Writing Your First Kernel Module

What Is a Kernel Module?

A kernel module is a piece of code that can be dynamically loaded into or removed from the running kernel without rebooting. It allows us to:

• Extend kernel functionality (e.g., new drivers)
• Keep the kernel slim and modular
• Easily debug or test features

Kernel Module Lifecycle

All kernel modules must implement two functions:

static int __init my_module_init(void);   // Called when module is loaded
static void __exit my_module_exit(void);  // Called when module is removed

You must register these with macros:

module_init(my_module_init);
module_exit(my_module_exit);

Our First Kernel Module: Hello World

hello.c:

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

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

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

module_init(hello_init);
module_exit(hello_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Your Name");
MODULE_DESCRIPTION("A Simple Hello World Kernel Module");

Explanation:

• KERN_INFO: Log level
• __init and __exit: Compiler hints to free memory if the functions aren't needed after boot
• MODULE_* macros add metadata (viewable with modinfo)

Creating the Makefile

Makefile:

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

Explanation:

• obj-m: Specifies the kernel object to be built
• -C: Tells make to run in the kernel build directory
• M=$(PWD): Points to your module's source directory

Building the Module

Step-by-Step:

1. Create the source files:

   mkdir hello_module && cd hello_module
   nano hello.c
   nano Makefile

2. Build the module:

   make

3. You should now have a file: hello.ko

Loading the Module

sudo insmod hello.ko

Check kernel logs:

dmesg | tail -n 10

Expected Output:

[ 1234.5678 ] Hello, Kernel World!

Unloading the Module

sudo rmmod hello

Check logs again:

dmesg | tail -n 10

Expected output:

[ 1234.9876 ] Goodbye, Kernel World!

Inspecting the Module

List all loaded modules:

lsmod

Show module info:

modinfo hello.ko

Expected metadata:

description:    A Simple Hello World Kernel Module
author:         Your Name
license:        GPL