user5406764 user5406764 - 1 month ago 6
C Question

How to call machine code stored in char array?

I'm trying to call native machine-language code. Here's what I have so far (it gets a bus error):

char prog[] = {'\xc3'}; // x86 ret instruction

int main()
{
typedef double (*dfunc)();

dfunc d = (dfunc)(&prog[0]);
(*d)();
return 0;
}


It does correctly call the function and it gets to the ret instruction. But when it tries to execute the ret instruction, it has a SIGBUS error. Is it because I'm executing code on a page that is not cleared for execution or something like that?

So what am I doing wrong here?

Answer

One first problem might be that the location where the prog data is stored is not executable.

On Linux at least, the resulting binary will place the contents of global variables in the "data" segment or here, which is not executable in most normal cases.

The second problem might be that the code you are invoking is invalid in some way. There's a certain procedure to calling a method in C, called the calling convention (you might be using the "cdecl" one, for example). It might not be enough for the called function to just "ret". It might also need to do some stack cleanup etc. otherwise the program will behave unexpectedly. This might prove an issue once you get past the first problem.