Re: [PATCH net-next v5 0/4] rtnetlink: dump link-layer multicast addresses

[Jakub Kicinski <kuba@kernel.org>]

On Mon, 14 Sep 2026 10:45:35 +0900 Yuyang Huang wrote:
> "ip maddr show" prints three kinds of entries: link-layer, IPv4 and
> IPv6. The IPv4 and IPv6 ones can be read over netlink today: IPv6 has
> had RTM_GETMULTICAST for a long time and IPv4 got it in eb4e17a1d915
> ("netlink: support dumping IPv4 multicast addresses"), with IFA_MC_USERS
> added later so the user count no longer has to come from procfs.

You compare the situation to IP but for L2 the UC addresses
are not exposed via RTM_GETADDR. Are you going to add it?

> The link-layer list is the missing piece. dev->mc, the addresses
> programmed into the device filter, is only exported via
> /proc/net/dev_mcast, so iproute2 still carries a procfs parser just for
> that. This series closes the gap so that "ip maddr show" can be served
> from rtnetlink alone.

Is that the only justification? To avoid reading proc?
Are you planning to extend this interface with new features?

> Patch 1 fixes the type of target-netnsid in the rt-addr spec. Patch 2
> handles RTM_GETMULTICAST dumps with ifa_family set to AF_PACKET and
> walks dev->mc under netif_addr_lock_bh(), no RTNL. The reply reuses
> the ifaddrmsg format of the IPv4 and IPv6 dumps: IFA_MULTICAST carries
> the raw link-layer address, IFA_MC_USERS the reference count, and a new
> IFA_F_GLOBAL flag in IFA_FLAGS marks entries added explicitly, which is
> the "static" column /proc/net/dev_mcast has and "ip maddr" prints. A
> non-zero ifa_index limits the dump to one device and IFA_TARGET_NETNSID
> selects another netns, like the IPv4 and IPv6 dumps.
> 
> Patch 3 updates the rt-addr spec and patch 4 adds a selftest that
> checks the filter, the user count, the global flag and target-netnsid.
> 
> Nothing changes for other families. AF_PACKET dumps returned
> -EOPNOTSUPP before, so iproute2 can keep the procfs fallback for older
> kernels. I have the iproute2 side ready and will post it once this is
> in; with it, "ip maddr show" does not open /proc/net at all.