net: add AF_PACKET multicast dumps

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 next to
the dev->mc helpers in dev_addr_lists.c 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_GLOBAL in IFA_FLAGS reports netdev_hw_addr::global_use, set
    by dev_mc_add_global() (SIOCADDMULTI) and dev_mc_add_excl()
    ("bridge fdb add ... self"), i.e. entries added explicitly rather
    than by a protocol join. This is the static column of
    /proc/net/dev_mcast
  - 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").

The default FDB dump also walks dev->mc, but only for Ethernet devices
and only if the driver uses it, vxlan for example does not, and it has
no users count or global_use bit. Extending it would change "bridge fdb
show" output and add NDA_* attributes.

Requests are always validated, there are no legacy users: prefixlen,
flags and scope must be zero, ifa_index selects one device and
IFA_TARGET_NETNSID is the only attribute accepted. The dump runs under
RCU and netif_addr_lock_bh() without RTNL. cb->seq is set from the
dev->mc generation counter and dev_base_seq before each round and
checked after it, so an entry added or removed since the previous
round sets NLM_F_DUMP_INTR. A round without a message skips the check
and cb->seq is set once more after the round, so the NLMSG_DONE check
reports a change made during the last round or before an empty one.

Signed-off-by: Yuyang Huang <sigefriedhyy@gmail.com>
Reviewed-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://patch.msgid.link/20261012011707.3378-5-sigefriedhyy@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/uapi/linux/if_addr.h b/include/uapi/linux/if_addr.h
index 7fb630b7fe31..0a1ad9ebb47b 100644
--- a/include/uapi/linux/if_addr.h
+++ b/include/uapi/linux/if_addr.h
@@ -57,6 +57,7 @@ enum {
 #define IFA_F_NOPREFIXROUTE	0x200
 #define IFA_F_MCAUTOJOIN	0x400
 #define IFA_F_STABLE_PRIVACY	0x800
+#define IFA_F_GLOBAL		0x1000
 
 struct ifa_cacheinfo {
 	__u32	ifa_prefered;