Linux Kernel Development

This is a complete guide, which walks through the complete process of building updated versions of a custom Linux kernel with BusyBox, creating an initramfs, and booting it using QEMU. This is useful for kernel development, testing, and learning about Linux internals.

Prerequisites

• Ubuntu 24.04
• Git
• Basic development tools (gcc, make, etc.)
• Approximately 10GB of free disk space

Recommended to have SSH and VS Code.

I've used 16 CPU cores 16GB RAM and 64GB disk in VM on VMWare Workstation.

1. Install Cross Compiler

Install the cross compiler (if not already installed):

# Install development packages
sudo apt install build-essential flex bison libssl-dev libelf-dev libncurses5-dev libncursesw5-dev

# Install the cross compiler on Ubuntu/Debian
sudo apt install gcc-x86-64-linux-gnu binutils-x86-64-linux-gnu

2. Download the kernel and BusyBox source codes:

Download Linux kernels using wget:

# Latest stable for now:
wget https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-6.16.tar.xz
# Latest LTS for now:
wget https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-6.12.40.tar.xz
# LTS 6.6 (6.8+ have some problem with BusyBox for now [1.36.1 and 1.37.0])
wget https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-6.6.100.tar.xz
# LTS 5.15:
wget https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.15.189.tar.xz

And BusyBox:

# Latest stable for now:
wget https://busybox.net/downloads/busybox-1.36.1.tar.bz2

Other for using with the image:

# Create utilities directory:
mkdir utilities
cd utilities

# Init script:
wget https://github.com/Arlu/Linux-Kernel-Development/raw/refs/heads/main/_init

## Leave:
cd ..

3. Build the Linux Kernel

Navigate to the Linux source directory and build the kernel:

# Extract some version (we use 5.15):
tar -xf linux-5.15.189.tar.xz
cd linux-5.15.189
# Or with version 6.16:
tar -xf linux-6.16.tar.xz
cd linux-6.16

# Set the cross-compiler (if you're compiling on x86_64 for x86_64)
export CROSS_COMPILE=x86_64-linux-gnu-

# Configure the kernel for x86_64
make x86_64_defconfig

# Build the kernel (adjust the number based on your CPU cores)
make -j$(nproc)

# Verify the kernel image is created
ls arch/x86/boot/bzImage

# Make headers for using with BusyBox later (skip for 6.8 and later):
make headers_install INSTALL_HDR_PATH=~/headers/linux-5.15

# Back to home folder:
cd ..

4. Download and Build BusyBox

BusyBox provides the essential user space utilities:

# Extract some version:
tar -xf busybox-1.36.1.tar.bz2
cd busybox-1.36.1

# Install required ncurses libraries if missing
# sudo apt-get install libncurses5-dev libncursesw5-dev

# Create a default configuration
make defconfig

# Configure BusyBox to build statically
make menuconfig
# In the menuconfig interface:
# Navigate to "Settings" → Find "Build static binary (no shared libs)"
# Press 'Y' to enable it
# (For 6.7 and earlier)
# Then navigate to "() Additional CFLAGS"
# Press enter, then add: -I/home/arie/headers/linux-5.15/include/
# (For 6.8 and later)
# Just disable some, press esc, then navigate to "Networking Utilities" → Find "tc (8.3 kb)"
# Press 'N' to disable it (because it isn't support by these kernels for now)
# Exit and save the configuration

# Build BusyBox
make -j$(nproc)

# Install BusyBox
make install

# Back to home folder:
cd ..

5. Create an Initramfs

Now create a minimal filesystem for the kernel to boot:

# Create a dedicated directory
mkdir -p /tmp/initramfs
cd /tmp/initramfs

# Create the basic directory structure
mkdir -p {bin,sbin,etc,proc,sys,dev,usr/{bin,sbin},root}

# Copy the BusyBox binary:
cp ~/busybox-1.36.1/busybox bin/

# Create essential symlinks
cd bin
ln -s busybox sh
cd ..

# Create a proper init script with explicit BusyBox paths
cp ~/utilities/_init ./init

# Make init executable
chmod +x init

# Create basic configuration files
cat > etc/passwd << EOF
root::0:0:root:/root:/bin/sh
EOF

cat > etc/group << EOF
root:x:0:
EOF

5. Package the Initramfs

Create the compressed initramfs image:

# Create images folder:
mkdir ~/images

# Package the initramfs with a clear filename to avoid confusion
find . -print0 | cpio --null -ov --format=newc | gzip -9 > ~/images/initramfs.cpio.gz

# Return to the project directory
cd ~/images/

6. Boot with QEMU

Finally, boot your custom Linux kernel with the initramfs using QEMU:

# Installation
sudo apt install virt-manager
Or
sudo apt-get install qemu-system-x86 qemu-utils

# Start QEMU with your kernel and initramfs
cd ~/images/
qemu-system-x86_64 -kernel ~/linux-5.15.189/arch/x86/boot/bzImage -initrd initramfs.cpio.gz -append "console=ttyS0 nokaslr" -nographic -m 512M
# Or:
qemu-system-x86_64 -kernel ~/linux-6.16/arch/x86/boot/bzImage -initrd initramfs.cpio.gz -append "console=ttyS0 nokaslr" -nographic -m 512M

7. Working with Your Custom Linux System

Basic Usage

After booting, you'll see a shell prompt (/ #). You can now use the various BusyBox commands.

Exiting QEMU

To exit the QEMU session:

• Press Ctrl+A followed by x