Writing a Simple misc Character Device Driver

No doubt, device drivers are a vast and interesting topic. Not only that, they are perhaps the most common use of the Loadable Kernel Module (LKM) framework that we have been using. Here, we shall introduce you to writing a few simple yet complete Linux character device drivers, within a class called misc; yes, that's short for miscellaneous. We wish to emphasize that this chapter is limited in its scope and coverage - here, we do not attempt to delve into the deep details regarding the Linux driver model and its many frameworks; instead, we refer you to several excellent books and tutorials on this topic via the Further reading section for this chapter. Our aim here is to quickly get you familiar with the overall concepts behind writing a simple character device driver.

Having said that, this book indeed has several chapters that are dedicated to what a driver author needs to know. Besides this introductory chapter, we cover (in detail) how a driver author works with hardware I/O memory, hardware interrupt handling (and its many sub-topics), and kernel mechanisms such as delays, timers, kernel threads, and work queues. Use of various user-kernel communication pathways or interfaces is covered in detail as well. The final two chapters of this book then focus on something very important for any kernel development, including drivers – kernel synchronization.

The other reasons we'd prefer to write a simple Linux character device driver and not just our "usual" kernel module are as follows:

• Until now, our kernel modules have been quite simplistic, having only init and cleanup functions, nothing more. A device driver provides several entry points into the kernel; these are the file-related system calls, known as the driver's methods. So, we can have an open() method, a read() method, a write() method, an llseek() method, an [unlocked|compat]_ioctl() method, a release() method, and so on.

FYI, all possible "methods" (functions) the driver author can hook into are in this key kernel data structure: include/linux/fs.h:file_operations (more on this in the Understanding the connection between the process, the driver, and the kernel section).

• This situation is simply more realistic, and more interesting.

In this chapter, we will cover the following topics:

• Getting started with writing a simple misc character device driver
• Copying data from kernel to user space and vice versa
• A misc driver with a secret
• Issues and security concerns