controlflow.c

#include <iostream>
#include <functional>
#include <memory>
#include <cstdio>

void* _saved_return;
void* _saved_bp;
#define NO_INLINE __attribute__((noinline))

#define SET_OLD_RBP()                     \
   asm("movq (%%rbp),%%rbp"               \
     :                                      \
     :                                  \
    );
    
#define GET_RETURN_VALUE64(VAR)  \
    asm ("movq %%rax,%0"         \
         : "=r" (VAR)            \
         :                       \
         : "rax"                 \
         );


  // "movq 0x16(%%rbp),%1\n"                   
#define STASH_RETURN_ADDRESS()                    \
    asm("movq 0x8(%%rbp),%0"                    \
     : "=r" (_saved_return)                        \
    ); 

#define UNSTASH_RETURN_ADDRESS()           \
    asm(                                    \
        "pushq (%0)\n"                       \
        "pushq (%1)\n"                       \
     :                                     \
     : "r" (&_saved_bp), "r" (&_saved_return)                \
    );


#define GET_RETURN_ADDRESS(LEVEL) \
        __builtin_return_address(LEVEL)

#define SET_RETURN_ADDRESS(LEVEL, ADDR)   \
    asm("movq %0,0x8(%%rbp)"              \
     :                                    \
     : "r" (ADDR)                         \
    );

uint64_t NO_INLINE ret_trampoline() {
    UNSTASH_RETURN_ADDRESS();
    uint64_t ret;
    GET_RETURN_VALUE64(ret);
    return ret;
}

int NO_INLINE foo () {
    SET_OLD_RBP();
    STASH_RETURN_ADDRESS()
    SET_RETURN_ADDRESS(0, &ret_trampoline);
    return 1234;
}

int main ( void ) {
    uint64_t ret = foo();
    std::cout << "foo() = " << ret;
    return 55;
}