Book Image

Linux Kernel Programming Part 2 - Char Device Drivers and Kernel Synchronization

By : Kaiwan N Billimoria
Book Image

Linux Kernel Programming Part 2 - Char Device Drivers and Kernel Synchronization

By: Kaiwan N Billimoria

Overview of this book

Linux Kernel Programming Part 2 - Char Device Drivers and Kernel Synchronization is an ideal companion guide to the Linux Kernel Programming book. This book provides a comprehensive introduction for those new to Linux device driver development and will have you up and running with writing misc class character device driver code (on the 5.4 LTS Linux kernel) in next to no time. You'll begin by learning how to write a simple and complete misc class character driver before interfacing your driver with user-mode processes via procfs, sysfs, debugfs, netlink sockets, and ioctl. You'll then find out how to work with hardware I/O memory. The book covers working with hardware interrupts in depth and helps you understand interrupt request (IRQ) allocation, threaded IRQ handlers, tasklets, and softirqs. You'll also explore the practical usage of useful kernel mechanisms, setting up delays, timers, kernel threads, and workqueues. Finally, you'll discover how to deal with the complexity of kernel synchronization with locking technologies (mutexes, spinlocks, and atomic/refcount operators), including more advanced topics such as cache effects, a primer on lock-free techniques, deadlock avoidance (with lockdep), and kernel lock debugging techniques. By the end of this Linux kernel book, you'll have learned the fundamentals of writing Linux character device driver code for real-world projects and products.
Table of Contents (11 chapters)
Section 1: Character Device Driver Basics
User-Kernel Communication Pathways
Handling Hardware Interrupts
Working with Kernel Timers, Threads, and Workqueues
Section 2: Delving Deeper

Our secret driver – the init code

In the init code of our secret device driver (a kernel module, of course, thus invoked upon insmod(8)), we first register the driver as a misc character driver with the kernel (via the misc_register() API, as seen in the Writing the misc driver code – part 1 section earlier; we won't repeat this code here).

Next, we allocate kernel memory for our driver's "context" structure – via the useful managed allocation  devm_kzalloc() API (as you learned in the companion guide Linux Kernel Programming, Chapter 8Kernel Memory Allocation for Module Authors – Part 1, in the Using the kernel's resource-managed memory allocation APIs section) – and initialize it. Notice that you must ensure you first get the device pointer dev before you can use this API; we retrieve it from our miscdevice structure's this_device member (as seen):

// ch1/miscdrv_rdwr/​miscdrv_rdwr.c
[ ... ]
static int __init miscdrv_rdwr_init(void)
int ret;
struct device *dev;

ret = misc_register(&llkd_miscdev);
[ ... ]
dev = llkd_miscdev.this_device;
[ ... ]
ctx = devm_kzalloc(dev, sizeof(struct drv_ctx), GFP_KERNEL);
if (unlikely(!ctx))
return -ENOMEM;

ctx->dev = dev;
strscpy(ctx->oursecret, "initmsg", 8);
[ ... ]
return 0; /* success */

Okay, clearly, we have initialized the dev member of our ctx private structure instance as well as the 'secret' string to the 'initmsg'  string (not a very convincing secret, but let's leave it at that). The idea here is that when a user space process (or thread) opens our device file and issues read(2) upon it, we pass back (copy) the secret to it; we do so by invoking the copy_to_user() helper function! Similarly, when the user-mode app writes data to us (yes, via the write(2) system call), we consider that data written to be the new secret. So, we fetch it from its user space buffer – via the copy_from_user() helper function – and update it in driver memory.

Why not simply use the strcpy() (or strncpy()) API to initialize the ctx->oursecret member? This is very important: they aren't safe enough security-wise. Also, the strlcpy() API has been marked as deprecated by the kernel community ( In general, always avoid using deprecated stuff, as documented in the kernel documentation here:

Quite clearly, the interesting parts of this new driver are the I/O functionality – the read and write methods; on with it!