Call function from executable

Question

I want to call a function from an executable. The only way to reach that process is to inject a dll in the parent process. I can inject a dll in the parent process but how do I

Answer 1

If you are running inside the process, you need to know the offset of the function you want to call from the base of the module (the exe) which contains the function. Then, you just need to make a function pointer and call it.

// assuming the function you're calling returns void and takes 0 params
typedef void(__stdcall * voidf_t)();

// make sure func_offset is the offset of the function when the module is loaded
voidf_t func = (voidf_t) (((uint8_t *)GetModuleHandle('module_name')) + func_offset);
func(); // the function you located is called here

The solution you have will work on 32bit systems (inline assembly is not permitted in 64 bit) if you know the address of the function, but you'll need to make sure you implement the calling convention properly. The code above uses GetModuleHandle to resolve the currently loaded base of the module whose function you want to call.

Once you've injected your module into the running process ASLR isn't really an issue, since you can just ask windows for the base of the module containing the code you wish to call. If you want to find the base of the exe running the current process, you can call GetModuleHandle with a parameter of NULL. If you are confident that the function offset is not going to change, you can hard code the offset of the function you wish to call, after you've found the offset in a disassembler or other tool. Assuming the exe containing the function isn't altered, that offset will be constant.

As mentioned in the comments, the calling convention is important in the function typedef, make sure it matches the calling convention of the function you're calling.