Guide to Linux Kernel Module Programming: Step-by-Step Hello World Program

Linux kernel modules are powerful tools that allow you to extend the functionality of the Linux operating system without modifying the core kernel code. In this guide, we’ll walk you through creating your first kernel module – a simple “Hello World” program. This hands-on approach will help you understand the basics of kernel module programming and its advantages.

What are Linux Kernel Modules?

Kernel modules are pieces of code that can be loaded and unloaded into the kernel on demand. They extend the functionality of the kernel without requiring a system reboot. Common uses for kernel modules include:

• Device drivers
• File system drivers
• System calls

Advantages of Loadable Kernel Modules (LKMs)

Using LKMs offers several benefits over modifying the base kernel:

1. Flexibility: Add or remove functionality without rebooting
2. Easier debugging: Isolate issues to specific modules
3. Memory efficiency: Only load modules when needed
4. Faster development: Test changes quickly without full kernel rebuilds

Creating Your First Kernel Module

Let’s create a simple “Hello World” kernel module. This module will print a message when loaded and unloaded.

Step 1: Set Up Your Development Environment

First, ensure you have the necessary tools installed:

sudo apt-get install build-essential linux-headers-$(uname -r)

Step 2: Write the Module Code

Create a file named hello.c with the following content:

#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Your Name");
MODULE_DESCRIPTION("A simple Hello World module");
MODULE_VERSION("0.1");

static int __init hello_init(void) {
    printk(KERN_INFO "Hello, World!\n");
    return 0;
}

static void __exit hello_exit(void) {
    printk(KERN_INFO "Goodbye, World!\n");
}

module_init(hello_init);
module_exit(hello_exit);

This code defines two functions:

• hello_init(): Called when the module is loaded
• hello_exit(): Called when the module is unloaded

Step 3: Create a Makefile

Create a file named Makefile in the same directory:

obj-m += hello.o

all:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

clean:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

This Makefile tells the kernel build system how to compile your module.

Step 4: Compile the Module

Run the following command to compile your module:

make

If successful, you’ll see a hello.ko file in your directory.

Step 5: Load and Test the Module

To load the module:

sudo insmod hello.ko

To see the “Hello, World!” message:

dmesg | tail

sudo rmmod hello