This started out as a personal project to build a very small Linux based operating system that has very few moving parts but is still very complete and useful. Along the way of figuring out how to get the damn thing to boot and making it do something useful I learned quite alot. Too much time has been spent reading very old and hard to find documentation, or when there was none the source code of how other people were doing it. So I thought, why not share what I have learned.

The Linux Kernel

The core component of our operating system, the kernel, that which manages the processes and talks to the hardware on our behalf. You can retrieve a copy of the source code easily from kernel.org. There are multiple versions to choose from, choosing one is usually a tradeoff between stability and wanting newer features. If you look at the Releases tab, you can see how long each version will be supported and keeps receiving updates. So you can usually just apply the update or use the updated version without changing anything else or having something break.

So just pick a version and download the tar.xz file and extract it with tar -xf linux-version.tar.xz. To build the kernel we obviously need a compiler and some build tools. Installing build-essential on Ubuntu (or base-devel on Arch Linux) will almost give you everything you need. You'll also need to install bc for some reason.

The next step is configuring you build, inside the untarred directory you do make defconfig. This will generate a default config for your currect architecture and place it in .config. You can edit it directly with a text editor but it's much better to do it with an interface by doing make nconfig (this needs libncurses5-dev on Ubuntu). Here you can enable/disable features and device drivers with the spacebar. * means that it will be compiled in your kernel image. M means it will be compiled inside a seprate kernel module. Which is a part of the kernel that will be put in a seperate file and can be loaded in dynamically in the kernel when they are required. The default config will do just fine for basic stuff like running in a virtual machine. But in our case, we don't really want to deal with kernel modules so we'll just do this: sed "s/=m/=y/" -i .config. Building the kernel is now just running make. Don't forget to add -jN with N the number of cores of this might take a while.

Other useful/interesting ways to configure the kernel are:

make localmodconfig will look at the modules that are currently loaded in the running kernel and change the config so that only those are enabled as module. Useful for when you only want to build the things you need without having to figure out what that is. % TODO LSMOD and caveat
make localyesconfig,the same as above but everything gets compiled in the kernel instead as a kernel module.
make allmodconfig generates a new config where all options are enabled and as much as possible as module.
make allyesconfig, same as above but with everything compiled in the kernel.
make randconfig generates a random config...