Question

I was experimenting with the following code to simulate GetProcAddress.

// Retrieve NT header from base address.
IMAGE_NT_HEADERS *GetNtHeaderFromBase( void *pBaseAddr )
{
 IMAGE_DOS_HEADER       *pDosHeader;
 IMAGE_NT_HEADERS       *pNtHeaders;

 pDosHeader = ((IMAGE_DOS_HEADER *)pBaseAddr);
 if(pDosHeader->e_magic != 0x5A4D)
  return NULL;

 pNtHeaders = ((IMAGE_NT_HEADERS *)((DWORD)pBaseAddr + pDosHeader->e_lfanew));
 if(pNtHeaders->Signature != 0x4550)
  return NULL;

 return ((pNtHeaders == NULL) ? NULL : pNtHeaders);
}


// This emulates GetProcAddress.
void *GetFuncAddr( DWORD pBaseAddr, char *lpszFuncName ) 
{
 IMAGE_NT_HEADERS       *pNtHeaders;
 IMAGE_DATA_DIRECTORY   *pDataDir;
 IMAGE_EXPORT_DIRECTORY *pExportDir;
 const char      **lpszNames;
 DWORD       *lpdwFuncs, dwIndex;

 pNtHeaders = GetNtHeaderFromBase((void *)pBaseAddr);
 if(pNtHeaders == NULL)
  return NULL;

 pDataDir = ((IMAGE_DATA_DIRECTORY *)(pNtHeaders->OptionalHeader.DataDirectory + IMAGE_DIRECTORY_ENTRY_EXPORT));
 if(pDataDir == NULL)
  return NULL;

 pExportDir = ((IMAGE_EXPORT_DIRECTORY *)(pBaseAddr + pDataDir->VirtualAddress));
 if(pExportDir == NULL)
  return NULL;

 lpdwFuncs  = ((DWORD *)(pBaseAddr + pExportDir->AddressOfFunctions));
 lpszNames  = ((const char **)(pBaseAddr + pExportDir->AddressOfNames));
 if(lpdwFuncs == NULL || lpszNames == NULL)
  return NULL;

 for(dwIndex = 0; dwIndex < pExportDir->NumberOfFunctions; dwIndex++)
 { 
  // decrypt funcname and get the address
  if(!cmpstr(((char *)(pBaseAddr + lpszNames[dwIndex])), lpszFuncName))
   return (void*)(pBaseAddr + lpdwFuncs[dwIndex]);
 }

 return NULL;
}

But when I run the program on a Windows Vista x64 I get an access violation. I thought it was the numbers on GetNtHeaderFromBase (the numbers passed on IMAGE_DOS_HEADER and IMAGE_NET_HEADER) but i can t find any reference anywhere as to what it might be on a x64 bit binary.

Anyone has any idea what changes do I need to make to this code to make it work under 64 bit Windows? Or, a better way to achieve a getprocaddress-like function that works on both x32 and x64?

Thank you for the help and the code.

jess.

Answer 1

The reason why is that you re storing a pointer sized value in a DWORD for the pBaseAddr method. Pointer values are 8 bytes on 64 bit windows while a DWORD is only 4 bytes. You need to do one of the following

Pass pBaseAddr as a pointer (preferred approach)
Pass it as a size_t which will correctly be 4 bytes on 32 bit windows and 8 bytes on 64 bit windows.

Answer 2

I m not sure if it s your only problem, but DWORD is 32 bits, even on 64-bit platforms. You re casting pointers to this, and on 64 bit that results in truncation. If you want a pointer-sized integer you should use DWORD_PTR or ULONG_PTR.

I would also check that your structures are 64-bit safe in that regard.

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