Booting a Minimal Linux Kernel on Ubuntu 24

This post documents my process of manually downloading, configuring, and compiling Linux kernel (v6.12) on an Ubuntu 24.04 machine.

Installing Required Tools

To compile the Linux kernel, several development tools and libraries are needed. You can install them with the apt package manager:

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

Brief explanation of each package:

• build-essential: Includes gcc, make, and other essential build tools
• flex, bison: Needed for building the configuration menu
• bc: A math utility used in kernel Makefiles
• libncurses-dev: Provides TUI interface for make menuconfig
• libssl-dev, libelf-dev: Required by some optional modules

Downloading the Linux Kernel Source Code

I chose the latest mainline version, v6.12. Use the following command to clone it:

git clone --depth=1 --branch=v6.12 git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
cd linux

The --depth=1 option fetches only the latest commit, which makes the download faster.

Generating a Minimal Configuration with tinyconfig

To quickly produce a minimal working kernel, I used tinyconfig:

make tinyconfig

This generates a .config file that enables only the most essential kernel features.

Compiling the Linux Kernel

Now compile the kernel:

make -j$(nproc)

$(nproc) uses all available CPU cores for faster build.

After compilation, the output image will be located at:

./arch/x86/boot/bzImage

bzImage is a bootable Linux kernel image, short for “big zImage”. It is a self-extracting compressed format mainly used for x86/x86_64 architectures.

You can check its file type with:

file arch/x86/boot/bzImage

Booting the Kernel with QEMU

First, install QEMU:

sudo apt install qemu-system-x86

Then boot the kernel:

qemu-system-x86_64 -kernel arch/x86/boot/bzImage

Unfortunately, the screen only showed:

Booting from ROM...

And nothing else followed—no kernel messages appeared. This usually means the kernel lacks the drivers necessary to output to the display.

Tip: After clicking into the QEMU window, your mouse will be captured. Press Ctrl + Alt + G to release it.

Configuring menuconfig

After closing the QEMU window, run menuconfig to modify kernel settings (make sure libncurses-dev is installed):

make menuconfig

Enable TTY Support

Navigate to:

Device Drivers  --->
  Character devices  --->
    <*> Enable TTY

Use arrow keys to navigate and press Y or the spacebar to enable the option.

This setting enables support for TTY devices (i.e., text console and serial output).

Enable printk Support

Also enable the kernel logging mechanism:

General setup  --->
  Configure standard kernel features (expert users)  --->
    [*] Enable support for printk

Save your configuration and recompile:

make -j$(nproc)
qemu-system-x86_64 -kernel arch/x86/boot/bzImage

Now the terminal finally shows kernel boot messages. Although it ends with:

panic - not syncing: No working init found

we have successfully booted the kernel and verified output is working.

To fix this panic, you need to provide a valid init process (e.g., systemd or busybox). This will not be covered in this post.

If you want to reset your kernel configuration to a clean tinyconfig state for further experiments, use:

make mrproper

References

• Building a tiny Linux from scratch