为字符设备编写设备驱动程序，实现简单的消息传递方式。 内核维护一个消息列表。 为了限制内存使用，我们对每条消息施加4KB的限制，并且还限制所有消息的大小，最初为2MB。(Write a device driver for a character device which implements a simple way of message passing. The kernel maintains a list of messages. To limit memory usage, we impose a limit of 4KB for each message, and also impose a limit of the size of all messages, which is initially 2MB.)