Today, I would like to show something different, than usual reverse-engineering, that appears on my blog usually. I needed to prepare a Linux distro for myself to be able to run it on my PC. But not the ordinary operating system that we download from webpage, then use fancy graphical installer to select, what we want and where. My goals were very specific. First was to have it custom-compiled. With that in mind there aren’t many choices left (maybe Gentoo?). Second was to not cross 16 MiB boundary. Why exactly that? That’s simple. I have old (15 years old to be precise) SD/MMC card made for Canon of exactly that size. Quick check showed me that this is possible. I tried buildroot and it failed to fulfill second requirement and I decided not to continue, despite the obvious optimizations on kernel modules, I could do. It’s simply too complex for such a simple task. If not buildroot, then let’s go and see how to do such thing from scratch!
Basically the plan is to have custom Linux distro compiled from scratch. It may sound like something incredibly complex and hard to do. But it’s not. There are just few problems one must learn on how to overcome. The most problematic constraint in my case is, obviously, 16 MiB limit. To not exceed it, I have to use busybox as my userspace. This by the way simplifies distro development significantly. Busybox works the way, that, if linked statically, requires only one, single binary to be able to work correctly. So, to sum up, on software side, we need Linux and busybox. You may wonder, how do I want to boot that system, then? Well. I said I need Linux 🙂 Maybe some people know, some does not, that Linux is itself a boot loader of some kind. At least, when using UEFI and this is what I want to use, it can be loaded directly by UEFI firmware. But that’s another thing to note – I will describe a way to prepare a distro for UEFI – it won’t be as simple as that, for legacy BIOS.
The whole plan will look as follows:
- Get compiler
- Compile Linux kernel
- Compile busybox (statically and stripped!)
- Prepare initramfs with whole userspace
- Format drive as EFI System Partition
- Combine kernel and initramfs into single binary
- Optionally sign the binary, in case we want Secure Boot to be enabled
- Add entry to embedded UEFI boot manager
In the meantime, I am going to show few ways to debug the system, in case of any problems. Continue reading “Busybox-based Linux distro from scratch”