hello_module.md

Introduction to Linux Kernel Module(LKM):

A kernel module is a piece of software that runs in the kernel space.
Advantages:

• Can access the hardware and kernel directly.
• Load dynamically into kernel and out of it.

Disadvantages:

• Can't use float variables.
• Smaller stack space than user space.
• Can only access specific header files inside the kernel modules.

But this is the right place to write the device drivers.

Creating the kernel module:

Create the 'hello_module.c' file in the directory which contains the WSL2-Linux-Kernel

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

//Meta informations
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Preethi");
MODULE_DESCRIPTION("Hello linux kernel module");

//Function that is executed when the module is loaded into the kernel (return int to know if successfully loaded(0) or not(-1))
static int __init InitModule(void)
{
	printk("Hello, module loaded\n");
	//printf prints to the command line but no command line is available in kernel, so printk is used where it prints to the linux kernel locks
	return 0;//to say successfully loaded
}

//Function that is executed when the module is removed from the kernel
static void __exit ExitModule(void)
{
	printk("Module unloaded\n");
}

module_init(InitModule);//defines which function to load at init
module_exit(ExitModule);//defines which function to load at exit

Creating makefile

Create the makefile in the same place as the module

vi Makefile

Content:

obj-m += hello_module.o

all:
		make -C $(shell pwd)/WSL2-Linux-Kernel M=$(shell pwd) modules
clean:
		make -C $(shell pwd)/WSL2-Linux-Kernel M=$(shell pwd) clean

Execute the makefile

make

Loading the kernel module into kernel

• insmod stands for insert module

sudo insmod hello_module.ko

• check if module is there in the list of loaded modules

lsmod | grep hello_module

• to check what is printed

dmesg | tail

Unloading the kernel module from the kernel

• rmmod stand for remove module

sudo rmmod hello_module.ko

lsmod now should not show this module

• to check print

dmesg | tail

Hello World module is successfully loaded and unloaded!