Get file offset from a loaded DLL's function - c++

Question

I'd like to ask, how could I locate a specific (exported) function inside a DLL. For example I'd like to locate ReadProcessMemory inside Kernel32. I wouldn't like to rely on Import table, I'd like to locate different APIs based on their addresses what I get with a custom function.
I tried to make a small research on VA, RVA & File offsets, but I didn't succeed. Here's an example which I tried, but it isn't working (returns 0 in all cases):
DWORD Rva2Offset(DWORD dwRva, UINT_PTR uiBaseAddress)
{
WORD wIndex = 0;
PIMAGE_SECTION_HEADER pSectionHeader = NULL;
PIMAGE_NT_HEADERS pNtHeaders = NULL;
pNtHeaders = (PIMAGE_NT_HEADERS) (uiBaseAddress + ((PIMAGE_DOS_HEADER) uiBaseAddress)->e_lfanew);
pSectionHeader = (PIMAGE_SECTION_HEADER) ((UINT_PTR) (&pNtHeaders->OptionalHeader) + pNtHeaders->FileHeader.SizeOfOptionalHeader);
if (dwRva < pSectionHeader[0].PointerToRawData)
return dwRva;
for (wIndex = 0; wIndex < pNtHeaders->FileHeader.NumberOfSections; wIndex++)
{
if (dwRva >= pSectionHeader[wIndex].VirtualAddress && dwRva < (pSectionHeader[wIndex].VirtualAddress + pSectionHeader[wIndex].SizeOfRawData))
return (dwRva - pSectionHeader[wIndex].VirtualAddress + pSectionHeader[wIndex].PointerToRawData);
}
return 0;
}
Could you help me how could I accomplish this simple task?
Thank you.
P.s.: I'm not sticking to the function above, both if you can point out what's the problem, or give a better source would be awesome.

Answer 1

This gives you the relative virtual address
uintptr_t baseAddr = (uintptr_t)GetModuleHandle("nameOfExe.exe");
uintptr_t relativeAddr = functionAddress - baseAddr;
This converts relative virtual address to File offset:
DWORD RVAToFileOffset(IMAGE_NT_HEADERS32* pNtHdr, DWORD dwRVA)
{
int i;
WORD wSections;
PIMAGE_SECTION_HEADER pSectionHdr;
pSectionHdr = IMAGE_FIRST_SECTION(pNtHdr);
wSections = pNtHdr->FileHeader.NumberOfSections;
for (i = 0; i < wSections; i++)
{
if (pSectionHdr->VirtualAddress <= dwRVA)
if ((pSectionHdr->VirtualAddress + pSectionHdr->Misc.VirtualSize) > dwRVA)
{
dwRVA -= pSectionHdr->VirtualAddress;
dwRVA += pSectionHdr->PointerToRawData;
return (dwRVA);
}
pSectionHdr++;
}
return (-1);
}