af_unix: don't post cmsg for SO_INQ unless explicitly asked for

A previous commit added SO_INQ support for AF_UNIX (SOCK_STREAM), but it
posts a SCM_INQ cmsg even if just msg->msg_get_inq is set. This is
incorrect, as ->msg_get_inq is just the caller asking for the remainder
to be passed back in msg->msg_inq, it has nothing to do with cmsg. The
original commit states that this is done to make sockets
io_uring-friendly", but it's actually incorrect as io_uring doesn't use
cmsg headers internally at all, and it's actively wrong as this means
that cmsg's are always posted if someone does recvmsg via io_uring.

Fix that up by only posting a cmsg if u->recvmsg_inq is set.

Additionally, mirror how TCP handles inquiry handling in that it should
only be done for a successful return. This makes the logic for the two
identical.

Cc: stable@vger.kernel.org
Fixes: df30285 ("af_unix: Introduce SO_INQ.")
Reported-by: Julian Orth <ju.orth@gmail.com>
Link: axboe/liburing#1509
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/07adc0c2-2c3b-4d08-8af1-1c466a40b6a8@kernel.dk
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: axboe

net/unix/af_unix.c

@@ -2904,6 +2904,7 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
 	unsigned int last_len;
 	struct unix_sock *u;
 	int copied = 0;
+	bool do_cmsg;
 	int err = 0;
 	long timeo;
 	int target;
@@ -2929,6 +2930,9 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
 
 	u = unix_sk(sk);
 
+	do_cmsg = READ_ONCE(u->recvmsg_inq);
+	if (do_cmsg)
+		msg->msg_get_inq = 1;
 redo:
 	/* Lock the socket to prevent queue disordering
 	 * while sleeps in memcpy_tomsg
@@ -3088,10 +3092,11 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
 	if (msg) {
 		scm_recv_unix(sock, msg, &scm, flags);
 
-		if (READ_ONCE(u->recvmsg_inq) || msg->msg_get_inq) {
+		if (msg->msg_get_inq && (copied ?: err) >= 0) {
 			msg->msg_inq = READ_ONCE(u->inq_len);
-			put_cmsg(msg, SOL_SOCKET, SCM_INQ,
-				 sizeof(msg->msg_inq), &msg->msg_inq);
+			if (do_cmsg)
+				put_cmsg(msg, SOL_SOCKET, SCM_INQ,
+					 sizeof(msg->msg_inq), &msg->msg_inq);
 		}
 	} else {
 		scm_destroy(&scm);