Linux Kernel Modules

Tutorial to insert & remove a Loadable kernel module [LKM].

Writing a kernel module

Create directory ‘linux-kernel-module’

$ mkdir linux-kernel-modules
$ cd /linux-kernel-modules

Create a file ‘hello.c’

$ touch hello.c

Write module program in ‘hello.c’ file.

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


static int __init print_hello(void){
	printk(KERN_INFO "Loading hello module...\n");
	printk(KERN_INFO "hello world\n");
	return 0;
}

static void __exit exit_hello(void){
	printk(KERN_INFO "bye bye kernel !\n");
}

module_init(print_hello);
module_exit(exit_hello);

Explaination,

• writing kernel module requires two important functions i.e. ‘__init’ & ‘__exit’.
  1. ‘__init’ function is called when the modules is loaded to the kernel.
  2. ‘__exit’ function is called when the module is removed from the kernel.
• ‘printk()’ function in C language is similar to ‘printf()’, but prints message to the ‘kernel log’ .

Compiling kernel module

• we are using ‘make’ program to compile our kernel module.
• we need to create a file named ‘Makefile’

$ touch Makefile

• with following set of commands

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

• Now compile our kernel module by executing ‘make’ .

$ make

Load the module to kernel.

• once the ‘make’ is successful, ‘hello.ko’ file is create.
• ‘.ko’ is the kernel object.
• insert this ‘.ko’ object to the kernel by,

$ sudo insmod hello.ko

Display kernel log

$ dmesg

Unload or remove kernel module we inserted,

$ sudo rmmod hello

• To display again kernel log

$ dmesg

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