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Adding A System Call To The Linux Kernel (5.8.1) In Ubuntu (20.04 LTS)

jasper profile image Jihan Jasper Al-rashid Updated on ・6 min read

In this guide, you will learn how to add a simple system call to the Linux kernel. Check the help sections on the table of contents if you need help with text editors.

Section 1 - Preparation
Section 2 - Creation
Section 3 - Installation
Section 4 - Result
Help - Text Editors
Help - nano

Section 1 - Preparation

In this section, you will download all necessary tools to add a basic system call to the Linux kernel and run it. This is the only part of the entire process where network connectivity is necessary.

1.1 - Fully update your operating system.

sudo apt update && sudo apt upgrade -y

1.2 - Download and install the essential packages to compile kernels.

sudo apt install build-essential libncurses-dev libssl-dev libelf-dev bison flex -y

If would rather use vim or any other text editor instead of nano, below is an example of how you install it.

sudo apt install vim -y

1.3 - Clean up your installed packages.

sudo apt clean && sudo apt autoremove -y

1.4 - Download the source code of the latest stable version of the Linux kernel (which is 5.8.1 as of 12 August 2020) to your home folder.

wget -P ~/ https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.8.1.tar.xz

If you have downloaded a newer version of the Linux kernel, refer to this documentation to learn about any relevant change made to system calls.

1.5 - Unpack the tarball you just downloaded to your home folder.

tar -xvf ~/linux-5.8.1.tar.xz -C ~/

1.6 - Reboot your computer.

Section 2 - Creation

In this section, you will write a basic system call in C and integrate it into the new kernel.

2.1 - Check the version of your current kernel.

uname -r

As of 12 August 2020, it should display the following.

5.4.0-42-generic

In section 4, it should be different.

2.2 - Change your working directory to the root directory of the recently unpacked source code.

cd ~/linux-5.8.1/

2.3 - Create the home directory of your system call.

Decide a name for your system call, and keep it consistent from this point onwards. I have chosen identity.

mkdir identity

2.4 - Create a C file for your system call.

Create the C file with the following command.

nano identity/identity.c

Write the following code in it.

#include <linux/kernel.h>
#include <linux/syscalls.h>

SYSCALL_DEFINE0(identity)

{
    printk("I am Jihan Jasper Al-rashid.\n");
    return 0;
}

You can write anything you like here.

Save it and exit the text editor.

2.5 - Create a Makefile for your system call.

Create the Makefile with the following command.

nano identity/Makefile

Write the following code in it.

obj-y := identity.o

Save it and exit the text editor.

2.6 - Add the home directory of your system call to the main Makefile of the kernel.

Open the Makefile with the following command.

nano Makefile

Search for core-y. In the second result, you will see a series of directories.

kernel/ certs/ mm/ fs/ ipc/ security/ crypto/ block/

In the fresh source code of Linux 5.8.1 kernel, it should be in line 1073.

Add the home directory of your system call at the end like the following.

kernel/ certs/ mm/ fs/ ipc/ security/ crypto/ block/ identity/

Save it and exit the editor.

2.7 - Add a corresponding function prototype for your system call to the header file of system calls.

Open the header file with the following command.

nano include/linux/syscalls.h

Navigate to the bottom of it and write the following code just above #endif.

asmlinkage long sys_identity(void);

Save it and exit the editor.

2.8 - Add your system call to the kernel's system call table.

Open the table with the following command.

nano arch/x86/entry/syscalls/syscall_64.tbl

Navigate to the bottom of it. You will find a series of x32 system calls. Scroll to the section above it. This is the section of your interest. Add the following code at the end of this section respecting the chronology of the row as well as the format of the column. Use Tab for space.

440     common  identity                sys_identity

In the fresh source code of Linux 5.8.1 kernel, the number for your system call should be 440.

Save it and exit the editor.

Section 3 - Installation

In this section, you will install the new kernel and prepare your operating system to boot into it.

3.1 - Configure the kernel.

Make sure the window of your terminal is maximized.

Open the configuration window with the following command.

make menuconfig

Use Tab to move between options. Make no changes to keep it in default settings.

Save and exit.

3.2 - Find out how many logical cores you have.

nproc

The following few commands require a long time to be executed. Parallel processing will greatly speed them up. For me, it is 12. Therefore, I will put 12 after -j in the following commands.

3.3 - Compile the kernel's source code.

make -j12

3.4 - Prepare the installer of the kernel.

sudo make modules_install -j12

3.5 - Install the kernel.

sudo make install -j12

3.6 - Update the bootloader of the operating system with the new kernel.

sudo update-grub

3.7 - Reboot your computer.

Section 4 - Result

In this section, you will write a C program to check whether your system call works or not. After that, you will see your system call in action.

4.1 - Check the version of your current kernel.

uname -r

It should display the following.

5.8.1

4.2 - Change your working directory to your home directory.

cd ~

4.3 - Create a C file to generate a report of the success or failure of your system call.

Create the C file with the following command.

nano report.c

Write the following code in it.

#include <linux/kernel.h>
#include <sys/syscall.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>

#define __NR_identity 440

long identity_syscall(void)
{
    return syscall(__NR_identity);
}

int main(int argc, char *argv[])
{
    long activity;
    activity = identity_syscall();

    if(activity < 0)
    {
        perror("Sorry, Jasper. Your system call appears to have failed.");
    }

    else
    {
        printf("Congratulations, Jasper! Your system call is functional. Run the command dmesg in the terminal and find out!\n");
    }

    return 0;
}

You can customize the messages for failure and success anyhow you like.

Save it and exit the editor.

4.4 - Compile the C file you just created.

gcc -o report report.c

4.5 - Run the C file you just compiled.

./report

If it displays the following, everything is working as intended.

Congratulations, Jasper! Your system call is functional. Run the command dmesg in the terminal and find out!

4.6 - Check the last line of the dmesg output.

dmesg

At the bottom, you should now see the following.

I am Jihan Jasper Al-rashid.

Congratulations! You have successfully added a system call to the Linux kernel!

Help - Text Editors

This guide has frequent instances of text editing. You can use whatever gives you comfort. For the purpose of this guide, I have used nano. If you would rather use vim or any other text editor, simply replace nano with either vim or any other text editor in all of the commands. If you would rather use a GUI-based text editor, the process is the same. gedit, GNOME's default GUI-based text editor, comes bundled with Ubuntu 20.04 LTS.

Help - Basic Controls For nano

Ctrl + O brings up the interface to save a document. Pressing Enter afterwards will complete the process.

Ctrl + W brings up the interface to search for anything in a document. You can write anything in here and press Enter to jump to it. The result will be highlighted.

Alt + W takes you to the next instance, if there is one, after you searched anything once.

Ctrl + C cancels any interface.

Alt + C turns on the display which shows you the line number based on the position of your cursor.

Ctrl + X exits the text editor.

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