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Hello World! Kernel Module

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Utkarsh Maurya
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Table of contents

Hey everyone! This script guides you through building and running a simple "Hello World" kernel module in Linux. We'll follow the approach explained in Robert Love's "Linux Kernel Development" and refer to the official Kernel Documentation (https://www.kernel.org/doc/html/latest/index.html).

Here are my solutions to the Eudyptula Challenge: https://github.com/pro-utkarshM/Kernel-Carnival.

Preparation:

Grab a mug of coffee, recall any regrets from your previous semester ( Organizational Behaviour could have saved you!), and let's jump in with some newfound experience!

The "Hello World" Module:

  1. Code:
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>

static int hello_init(void) {
  printk(KERN_DEBUG "Hello World!\n");
  return 0;
}

static void hello_exit(void) {
  printk(KERN_DEBUG "See you later.\n");
}

module_init(hello_init);
module_exit(hello_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("projects.utkarshMaurya@gmail.com");
MODULE_DESCRIPTION("Just a module");

obj-m += helloworld.o
KDIR := /lib/modules/$(shell uname -r)/build

all:
    $(MAKE) -C $(KDIR) M=$(PWD) modules
clean:
    $(MAKE) -C $(KDIR) M=$(PWD) clean
  1. Compilation:

Use the make command in the module directory.

  1. Loading:

Run sudo insmod FILE_NAME.ko after replacing FILE_NAME with your actual module file name (e.g., insmod helloworld.ko).

  1. Viewing Output:

Check kernel messages with sudo dmesg | grep tail.

Understanding the Process:

  • module_init and module_exit macros define functions to run when loading and unloading the module.

  • The printk function logs messages to the kernel ring buffer.

Example:

Here's a demonstration of compiling and running the module:

$ ls
Kbuild Makefile modul.c

$ make
make -C /lib/modules/`uname -r`/build M=`pwd`
# ... compilation output ...

$ ls
built-in.o Kbuild Makefile modul.c Module.markers
modules.order Module.symvers modul.ko modul.mod.c
modul.mod.o modul.o

$ insmod modul.ko

$ dmesg | tail -1
Hi

$ rmmod modul

$ dmesg | tail -2
Hi
Bye

Additional Information:

  • For a list of loaded modules, use lsmod or explore /proc/modules and /sys/module directories.

I hope it's easier to understand and follow!

Let me know if you have any questions about specific parts or need further clarification.

Linuxlinux kernel