x/net/ipv4: PacketConn.MulticastInterface is implemented incorrectly on Linux

[tamird]

Per the man page:

       IP_MULTICAST_IF (since Linux 1.2)
              Set the local device for a multicast socket.  The argument for
              setsockopt(2) is an ip_mreqn or (since Linux 3.5) ip_mreq
              structure similar to IP_ADD_MEMBERSHIP, or an in_addr struc‐
              ture.  (The kernel determines which structure is being passed
              based on the size passed in optlen.)  For getsockopt(2), the
              argument is an in_addr structure.

However, the implementation uses the wrong type for retrieval:

https://github.com/golang/net/blob/a7d1128ccaa05167332a625f69773af1a0c6b39c/ipv4/dgramopt.go#L41

https://github.com/golang/net/blob/a7d1128ccaa05167332a625f69773af1a0c6b39c/ipv4/sys_linux.go#L27

https://github.com/golang/net/blob/a7d1128ccaa05167332a625f69773af1a0c6b39c/ipv4/sockopt_posix.go#L17

Oddly, the default case in getMulticastInterface would do the right thing.

This looks to have been broken in golang/net@353547e

[gopherbot]

Change https://golang.org/cl/260680 mentions this issue: ipv4: Correctly MulticastInterface on Linux

[networkimprov]

cc @odeke-em

[dmitshur]

CC @mikioh, @ianlancetaylor per owners.

[randomizedcoder]

A related observation, is that Linux has "ip_mreq_source":

           struct ip_mreq_source {
               struct in_addr imr_multiaddr;  /* IP multicast group
                                                 address */
               struct in_addr imr_interface;  /* IP address of local
                                                 interface */
               struct in_addr imr_sourceaddr; /* IP address of
                                                 multicast source */
           };

https://man7.org/linux/man-pages/man7/ip.7.html
https://github.com/torvalds/linux/blob/9b6de136b5f0158c60844f85286a593cb70fb364/include/uapi/linux/in.h#L189

Go doesn't seem to have the equivalent of ip_mreq_source
https://cs.opensource.google/go/go/+/refs/tags/go1.21.4:src/syscall/ztypes_linux_amd64.go;l=216