[vbezhenar]

There's a security feature called W^X [0] (also called DEP in Windows). Basically you can use special mode which prevents memory pages to be writeable and executable at the same time, so self-modifying code is not allowed, but it prevents exploits from modifying memory containing executable code. OpenBSD uses it as well.

0: https://en.wikipedia.org/wiki/W%5EX

[Avery3R]

That's not what DEP is, but it is dependent on DEP. DEP is just another word for the NX bit, that allows a page to be marked as non-executable. With DEP you can still have a page that is RWX if you set its permissions that way.