The most trivial incarnation of return pointer protection consists of XORing a fixed cookie into the return pointer. XORing the cookie with the pointer before it is saved onto the stack and again after it is popped off preserves the legitimate pointer but will distort any attack.
By setting one or both of the two least significant bits (LSBs) in the cookie, the XOR not only inhibits exploit, it also can detect a corrupt return pointer. If an attacker does not know which of the LSBs are set on the stack, the corrupt return pointer will cause an alignment fault unless the attacker gets lucky. Even if each state of the LSBs are tried, the interaction with the remainder of the cookie shound hinder the predictable operation of a corrupted return pointer.
A Per-Kernel XOR cookie can be built into OpenBSD by adding about a dozen assembler instructions. A sign-extended 13-bit cookie can be built into the kernel as an immediate operand that will cost one extra instruction per window pushed and another instruction when the window is popped.
The Per-Kernel XOR cookie can also be trivially defeated. It is constant for every process on the system. The cookie can be determined if an attacker is able to run arbitrary programs. Even without a priori knowledge of the cookie, an attacker could use a large shell code sled to slide into the main exploit code .
The Per-Kernel XOR cookie will not be enough to stop a competent attacker.