Question

How can I run some machine code from within my C program?

Lets say I have the instruction ‘B2’, how can I execute this? (note that the instructions will change at run time)

Answer 1

1. Load code into memory.
2. Either create a function pointer to this memory and call it (void (*foo)(void) = mmap(...), foo();) , or use inline assembly to “jmp” to the code.

Note that on newer systems you will need to make sure you have requested memory which does NOT have the NX (no execute) bit set. If NX is set, jumping to your code will produce a processor exception and your process will be killed.

On Linux this is an mmap flag, on Windows there are other means to request DEP-unprotected memory.

Your code should also not rely on fixed addresses, that is it should be position independent. You cannot guarantee the same load address.

If your code needs to call into your program, it is best to provide it a table via the function call where it can resolve function addresses of your executable or C library, or attempt to use the system linker (you might have some luck using ld.so functionality on Linux, but this is of course non-portable).