How to Create a Linux Kernel Module

Prerequisites

Before you begin, make sure you have:

A Linux system (with root access)
Basic knowledge of C programming
Kernel headers and development tools installed

To install the necessary tools on Ubuntu/Debian:

sudo apt-get updatesudo apt-get install gcc-12 build-essential linux-headers-$(uname -r)

Step 1: Create a Simple Kernel Module

Begin by creating a basic kernel module that can be loaded into and unloaded from the kernel. While it won’t perform any special functions, it serves as a “Hello World” example. Here’s how to do it:

Create a file named hello.c and add the following code:

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

// Initialization function (called when the module is loaded)
static int __init hello_init(void)
{
  printk(KERN_INFO "Hello, world!\n");
  return 0;  // Return 0 means success
}

// Exit function (called when the module is removed)
static void __exit hello_exit(void)
{
  printk(KERN_INFO "Goodbye, world!\n");
}

// Register the functions
module_init(hello_init);
module_exit(hello_exit);

MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Simple Hello World Module");
MODULE_AUTHOR("Tecadmin.net");

Step 2: Create a Makefile

To compile the module, you’ll need a Makefile—a tool that aids in correctly building kernel modules.

Create a file named Makefile in the same directory as your hello.c file:

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

Step 3: Compile the Kernel Module

Now, compile the kernel module using the make command.

Open a terminal and navigate to the directory where your files are located.
Run the following command to compile the module:
```
make
```

If everything is set up correctly, this will generate a file named hello.ko. This is your compiled kernel module.

Step 4: Test the Kernel Module

Now, it’s time to test your kernel module by loading it into the running kernel and checking its output.

Load the module: Use the insmod command to insert the module into the kernel.
```
sudo insmod hello.ko
```
After running this, your kernel module should be loaded, and the hello_init() function will run.
Check the output: Use the dmesg command to check the messages logged by the module.
```
dmesg | tail
```
You should see the message "Hello, world!" in the output.
Remove the module: Use the rmmod command to remove the module.
```
sudo rmmod hello
```
This will unload the module and trigger the hello_exit() function.
Check the output again: Run dmesg | tail again to see the "Goodbye, world!" message in the output.

Step 5: Delete the Kernel Module

If you no longer need the kernel module, you can clean up the files using the make clean command.

In the terminal, run:

make clean

This will remove the generated files, including the hello.ko module, leaving only your source code (hello.c and Makefile).

https://tecadmin.net/how-to-create-a-kernel-module-for-linux/

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