and AF_PACKET multicast dumps
(dev_mc_fill_addr()) all report IFA_TARGET_NETNSID when a netns id is
specified. Adding the same behavior to IPv4 multicast dumps aligns
reporting across address families and helps userspace consistently
identify target-netns replies.

Emit IFA_TARGET_NETNSID in inet_fill_ifmcaddr() when args->netnsid >= 0,
and initialize fillargs.netnsid = -1 in inet_ifmcaddr_notify() so that
multicast join/leave notifications do not include IFA_TARGET_NETNSID or
exceed their preallocated skb size.

This alignment was identified with the assistance of an LLM-based code
review tool, confirmed in a User-Mode Linux (UML) environment where AF_INET
RTM_GETMULTICAST dumps with IFA_TARGET_NETNSID omitted the attribute while
AF_INET6 included it, and verified in UML that AF_INET multicast dumps
now report IFA_TARGET_NETNSID, regular dumps without netnsid continue to
work, and multicast join/leave notifications succeed without warning.

Signed-off-by: Wenbo Sun <sunwenbo@xxxxxxxxxxxxxxxxxxxx>
---
Changes in v2:
- Target net-next as an enhancement/alignment change and drop the Fixes
  tag, per Hangbin Liu's review.
- Soften and clarify the commit message to reflect feature alignment with
  inet_fill_ifaddr(), inet6_fill_ifmcaddr(), and dev_mc_fill_addr().
- Link to v1: https://lore.kernel.org/r/20261111082222.327963-1-sunwenbo@xxxxxxxxxxxxxxxxxxxx

 net/ipv4/igmp.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/ipv4/igmp.c b/net/ipv4/igmp.c
index 144fca158adc..831d003677ba 100644
--- a/net/ipv4/igmp.c
+++ b/net/ipv4/igmp.c
@@ -1517,6 +1517,12 @@ int inet_fill_ifmcaddr(struct sk_buff *skb, struct net_device *dev,
 	ifm->ifa_scope = RT_SCOPE_UNIVERSE;
 	ifm->ifa_index = dev->ifindex;
 
+	if (args->netnsid >= 0 &&
+	    nla_put_s32(skb, IFA_TARGET_NETNSID, args->netnsid)) {
+		nlmsg_cancel(skb, nlh);
+		return -EMSGSIZE;
+	}
+
 	ci.cstamp = (READ_ONCE(im->mca_cstamp) - INITIAL_JIFFIES) * 100UL / HZ;
 	ci.tstamp = ci.cstamp;
 	ci.ifa_prefered = INFINITY_LIFE_TIME;
@@ -1538,6 +1544,7 @@ static void inet_ifmcaddr_notify(struct net_device *dev,
 {
 	struct inet_fill_args fillargs = {
 		.event = event,
+		.netnsid = -1,
 	};
 	struct net *net = dev_net(dev);
 	struct sk_buff *skb;
-- 
2.33.0