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Let’s make a hello-world type Linux kernel module.

Get Linux kernel sources

First we need to integrate a Makefile, such as hello.mk into the Linux kernel source tree. Thus you'll first need to obtain the Linux kernel source code from the official website or your distribution's package manager and get it extracted.

Here's an example for Arch (or Manjaro)

sudo pacman -S base-devel git xmlto cpio pahole
mkdir ~/kernel_build/
cd kernel_build
git clone https://gitlab.manjaro.org/packages/core/linux612.git
cd linux612
makepkg --nobuild
KERNEL_SRC=~/kernel_build/linux612/src/linux-6.12
ln -s /usr/lib/modules/$(uname -r)/build/Module.symvers "$KERNEL_SRC/"
zcat /proc/config.gz > "$KERNEL_SRC/.config"
make -C "$KERNEL_SRC" modules_prepare
# build a kernel module (usbip) example
make --directory="$KERNEL_SRC" M=drivers/usb/usbip/

Add driver directory

So, one thing is if you want to make changes to an existing module, but we will be creating a totally fresh one. For that, navigate to the drivers directory and create a new directory for your module. For example, if your module is named "hello," you create a directory called hello.

cd $KERNEL_SRC
mkdir $KERNEL_SRC/drivers/hello

nano $KERNEL_SRC/drivers/hello/hello.c
nano $KERNEL_SRC/drivers/hello/hello.mk
nano $KERNEL_SRC/drivers/hello/Kconfig

Create a Kconfig file if you want your module to be selectable through the kernel configuration menu, create a file named Kconfig inside your module directory (drivers/hello/Kconfig). The Kconfig file should define the configuration options for your module.

nano $KERNEL_SRC/drivers/hello/Kconfig

with this content

config HELLO_MODULE
    tristate "Hello Module"
    default m
    help
        This is a sample kernel module.

config HELLO_MODULE_OPTION
    bool "Enable additional option"
    depends on HELLO_MODULE
    default n
    help
        Enable an additional option for Hello Module.
  1. Add module information to the top-level Kconfig (drivers/Kconfig). Open the file
nano $KERNEL_SRC/drivers/Kconfig
  1. and add an source entry pointing to your module's Kconfig file.
source "drivers/hello/Kconfig"

A simple hello.c kernel module that prints a message to the kernel log using printk at log level 3:

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

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Your Name");
MODULE_DESCRIPTION("A simple hello world kernel module");

static int __init hello_init(void)
{
    printk(KERN_INFO "Hello, world!\n");
    return 0;
}

static void __exit hello_exit(void)
{
    printk(KERN_INFO "Goodbye, world!\n");
}

module_init(hello_init);
module_exit(hello_exit);

The module_init and module_exit macros are used to specify the module initialization and cleanup function accordingly (hello_init, hello_exit).

Makefile for building the hello.c kernel module, hello.mk:

obj-m += hello.o

KERNELDIR ?= /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)

default:
	$(MAKE) -C $(KERNELDIR) M=$(PWD) modules

clean:
	$(MAKE) -C $(KERNELDIR) M=$(PWD) clean

What kernel modules can do?

  • Device Drivers
  • Filesystems
  • Network Protocols (such as TCP/IP stack, IP routing protocols (like OSPF or BGP)), network packet filtering (firewalling)
  • Virtualization (such as Kernel-based Virtual Machine (KVM), Xen, or containerization technologies like Docker or LXC)
  • Security and encryption
  • Performance Monitoring
  • Debugging and Tracing
  • Modify system calls
  • Power Management

Simple example of Kernel-based Virtual Machine (KVM)

KVM has the capability to emulate different CPU architectures with their respective instruction sets. For example, we can create a virtual machine with an ARM CPU architecture as the guest, even if the host CPU architecture is x86.

// to be continued //

Reference

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