The Linux Kernel

Firstly, configure, build, compile the Linux kernel, https://kernel.org/ and grab the latest Linux kernel source. To configure the kernel use make decofig which takes the default config file from your running Linux system. To enter a TUI to edit the config file. One thing for older devices is unchecking 64 bit kernel. Use make -j $(nproc) to compile using all the CPU cores.

# Replace with updated version numbers when required
wget https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-6.7.1.tar.xz
tar -xvf linux-6.7.1.tar.xz 
cd linux-6.7.1

make defconfig
make menuconfig

make -j4

After the build completes successfully, you should have a bzImage in the arch/x86_64/boot directory (your architecture may differ). If not, check the make file log.

That's done. cd ~/custom_linux

Now, create a directory structure: We could manually create the Linux directory structure using mkdir and copy the over the system programs like cat and ls, the majority of files on a modern Linux system are not statically linked, they rely on of libs. This can be checked with command ldd. BusyBox combines tiny versions of many common UNIX utilities into a single small executable and ln the commands.

wget https://busybox.net/downloads/busybox-1.36.1.tar.bz2
tar xf busybox-1.36.1.tar.bz2
cd busybox-1.36.1

Run...

make defconfig

to make a default configuration for the BusyBox. Then...

make menuconfig

to bring the TUI and edit the configs. The option which you must edit is located in Settings, and then Build static binary (no shared libs). The reason to enable this option is we do not want to compile Glibc into the Linux distro.

Then...

make -j $(nproc)

to compile BusyBox. To check if the compiled file is fine, use command...

file busybox

The file must be statically linked.

Creating The File System

Busybox also generates the diretories, run make install to create a folder called _install and change directory to _install and you will see a hierarchy like Linux file system.

In this directory, run the following command to create the folders needed for kernel.

mkdir dev proc sys

Now create a file called init and open it with a text editor. Copy and paste the following data to it:

#!/bin/sh
mount -t devtmpfs none /dev
mount -t proc none /proc
mount -t sysfs none /sys
echo "Welcome to my Linux!"
exec /bin/sh

Then make the script executable with...

chmod +x init

Done. BusyBox made one executable which is capable of providing us a lot of Linux utilities such as sh , echo , vi and so on. With make install we made a filesystem hierarchy which contains these programs as links to BusyBox executable. Next, we make a shell script called init. This script will be ran after kernel loads. At first, it mounts dev, proc and sys directories. It then prints a welcome message and runs sh to open a shell.

You can put ANY executable as init file. You might also want to mount dev, sys and proc directories when the program starts using mount.h header. Last thing we need to do is to create the filesystem itself... To do so, run these commands inside _install directory.

find . -print0 | cpio --null -ov --format=newc | gzip -9 > ../initramfs.cpio.gz

This will make the file initramfs.cpio.gz in upper directory.

Testing The Compiled Kernel With QEMU

Before creating the ISO file let us check if the kernel is fine or not. To do so, we use QEMU. Just run the following command: (make sure that you change the path of bzImage and initramfs)

qemu-system-x86_64 -kernel bzImage -initrd initramfs.cpio.gz
qemu-system-x86_64 -kernel ../../linux-6.7.1/arch/x86/boot/bzImage -initrd ../initramfs.cpio.gz

Running our Linux in QEMU, we know that our kernel boots, we can create a bootable ISO for it.

Creating a Bootable ISO

Create a folder somewhere with any name. I name it iso, then create a folder called boot in it and inside boot create a folder called grub. Then copy bzImage and initramfs.cpio.gz into the boot folder.

mkdir iso
mkdir iso/boot
mkdir iso/boot/grub
cp ../linux-6.7.1/arch/x86_64/boot/bzImage boot/
cp ../busybox-1.36.1/initramfs.cpio.gz  boot/

Creating the Grub Config File

We will use grub-mkrescue to create our bootable ISO. But before doing so, we have to know if our current host is booted with UEFI or BIOS. To do so, check if the folder /sys/firmware/efi exists on your system or not. If it does, your computer uses UEFI otherwise it’s BIOS. So why knowing this is important? The grub-mkrescue uses the currently installed grub stuff to create the ISO image. This means that if your operating system is booted in BIOS, the chances are that the ISO created from grub-mkrescue does not support UEFI at all. In some cases, UEFI motherboards support booting BIOS images using CMS. But that is not always the case. If you want to make images for BIOS from UEFI host or vice versa, I suggest you to create a Debian virtual machine in VirtualBox. VirtualBox supports both BIOS and UEFI in it’smotherboard settings. After choosing the appropriate one, install Debian (net install is sufficient) and move the folder which contains boot and grub folders to virtual machine. Then continue reading the guide to configure the grub and create the ISO file.

Now we have to configure grub itself. Create a file named grub.cfg in grub folder of the boot folder. If your host is booted using BIOS (and thus the output ISO is BIOS too) put these lines in the config file:

set default=0
set timeout=10
menuentry 'myos' --class os {
insmod gzio
insmod part_msdos
linux /boot/bzImage
initrd /boot/initramfs.cpio.gz
}

If you are using UEFI, put these lines in it:

set default=0
set timeout=10
# Load EFI video drivers. This device is EFI so keep the
# video mode while booting the linux kernel.
insmod efi_gop
insmod font
if loadfont /boot/grub/fonts/unicode.pf2
then
insmod gfxterm
set gfxmode=auto
set gfxpayload=keep
terminal_output gfxterm
fi
menuentry 'myos' --class os {
insmod gzio
insmod part_msdos
linux /boot/bzImage
initrd /boot/initramfs.cpio.gz
}

At last run this command to create the ISO file. Replace the last argument with the folder name which you created at first step.

grub-mkrescue -o myos.iso iso/

Testing The ISO With VirtualBox

Before testing the ISO on a real computer, boot it in VirtualBox. To do so, make a new virtual machine and choose the ISO you have just created as the content of optical disk. Start the operating system. You must see the grub menu and then the operating system must boot. Do not forget the select EFI in motherboard settings if needed.