Another interesting thing about iOS- they enforce code signing on the page level. You cannot create a RW page, write new code (from the network) into it, and then remap it as RX; every instruction needs to be approved by Apple. So changing behavior at runtime in a way that's subtle enough to pass certification is a lot harder than it would be on some hypothetical walled-garden Android (where you can, AFAIK, pull down a JNI extension over HTTP and load it.)