Re: [PATCH net-next v2 1/3] rtnetlink: add AF_PACKET multicast dumps

[Nicolas Dichtel <nicolas.dichtel@6wind.com>]

Le 09/09/2026 à 03:33, Yuyang Huang a écrit :
> RTM_GETMULTICAST dumps IPv4 and IPv6 multicast group memberships, but
> the device multicast list (dev->mc) is only available through
> /proc/net/dev_mcast, so "ip maddr show" still has to parse procfs for
> its link-layer entries.
> 
> Handle RTM_GETMULTICAST dumps with ifa_family set to AF_PACKET and
> report every entry of dev->mc in the existing ifaddrmsg format:
> 
>   - IFA_MULTICAST carries the raw link-layer address
>   - IFA_MC_USERS carries the entry reference count
>   - IFA_F_PERMANENT marks entries added with SIOCADDMULTI
>     (netdev_hw_addr::global_use, "static" in "ip maddr")
The global flag is also set for addresses added via dev_mc_add_excl(), ie by
some drivers.

>   - ifa_scope is RT_SCOPE_LINK
> 
> This covers every column of /proc/net/dev_mcast. AF_PACKET is the
> family iproute2 already uses for link-layer addresses ("ip -0"), and
> AF_UNSPEC keeps its "all families" meaning from RTM_GETADDR.
> 
> The default FDB dump also walks dev->mc, but only for Ethernet devices
> without an ndo_fdb_dump of their own, so bridge, vxlan or macvlan
> devices never show their multicast filter there, and it has no users
> count or SIOCADDMULTI bit. Extending it would change "bridge fdb show"
> output and add NDA_* attributes, while this dump needs no new uAPI.
> 
> There are no legacy users of AF_PACKET requests, so they are always
> validated: prefixlen, flags and scope must be zero, no attributes are
> accepted, and a non-zero ifa_index restricts the dump to that device.
> The dump runs under RCU and netif_addr_lock_bh() and does not need
> RTNL.
> 
> Signed-off-by: Yuyang Huang <sigefriedhyy@gmail.com>
> ---
>  net/core/rtnetlink.c | 129 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 129 insertions(+)
> 
> diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
> index 81c5a6104dea..b2febdb6915e 100644
> --- a/net/core/rtnetlink.c
> +++ b/net/core/rtnetlink.c
> @@ -4566,6 +4566,133 @@ static int rtnl_dump_all(struct sk_buff *skb, struct netlink_callback *cb)
>  	return skb->len ? : ret;
>  }
>  
> +static int rtnl_fill_mcaddr(struct sk_buff *skb, const struct net_device *dev,
> +			    const struct netdev_hw_addr *ha, u32 portid,
> +			    u32 seq, unsigned int flags)
> +{
> +	struct ifaddrmsg *ifm;
> +	struct nlmsghdr *nlh;
> +
> +	nlh = nlmsg_put(skb, portid, seq, RTM_GETMULTICAST, sizeof(*ifm),
> +			flags);
> +	if (!nlh)
> +		return -EMSGSIZE;
> +
> +	ifm = nlmsg_data(nlh);
> +	ifm->ifa_family = AF_PACKET;
> +	ifm->ifa_prefixlen = 0;
> +	ifm->ifa_flags = ha->global_use ? IFA_F_PERMANENT : 0;
I wonder if adding a new attribute IFA_F_GLOBAL would not make the API more
understandable.

