How to Write Kernel Modules

  • Kernel documentation
    • Hacking index
    • Driver API
    • Module building
  • Source analysis & books
  • Practice!

Hello World Kernel Module

$ cat hello.c
#include <linux/init.h>
#include <linux/module.h>


static int hello_init(void)
    printk(KERN_ALERT "Hello, world\n");
    return 0;

static void hello_exit(void)
    printk(KERN_ALERT "Goodbye, cruel world\n");


We are not in Kansas anymore

  • Goodbye main
  • Goodbye C standard library
  • Goodbye IEEE754
  • Hello ring 0 (x86_64) / EL1 (aarch64)
  • Hello asynchronous execution
  • Hello kernel API

Kernelspace vs Userspace

  • Small, fixed-size stack
  • Each mode can have it’s own address space
  • Kernel space handles access to hardware
  • Execution is transferred to kernel space after a system call

Reentrant code

Why not stick with userspace?

  • Context switches makes response time slower
  • Access to hardware requires priveledged instructions
    • The most important drivers can’t be handled in user space

Symbols, Versions & Platform dependence

  • Kernel symbol table
    • Contains address of global kernel items (functions and variables)
    • Changes every time the kernel is recompiled
    • Modules can access symbols already exported by other code
    • Modules can introduce their own exported symbols
  • A Module must be recompiled for each version of the kernel that wants to link with
    • Module has to be built with same understanding of target processor as kernel
    • Easier to distribute driver for general distribution by contributing to the mainline kernel

Compiling Modules (out-of-tree)

  • Develop your code in own folder
  • Compile by referencing the kernel makefile
  • If you build your own kernel, the makefile will already be installed
  • To get the makefile for a kernel from fedora run
sudo dnf install kernel-devel-`uname -r`

Hello World Kernel Module continued

$ cat Makefile
obj-m += hello.o

.PHONY: build clean load unload

    make -C /lib/modules/$(shell uname -r)/build modules M=$(shell pwd)

    make -C /lib/modules/$(shell uname -r)/build clean M=$(shell pwd)

    sudo insmod hello.ko
    -sudo rmmod hello

Loading and Unloading Modules

  • Modules can be added/removed at runtime using insmod and rmmod
  • Adding a module calls its init function, and removing it calls its exit function.
$ make load
sudo insmod hello.ko
$ dmesg -t | tail -3
hello: loading out-of-tree module taints kernel.
hello: module verification failed: signature and/or required key missing - tainting kernel
Hello, world
$ make unload
sudo rmmod hello
$ dmesg | tail -1
Goodbye, cruel world

Compiling Modules (in-tree)

  • Add your code directly to the kernel, and add them to the Makefile(s)
  • Compile the kernel
  • Init is called at boot

The End