The user-space interface

Once upon a time, the ABI was simple
System calls

Now it is rather more complicated
Lots of system calls
/proc (100's of files)
/sys (1000's of files)
Netlink

Breaking this ABI is against the rules
The promise made by the kernel developers

But breaking it is increasingly easy to do
Changes in /sys can be surprising
The real shape of these ABIs is still changing