> +	ifm->ifa_scope = RT_SCOPE_LINK;
> +	ifm->ifa_index = dev->ifindex;
> +
> +	if (nla_put(skb, IFA_MULTICAST, dev->addr_len, ha->addr) ||
> +	    nla_put_u32(skb, IFA_MC_USERS, ha->refcount)) {
> +		nlmsg_cancel(skb, nlh);
> +		return -EMSGSIZE;
> +	}
> +
> +	nlmsg_end(skb, nlh);
> +	return 0;
> +}
> +
> +static int rtnl_dump_mcaddr_dev(struct net_device *dev, struct sk_buff *skb,
> +				struct netlink_callback *cb, int *s_addr_idx,
> +				unsigned int flags)
> +{
> +	struct netdev_hw_addr *ha;
> +	int addr_idx = 0;
> +	int err = 0;
> +
> +	netif_addr_lock_bh(dev);
> +	netdev_for_each_mc_addr(ha, dev) {
> +		if (addr_idx < *s_addr_idx) {
> +			addr_idx++;
> +			continue;
> +		}
> +		err = rtnl_fill_mcaddr(skb, dev, ha, NETLINK_CB(cb->skb).portid,
> +				       cb->nlh->nlmsg_seq, flags);
> +		if (err < 0)
> +			break;
> +		addr_idx++;
> +	}
> +	netif_addr_unlock_bh(dev);
> +
> +	*s_addr_idx = err < 0 ? addr_idx : 0;
> +
> +	return err;
> +}
> +
> +static int rtnl_valid_dump_mcaddr_req(const struct nlmsghdr *nlh,
> +				      struct netlink_ext_ack *extack,
> +				      int *pifindex)
> +{
> +	struct ifaddrmsg *ifm;
> +
> +	ifm = nlmsg_payload(nlh, sizeof(*ifm));
> +	if (!ifm) {
> +		NL_SET_ERR_MSG(extack,
> +			       "Invalid header for multicast dump request");
> +		return -EINVAL;
> +	}
> +
> +	if (ifm->ifa_prefixlen || ifm->ifa_flags || ifm->ifa_scope) {
> +		NL_SET_ERR_MSG(extack,
> +			       "Invalid values in multicast dump header");
> +		return -EINVAL;
> +	}
> +
> +	if (nlmsg_attrlen(nlh, sizeof(*ifm))) {
> +		NL_SET_ERR_MSG(extack,
> +			       "Invalid data after multicast dump header");
> +		return -EINVAL;
> +	}
> +
> +	*pifindex = ifm->ifa_index;
> +
> +	return 0;
> +}
> +
> +static int rtnl_dump_mcaddr(struct sk_buff *skb, struct netlink_callback *cb)
> +{
> +	struct net *net = sock_net(skb->sk);
For consistency with ipv4/ipv6, it would be nice to handle IFA_TARGET_NETNSID.

> +	unsigned int flags = NLM_F_MULTI;
> +	struct {
> +		unsigned long ifindex;
> +		int addr_idx;
> +	} *ctx = (void *)cb->ctx;
> +	struct net_device *dev;
> +	int ifindex;
> +	int err;
> +
> +	err = rtnl_valid_dump_mcaddr_req(cb->nlh, cb->extack, &ifindex);
> +	if (err < 0)
> +		return err;
> +
> +	rcu_read_lock();
> +
> +	if (ifindex) {
> +		cb->answer_flags |= NLM_F_DUMP_FILTERED;
> +		flags |= NLM_F_DUMP_FILTERED;
> +		dev = dev_get_by_index_rcu(net, ifindex);
> +		if (!dev) {
> +			err = -ENODEV;
> +			goto out;
> +		}
> +		err = rtnl_dump_mcaddr_dev(dev, skb, cb, &ctx->addr_idx, flags);
> +		goto out;
> +	}
> +
> +	for_each_netdev_dump(net, dev, ctx->ifindex) {
> +		err = rtnl_dump_mcaddr_dev(dev, skb, cb, &ctx->addr_idx,
> +					   flags);
> +		if (err < 0)
> +			break;
> +	}
> +out:
> +	rcu_read_unlock();
> +	return err;
> +}
> +
>  struct sk_buff *rtmsg_ifinfo_build_skb(int type, struct net_device *dev,
>  				       unsigned int change,
>  				       u32 event, gfp_t flags, int *new_nsid,
> @@ -7251,6 +7378,8 @@ static const struct rtnl_msg_handler rtnetlink_rtnl_msg_handlers[] __initconst =
>  	{.msgtype = RTM_SETSTATS, .doit = rtnl_stats_set},
>  	{.msgtype = RTM_NEWLINKPROP, .doit = rtnl_newlinkprop},
>  	{.msgtype = RTM_DELLINKPROP, .doit = rtnl_dellinkprop},
> +	{.protocol = PF_PACKET, .msgtype = RTM_GETMULTICAST,
> +	 .dumpit = rtnl_dump_mcaddr, .flags = RTNL_FLAG_DUMP_UNLOCKED},
>  	{.protocol = PF_BRIDGE, .msgtype = RTM_GETLINK,
>  	 .dumpit = rtnl_bridge_getlink},
>  	{.protocol = PF_BRIDGE, .msgtype = RTM_DELLINK,