Linux Kernel Module (LKM)

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Overview

A Linux Kernel Module (LKM) is a piece of code that can be dynamically loaded into the kernel.

It allows extending kernel functionality without rebuilding the kernel.

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Why Kernel Modules?

• Add device drivers
• Extend kernel features
• Debug kernel behavior

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Kernel Space vs User Space

Type	Description
User Space	Applications
Kernel Space	Core OS + drivers

Kernel modules run in kernel space.

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Basic Structure

Minimal example:

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

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

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

module_init(hello_init);
module_exit(hello_exit);

MODULE_LICENSE("GPL");

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Build System

Makefile:

obj-m += hello.o

KDIR := /lib/modules/$(shell uname -r)/build
PWD  := $(shell pwd)

all:
$(MAKE) -C $(KDIR) M=$(PWD) modules

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Build Output

After compilation:

• .ko → kernel object (module)
• .o, .mod.c, .symvers → build artifacts

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Load / Unload Module

Load module:

sudo insmod hello.ko

Unload module:

sudo rmmod hello

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Check Module Status

lsmod
modinfo hello.ko

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Kernel Logging

Use printk instead of printf.

printk(KERN_INFO "info message\n");
printk(KERN_ERR  "error message\n");

View logs:

dmesg | tail

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Important Notes

1. No standard C library

• No printf
• No malloc (use kmalloc)

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2. Kernel crash risk

• Bugs can crash the entire system
• No memory protection like user space

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3. Licensing

MODULE_LICENSE("GPL");

Required to avoid kernel warnings.

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Summary

• Kernel modules extend kernel dynamically
• .ko files are loadable modules
• printk is used for debugging
• Runs with highest privilege

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Next Step

• Character device driver
• file_operations
• User ↔ Kernel communication

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