Get current Image Base address - c++

Question

I made some computations to get a relative virtual address(RVA).
I compute a correct RVA (according to the .map file) and I want to translate it to a callable function pointer.
Is there a way to translate it to a physical address?
I have thought of taking the image base address and add it. According to this thread it should be possible via GetModuleHandle(NULL), but the result is "wrong". I only have a good result when I subtract a pointer from a function from its RVA defined in the .map file.
Is there a WINAPI to either convert a RVA to a physical address, or get the image base address, or get the RVA of a physical address?
Here's some example code:
#include <stdio.h>
#include <Windows.h>
#include <WinNT.h>
static int count = 0;
void reference() // According to .map RVA = 0x00401000
{
}
void toCall() // According to .map RVA = 0x00401030
{
printf("Called %d\n", ++count);
}
int main()
{
typedef void (*fnct_t)();
fnct_t fnct;
fnct = (fnct_t) (0x00401030 + (((int) reference) - 0x00401000));
fnct(); // works
fnct = (fnct_t) (0x00401030 + ((int) GetModuleHandle(NULL)) - 0x00400000);
fnct(); // often works
return 0;
}
My main concern is that it seems that sometimes (maybe in threaded contexts) GetModuleHandle(NULL) isn't correct.

Answer 1

To get the image base without the entry point being predefined directly at compile-time you can do, a simple search from aligning eax#1000h from the current VA and looping until a valid PE signature 'MZ' is found in the current memory page.
Assuming the base address is not relocated into another PE image. I've prepared a function for you:
DWORD FindImageBase() {
DWORD* VA = (DWORD *) &FindImageBase, ImageBase;
__asm {
mov eax, VA
and eax, 0FFFF0000h
search:
cmp word ptr [eax], 0x5a4d
je stop
sub eax, 00010000h
jmp search
stop:
mov [ImageBase], 0
mov [ImageBase], eax
}
return ImageBase;
}