Littinit: little init programs

A simple init process designed to launch a shell or other processes on the console tty after the Linux kernel boots. Compared to using sh as the init process, Littinit automatically mounts directories like proc, sys, and dev during startup and configures the tty device, avoiding the no job control in this shell warning.

Build

cargo b --release

The default target is x86_64-unknown-linux-musl. You can also specify other targets using the --target option, but it is recommended to use a musl-based libc to obtain a statically linked Littinit directly.

cargo b --release --target aarch64-unknown-linux-musl

Then you will have a dependency-free executable:

ldd target/x86_64-unknown-linux-musl/release/littinit

-> statically linked

Testing in QEMU

To test using QEMU with an Ubuntu Base rootfs, prepare the following files in advance:

• Linux kernel: bzImage
• Ubuntu Base: ubuntu-base-22.04-base-amd64.tar.gz
• QEMU

Installing Dependencies

# for ubuntu-base
sudo apt install -y qemu-system-x86 cpio wget tar gzip

# for linux kernel
sudo apt install -y make clang llvm lld flex bison bc automake libelf-dev libssl-dev

Downloading Ubuntu Base

mkdir ubuntu-rootfs

wget -qO - https://cdimage.ubuntu.com/ubuntu-base/releases/22.04/release/ubuntu-base-22.04-base-amd64.tar.gz | tar -C ubuntu-rootfs -xvzf -

After completion, the directory structure will look like this:

tree -L 1 ubuntu-rootfs/

ubuntu-rootfs/
├── bin -> usr/bin
├── boot
├── dev
├── etc
├── home
├── lib -> usr/lib
├── lib32 -> usr/lib32
├── lib64 -> usr/lib64
├── libx32 -> usr/libx32
├── media
├── mnt
├── opt
├── proc
├── root
├── run
├── sbin -> usr/sbin
├── srv
├── sys
├── tmp
├── usr
└── var

21 directories, 0 files

Copying Littinit as the Init Process

cp target/x86_64-unknown-linux-musl/release/littinit ubuntu-rootfs/init

Packing the Initrd

find ubuntu-rootfs -printf "%P\n" | cpio -D ubuntu-rootfs -R root:root -H newc -o -F ubuntu-rootfs.cpio

Compiling the Linux Kernel

1. Download the source code

git clone https://github.com/torvalds/linux.git -b v6.14 --depth 1

2. Compile the kernel

export LLVM=1
make -C linux O=../linux-build defconfig
make -C linux O=../linux-build bzImage -j$(nproc)

he compiled kernel file will be located at linux-buildarch/x86/boot/bzImage

Running the Kernel and Littinit with QEMU

qemu-system-x86_64 --nographic \
-smp $(nproc) -m 2G \
-kernel linux-build/arch/x86/boot/bzImage \
-initrd ubuntu-rootfs.cpio \
-append "console=ttyS0"

The output will show that Littinit has successfully launched Bash

......
Trying run /bin/bash...
......
root@(none):/# ps
  PID TTY          TIME CMD
    1 ttyS0    00:00:01 bash
  131 ttyS0    00:00:00 ps
root@(none):/#