Re: kernel module to list sockets with their multicast group joins and respective processes

sockets that join different groups receive messages from the respective 
other group (if they are only bound to the wildcard address). Obviously 
this is handled differently in Linux for IPv4, where the socket matching 

for incoming message is done solely on the 4-tuple of addresses and 

ports, 

and IPv6, where the explicit socket joins are taken into account [1,2]. 

        These are not different in v4 vs v6. Group membership is 
per-interface,
delivery depends on the binding (for both v4 and v6), and this is the same
as BSD behavior since IP multicasting was invented.
        There are two levels of checks, and I think you're confusing them.
inet6_mc_check() is checking that somebody (*anybody*) has joined the
group on the receiving interface, before delivering the packet to IP.
Delivery to a particular socket depends on source filter settings, if
they are in use, and bindings (only). The socket has a list of joined
addresses to track source filters and to check for correct join/leave
(a socket can't leave a group it hasn't joined), but that list does not
determine delivery of a multicast packet to the socket when source 
filtering
is not in use.
 +-DLS