simple

Simple

Let's make our first Kernel Module named simple, which will essentially just log some messsages when it's loaded and is removed. We'll also explore some of Kernel headers and utilize them in the module.

Pre-requisites

• Finish workspace setup
• Understand workflow
• Download the folder simple to local workspace

Steps

• Navigate to Project directory

cd simple

• compile Module

sudo make

• Look for simple in loaded Modules

sudo lsmod | grep simple

You will get no results

• Insert Module

sudo insmod simple.ko

• Again check for simple in module list

sudo lsmod | grep simple

Now you will see the simple module listed

• Check the ouput in Kernel Buffer

sudo dmesg

You can find it with a bit of searching.
Note : You can safely ignore the message module verification failed and tainting Kernel, this arises because this module is custom made and not signed.

• Remove the Module and verify it's gone

sudo rmmod simple
sudo lsmod | grep simple

No output means module is removed

• Check for message again in Kernel Buffer

sudo dmesg

• Clean Kernel Buffer

sudo dmesg -c

Understanding the Code

• Necessary Headers

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

• This function is called when the module is loaded

static int simple_init(void)
{
printk(KERN_INFO "%s\n", "Loading Kernel Module");
printk(KERN_INFO "golden ratio is %llu\n",GOLDEN_RATIO_PRIME);
printk(KERN_INFO "gcd(3300,24) = %lu\n", gcd(3300,24));
return 0;
}

printk is kernel equivalent of printf, and KERN_INFO is a level of Kernel log which represents informational message, there are 8 levels of logs depending on severity.
We utilize the constant GOLDEN_RATIO_PRIME and function gcd for outputting some useful stuff.

• This function is called when the module is removed

static void simple_exit(void)
{
printk(KERN_INFO "%s\n", "Removing Kernel Module");
}

• Macros for registering module entry and exit points

module_init(simple_init);
module_exit(simple_exit);

• Defining additional module info

MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Simple Module");
MODULE_AUTHOR("SGG");

Note : We declare custom functions and variable as static, this is to limit their scope to current module,otherwise they would be accessible in whole of Kernel and pollute global namespace!

Next up : hello Module