[PATCH net-next 0/3] rtnetlink: dump link-layer multicast addresses

[Yuyang Huang]

"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.

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.

Patch 1 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
IFA_F_PERMANENT is set for entries added with SIOCADDMULTI, which is the
"static" column /proc/net/dev_mcast has and "ip maddr" prints. A
non-zero ifa_index in a strict request limits the dump to one device.

Patch 2 updates the rt-addr spec and patch 3 adds a selftest that
checks the filter, the user count and the permanent flag on a dummy
device.

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.

Yuyang Huang (3):
  rtnetlink: add AF_PACKET multicast dumps
  netlink: specs: rt-addr: document AF_PACKET multicast dumps
  selftests: net: test AF_PACKET multicast dumps

 Documentation/netlink/specs/rt-addr.yaml |   7 +-
 net/core/rtnetlink.c                     | 132 +++++++++++++++++++++++
 tools/testing/selftests/net/rtnetlink.py |  58 +++++++++-
 3 files changed, 193 insertions(+), 4 deletions(-)

-- 
2.43.0