AWS - How to Write a Simple Kernel Module?

In this blog, you will learn how to create a kernel module and load it into Linux Kernel in AWS EC2 environment. Since you are directly interact with the Kernel, there is a possibility that you could crash the kernel, well, another reason why you should use a cloud platform such as AWS EC2. You can simply spin up and tear down servers.

Preparation:

• OS: Ubuntu 16.04.3 LTS (I tried RedHat 7 and Amazon Linux AMI 2017.09.1, didn't work for me, so hard to get the necessary lib installed)
• You need to install the following packages:

# apt-get install build-essential linux-headers-$(uname -r)

• At least a open SSH port (22) on your AWS EC2 server.

The Kernel Module

Creating the kernel module called "small.c":

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

/* This function is called when the module is loaded .*/
int small_init(void)
{
    printk(KERN_INFO "Loading Module small\n");
    return 0;
}

/* This function is called when the module is removed. */
void small_exit(void)
{
    printk(KERN_INFO "Removing Module\n");
}

/* Macros for registering module entry and exit points. */
module_init(small_init);
module_exit(small_exit);

MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Small Simple Module");
MODULE_AUTHOR("TONY");

The Makefile

obj-m += small.o
all:
        make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
        make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

Note: Make sure you use tab instead of spaces. Otherwise you will see "Makefile:3: *** missing separator.  Stop." error.

Compile and Generate Kernel Module

$ make
make -C /lib/modules/4.4.0-1041-aws/build M=/home/ubuntu modules
make[1]: Entering directory '/usr/src/linux-headers-4.4.0-1041-aws'
  Building modules, stage 2.
  MODPOST 1 modules
make[1]: Leaving directory '/usr/src/linux-headers-4.4.0-1041-aws'
$ ls
Makefile  modules.order  Module.symvers  small.c  small.ko  small.mod.c  small.mod.o  small.o

The "make" command will create a "small.ko" file, this is the compiled module file.

Load and Unload Module

$ sudo insmod small.ko
$ dmesg | tail -1
[ 6751.246020] Loading Module small
$ sudo rmmod small.ko
$ dmesg | tail -1
[ 6796.304281] Removing Module

When a module is inserted into the kernel, the module_init macro will be invoked, which will call the function small_init. Similarly, when the module is removed with rmmod, module_exit macro will be invoked, which will call the small_exit. Using dmesg command, we can see the output from the sample Kernel module.