# What

This is a minimal set of software running with a stripped down Linux kernel to 
provide a very basic operating system for the x86_64 architecture. Currently, it 
boots to a stub init process which executes a rudimentary shell.

# Why

Mostly for educational purposes. In order to understand what is needed to build 
a basic Linux kernel and how it operates.

# How

See the Makefile. Basically, kernel sources are downloaded, verified and built 
with a minimal configuration (make tinyconfig). This should, in theory, switch 
off most non essential features in the kernel resulting in the smallest 
possible kernel. Necessary features are switched on explicitly with an 
architecture specific config file under kernel/. This produces a small compressed 
Linux kernel image (~2MB).

The tool source files are compiled as static ELF binaries and placed in a root 
filesystem structure. This filesystem is archived and compressed with cpio and 
gzip to provide an initramfs image. The image is unpacked and mounted as the 
root filesystem by the kernel during boot.

The kernel then runs /init as PID 1. Init forks and execs /bin/sh. This shell
is able to take user input and will attempt to exec it.

Testing is done on a virtualized system using QEMU. The kernel is compiled with 
EFI_STUB. This adds a PE header to the kernel which allows it to be loaded directly
by UEFI compliant firmware negating the need for a separate bootloader.

# Dependencies

The build process using the Makefile has the following dependencies:

* POSIX headers
* make
* gcc
  
Additionally the build process for the kernel requires:

* Linux headers
* ELF headers
* bash
* flex
* bison
* tar
* wget
* xz

# Acknowledgements

Heavily inspired by other, much better work:

    linux from scratch          https://www.linuxfromscratch.org/lfs/
    minimal linux live          https://github.com/ivandavidov/minimal
    mkroot                      https://github.com/landley/mkroot
    aboriginal linux            https://github.com/landley/aboriginal
    tldr linux from scratch     https://github.com/comfies/tldrlfs