[PATCH net-next v4 0/4] rtnetlink: dump link-layer multicast addresses

[Yuyang Huang <sigefriedhyy@gmail.com>]

"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 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.

Changes in v4:
- Reset the resume offset when the device the dump stopped at is gone
- Use a tracked netns reference (put_net_track)
- Say ifa-family must be set in the spec doc, drop the AF_UNSPEC remark
- Close the netlink socket and guard the checks in the selftest
- Drop the Fixes tag

Changes in v3:
- Report the static bit as a new IFA_F_GLOBAL flag in IFA_FLAGS
  instead of IFA_F_PERMANENT
- Support IFA_TARGET_NETNSID and test it
- Describe global_use accurately, it is also set by dev_mc_add_excl()
- Fix the target-netnsid type in the rt-addr spec, as its own patch

Changes in v2:
- Always validate the request header, not only with strict checking
- Use a single "with" statement in the selftest (ruff)

Yuyang Huang (4):
  netlink: specs: rt-addr: fix the type of target-netnsid
  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 |  17 ++-
 include/uapi/linux/if_addr.h             |   1 +
 net/core/rtnetlink.c                     | 174 +++++++++++++++++++++++
 tools/testing/selftests/net/rtnetlink.py |  75 +++++++++-
 4 files changed, 261 insertions(+), 6 deletions(-)

-- 
2.43.0