Linux Loadable Kernel Modules (LKM)

Linux Loadable Kernel Modules (LKMs) are pieces of code that can be dynamically added to or removed from the Linux kernel while the system is running. They extend the kernel‘s functionality without requiring a full system reboot, providing flexibility and modularity to the Linux operating system.

Advantages of LKMs

Using LKMs offers several benefits over built-in kernel modules:

1. Flexibility: Modules can be loaded and unloaded as needed, saving memory and resources.
2. Easy maintenance: Updates and fixes can be applied without rebooting the entire system.
3. Simplified debugging: Issues with LKMs are easier to isolate and diagnose compared to problems in the base kernel.
4. Reduced kernel rebuilds: New functionality can be added without recompiling the entire kernel.

Common Use Cases

LKMs are typically used for:

1. Device drivers
2. File system drivers
3. System calls
4. Network protocols
5. Security features

Creating a Basic LKM

Here’s a simple example of an LKM that prints a message when loaded and unloaded:

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

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Your Name");
MODULE_DESCRIPTION("A simple Linux kernel 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 module defines two functions: hello_init() runs when the module is loaded, and hello_exit() runs when it’s unloaded.

Building and Using the Module

To build the module, create a 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

Then, compile the module:

make

sudo insmod hello.ko

sudo rmmod hello

dmesg | tail