io_uring/net: disable partial retries for recvmsg with cmsg

We cannot sanely handle partial retries for recvmsg if we have cmsg
attached. If we don't, then we'd just be overwriting the initial cmsg
header on retries. Alternatively we could increment and handle this
appropriately, but it doesn't seem worth the complication.

Move the MSG_WAITALL check into the non-multishot case while at it,
since MSG_WAITALL is explicitly disabled for multishot anyway.

Link: https://lore.kernel.org/io-uring/0b0d4411-c8fd-4272-770b-e030af6919a0@kernel.dk/
Cc: stable@vger.kernel.org # 5.10+
Reported-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/net.c

@@ -789,16 +789,19 @@ int io_recvmsg(struct io_kiocb *req, unsigned int issue_flags)
 	flags = sr->msg_flags;
 	if (force_nonblock)
 		flags |= MSG_DONTWAIT;
-	if (flags & MSG_WAITALL)
-		min_ret = iov_iter_count(&kmsg->msg.msg_iter);
 
 	kmsg->msg.msg_get_inq = 1;
-	if (req->flags & REQ_F_APOLL_MULTISHOT)
+	if (req->flags & REQ_F_APOLL_MULTISHOT) {
 		ret = io_recvmsg_multishot(sock, sr, kmsg, flags,
 					   &mshot_finished);
-	else
+	} else {
+		/* disable partial retry for recvmsg with cmsg attached */
+		if (flags & MSG_WAITALL && !kmsg->msg.msg_controllen)
+			min_ret = iov_iter_count(&kmsg->msg.msg_iter);
+
 		ret = __sys_recvmsg_sock(sock, &kmsg->msg, sr->umsg,
 					 kmsg->uaddr, flags);
+	}
 
 	if (ret < min_ret) {
 		if (ret == -EAGAIN && force_nonblock) {