Basic Concepts for Code

Everything is a system call
All the OS kernel offers, code-wise, is system calls
A program is a linear sequence of instructions and system calls
And a program is nothing more than a data file being run

The system call interface is very simple and low-level
It relies on some hardware mechanism for privilege escalation
All arguments are passed through registers
Errors are standardized in a few dozens integer codes
Most other items are integers as well: __NR_*, fd, uid, time, ...
Complex parameter passing, if any, happens through pointers

Mechanism, not policy
Any user-visible mapping of those integers is policy
The kernel interface includes as little policy as possible
All policy choices (and blame) are left to upper levels