Bare Minimal Linux Kernel & RootFS

OS: Ubuntu 20.04 LTS

Linux Kernel: Version-6.5.7

BusyBox: Version-1.36.1

Creating Linux Kernel

Download

$ wget https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-6.5.7.tar.xz
$ tar -xvf linux-6.5.7.tar.xz
$ cd linux-6.5.7

Configure tiniest possible kernel

$ make allnoconfig

This will create .config file setting values to 'n' as much as possible.

Customization

$ make menuconfig 

This will open a window with many Linux kernel configuration settings. You can enable or disable those settings and customize the Linux kernel as needed.

Tips: use left, right, up and down arrow key to navigate

Now set following options-

Option 1: Enable 64 bit support

Enable 64 support

Option 2: Hostname

General setup >> Default hostname Set a Host name Embedded_linux

Option 3: Enable support for RAM disk

General Setup >> Initial RAM filesystem and RAM disk (initramfs/initrd) support

Option 4: Configure standard kernel features

General Setup > Configure standard kernel features (expert users)

Option 5: Ensure Gzip Kernel compression

General Setup >kernel compression mode (Gzip)

Option 6: ELF binary and script

Executable file formats > Kernel support for ELF binaries Executable file formats > Kernel support for scripts starting with #!

Option 7: Enable devtmpfs

Device Driver > Generic Driver Options > Maintain a devtmpfs filesystem to mount at /dev

Device Driver > Generic Driver Options > Automount devtmpfs at /dev, after the kernel mounted the rootfs

Option 8: Enable TTY

Device Driver > Character devices > Enable TTY

Option 9: Enable Serial Drivers

Device Driver > Character devices > Serial Drivers > 8250/16550 and compatible serial support

Device Driver > Character devices > Serial Drivers > Console on 8250/16550 and compatible serial port

Option 10: Pseudo filesystems

File systems > Pseudo filesystems > /proc file system support File systems > Pseudo filesystems > /sysfs file system support

Now, save and close the configuration window.

Building Linux Kernel

Here is the build command to build Linux kernel.

$ make -j4

Here make -j <number of cpu>, it will take 1.28min for me.

To see how many CPU Core or How mane processor you have type:

$ nproc

Your Linux kernel is now ready and can be found in the linux-6.5.7/arch/x86/boot directory.

$ cd linux-6.5.7/arch/x86/boot
$ ls -sh bzImage

Linux Kernel Size: 1.7MB

Create a working Directory and put the linux kernel image

$ mkdir -p ~/workspace_kernel/linux-kernel
$ cp linux-6.5.7/arch/x86/boot/bzImage ~/workspace_kernel/linux-kernel

Creating Initramfs

Downloading latest Busybox

$ wget https://busybox.net/downloads/busybox-1.36.0.tar.bz2

cd to workspace directory

$ cd ~/workspace_kernel

extracting the Busybox source tree

$ tar -xvf busybox-1.36.0.tar.bz2

Cd to busybox

$ cd busybox-1.33.1

Customize busybox

$ make menuconfig

This will start configuration menu for BusyBox. We need only one setting.

Settings > Build static binary (no shared libs)

Now, exit and save.

It is time to build busybox.

Build

$ make -j4
$ make -j <number of CPU core>

To see how many CPU Core or How mane processor you have type:

$ nproc

More general command

$ make -j ${nproc}

Install

$ make install

This will install binaries in “./_install” directory

# another command to install busybox in user specific directory
$ make CONFIG_PREFIX=$PWD/woris install

Create The RAM DISK Image

Cd to workspace directory

$  cd ~/workspace_kernel

creating embedded_linux directory and cd to embedded_linux

$ mkdir embedded_linux && cd embedded_linux

Now Craete etc, proc, sys and dev directory.

$  mkdir -p etc proc sys dev

Coping all busybox installed files to ~/workspace_linux/embedded_linux

$  cp -a <busybox install dir>/_install/* .

Create init script in “embedded_linux” directory. This is the content of init script.

$ cd ~/workspace_kernel/embedded_linux
$ vim init

#!/bin/sh
mount -t proc none /proc
mount -t sysfs none /sys
cat <<!
boot took $(cut -d' ' -f1 /proc/uptime) seconds
Welcome to EmbeddedCraft Mini Linux for Learners !!!
!
exec /bin/sh

It is time to make init file executable. Give executable permission to init file

$  chmod +x init

Creating initramfs as cpio archieve

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

Now the Directory structure look like

.
├── bin
├── dev
├── etc
├── init
├── initramfs.cpio.gz
├── linuxrc -> bin/busybox
├── proc
├── sbin
├── sys
└── usr

7 directories, 3 files

Booting Linux in QEMU

it is time to start QEMU and booting our mini Linux.

$ qemu-system-x86_64 \
-kernel <linux kernel dir>/arch/x86_64/boot/bzImage \
-initrd <busybox dir>/embedded_linux/initramfs.cpio.gz \
-append "init=/bin/sh console=ttyS0"  -nographic -no-reboot

if you download the zip archive so the command look like

$ qemu-system-x86_64 \
-kernel linux_kernel/bzImage -initrd embedded_linux/initramfs.cpio.gz \
-append "init=/bin/sh console=ttyS0" -nographic

You Successfully build a custom linux kernel & RootFS

Here some Command you may try

$ uname -a
$ cat /proc/cpuinfo
$ top
$ ls
$ cd
$ mkdir
$ grep
$ find

# typelinux commands

To kill qemu open a new terminal and type

$ killall qemu-system-x86_64

Project Screen Shots:

Reference:

• Mastering Embedded Linux Programming - Second Edition.pdf

• Tutorial: Building the Simplest Possible Linux System - Rob Landley, se-instruments.com

• landlay.net

• https://www.kernel.org/