sockets: implement MSG_TRUNC recv/recvmsg flag

[melifaro]

Summary

Implement Linux-variant of MSG_TRUNC flag used in recv(), recvfrom() and recvmsg().

Posix defines MSG_TRUNC as an output flag, indicating packet/datagram truncation. Linux extended it a while (~15+ years) ago to act as input flag, making the returned number of bytes being the full packet size.

It's a (relatively) popular pattern to do recvmsg( MSG_PEEK | MSG_TRUNC) to get the packet size, allocate the buffer and issue another call to fetch the packet. In particular, it's popular in userland netlink code, which is the primary driving factor of this change.

This diff implements the aforementioned behaviour for SOCK_DGRAM sockets (udp, unix, all soreceive_generic() users).

Test Plan

20:09 [2] m@devel0 s kyua debug -k /usr/tests/sys/netinet/Kyuafile socket_afinet:socket_afinet_unix_recv_trunc
socket_afinet:socket_afinet_unix_recv_trunc  ->  passed
20:09 [2] m@devel0 s kyua debug -k /usr/tests/sys/netinet/Kyuafile socket_afinet:socket_afinet_udp_recv_trunc
socket_afinet:socket_afinet_udp_recv_trunc  ->  passed

[melifaro]

.

[pauamma_gundo.com]

Both are minor and can be changed on commit. (And as always, I can't review the C source.)

lib/libc/sys/recv.2

31

Bump.

190

[melifaro]

Fix mbuf leak & MSG_TRUNC flag clearning, address comments.

[melifaro]

.