A proposal for preventing stack buffer overflow attacks is based on making a backup copy of the return address when a function starts. The backup copy is written to a shadow stack located at some random location L on the heap. In the function epilog, just before the function is about to return, the backup copy of the return address is compared to the return address on the stack and if they differ the program exits. Otherwise, the return instruction is executed normally.

(a) Explain why this mechanism can make it harder to mount a stack buffer overflow attack.

(b) Give sample C code that is vulnerable to a stack buffer overflow attack even if this mechanism is used. For example, consider the case of function pointers stored on the stack.