How to Write Kernel Modules

• Kernel documentation
  • Hacking index
  • Driver API
  • Module building
• Source analysis & books
  • https://elixir.bootlin.com
  • LDD3
  • The master documentation is the code
• Practice!

Hello World Kernel Module

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

MODULE_LICENSE("GPL");

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");
}

module_init(hello_init);
module_exit(hello_exit);

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

• Potential confusion: What is called when?
• Kernel code must be able to run in more than one context
• Mutexes mandatory
  
  https://www.cs.mcgill.ca/~cs573/fall2002/notes/lec273/lecture15/re-ent2.GIF

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

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

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

load:
    sudo insmod hello.ko
unload:
    -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