Comment by JoshTriplett

> You look at Linux's syscall table[0], read through the documentation to figure out the arguments, data types, flags, return values, etc., and then implement that in your kernel.

As well as some subset of the files expected in /dev, /proc, /sys, and similar, which are also part of the userspace ABI. And the startup mechanisms for processes, and the layout of AUXV...

It's absolutely doable, but the interface is wider than just the syscall layer.