why- Open BSD .rocks

stack-register checking (∞)

A memory object should have the fewest permissions possible: typically read, write and execute. OpenBSD introduced a new permission flag known as stack.

If you want to use memory as a stack, you must mmap it with that flag bit.

When a system call happens, we check to see if the stack-pointer register points to such a page. If not, the program is killed. The ABI is tighter as a result. You may no longer point your stack register at non-stack memory, or your program will die.