Bryan Steele 
A draft proposal from Theo de Raadt on #OpenBSD tech@:"openat(2) is mostly useless, sadly."
To keep it simple, these calls were not designed to assist any security model.
[...]
Let's create directory fd's which cannot traverse upwards. Mark the object, instead of requiring a programmer to put a flag on every system call acting upon the object. Two operational flags are added, O_BELOW and F_BELOW.
Tim Chase