2.5. Linux Kernel Modules

2.5.1. References

• Bootlin slides, chapter Linux kernel introduction
• Mainline kernel: https://www.kernel.org/
• Kernel documentation on out of tree builds: https://www.kernel.org/doc/Documentation/kbuild/modules.txt
• http://derekmolloy.ie/writing-a-linux-kernel-module-part-1-introduction/

2.5.2. Goals

• Make a simple out-of-tree kernel module
• Build the kernel module for a target (e.g. qemu-system-arm)
• Test the module and module commands on a target (e.g. qemu-system-arm)

2.5.3. Steps

1. Create a hello.c file containing the following code inside a new directory hello-module/:

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

static int __init hello_init(void)
{
        printk(KERN_INFO "########################### hello module\n");
        return 0;
}

static void __exit hello_exit(void)
{
        printk(KERN_INFO "########################### goodbye module\n");
}

module_init(hello_init);
module_exit(hello_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("foo");
MODULE_DESCRIPTION("a simple hello kernel module");
MODULE_VERSION("0.01");

2. Create a Makefile file containing the following code:

obj-m += hello.o

KBUILD=$(LINUX_PATH)

all:
        make -C $(KBUILD) M=$(PWD) modules
install:
        make -C $(KBUILD) M=$(PWD) INSTALL_MOD_PATH=$(STAGING_PATH) modules_install
clean:
        make -C $(KBUILD) M=$(PWD) clean

Important

Makefile commands do not include architecture and cross compiler info

3. Build the module:

user@host: make && make install

2.5.4. Assignments

• Create a root file system containing the module (use ${STAGING_PATH})
• Test the module on a target, e.g. qemu-system-arm
• List all shell commands that manipulate kernel modules
• Make the module load automatically at boot

2.5.5. Questions

• What are the purpose of kernel modules?
• What well known external kernel modules exist?
• Where are out-of-tree modules installed?
• What are the risks of using/depending on externally maintained kernel modules?