recvmsg() reads one byte more than the guest buffers hold to see
whether a record did not fit, but only for SOCK_DGRAM sockets. On a
SOCK_SEQPACKET socketpair a record longer than the buffers was cut
short with msg_flags 0: a 6-byte record received into 2 bytes returned
2 and no MSG_TRUNC, and the rest of the record was gone.
FreeBSD's SOCK_SEQPACKET protocol is atomic, sys/kern/uipc_usrreq.c:
.pr_type = SOCK_SEQPACKET,
...
.pr_flags = PR_ADDR|PR_ATOMIC|PR_CONNREQUIRED|PR_WANTRCVD|
PR_RIGHTS,
and its receive routine, soreceive_generic() in sys/kern/uipc_socket.c,
flags the truncation of an atomic record and drops its rest:
if (m != NULL && pr->pr_flags & PR_ATOMIC) {
flags |= MSG_TRUNC;
if ((flags & MSG_PEEK) == 0)
(void) sbdroprecord_locked(&so->so_rcv);
}
which kern_recvit() hands back in msg_flags. recvmsg() now probes for a
longer record on SOCK_SEQPACKET sockets as it does for SOCK_DGRAM.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
AF_UNIX pairs as FreeBSD 11 makes them: SOCK_STREAM, SOCK_DGRAM and
SOCK_SEQPACKET with protocol 0, SOCK_NONBLOCK applied to both ends and
SOCK_CLOEXEC accepted, EAFNOSUPPORT for an unknown family, EOPNOTSUPP
for AF_INET and AF_INET6, EPROTONOSUPPORT for type 0 or a protocol,
EPROTOTYPE for another type, EFAULT for a null output, and nothing
written on failure. The ends are guest sockets, so send, recv,
shutdown, fcntl, poll and close work on them; getsockname and
getpeername report the unnamed AF_UNIX address, recvfrom and recvmsg
no address for a stream and the unnamed one for a datagram. On Linux
the pair is the host's socketpair; on Windows a loopback TCP connection
or two connected UDP sockets, and SOCK_SEQPACKET throws. The remaining
differences from the console are recorded in TechnicalDebt.
OpenSSL's QUIC notifier in 15 catalog titles creates such a pair.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
milescoviello
Jan Šlégl