CODEWITHSUNDEEP
access-violation
Viotto01
Viotto01

Reputation: 31

LdrLoadDll problem

I am trying to code an alternative to LoadLibrary function, based on the idea of calling the function LdrLoadDll from ntdll. This function needs as a parameter the dll file to load, in a UNICODE_STRING format. I really can't get what I am doing wrong here (string seems to be correctly initialized), but when LdrLoadDll is called, I get the following error:

Unhandled exception in "Test.exe" (NTDLL.DLL): 0xC0000005: Access Violation.

I use Visual C++ 6.0 for this test, and I am using Windows 7 64 bit.

I post full code here, thanks in advance for any help:

#include <Windows.h>


typedef LONG NTSTATUS; //To be used with VC++ 6, since NTSTATUS type is not defined



typedef struct _UNICODE_STRING { //UNICODE_STRING structure
  USHORT  Length;
  USHORT  MaximumLength;
  PWSTR  Buffer;
} UNICODE_STRING;
typedef UNICODE_STRING *PUNICODE_STRING;




typedef NTSTATUS (WINAPI *fLdrLoadDll) //LdrLoadDll function prototype 
    (
         IN PWCHAR PathToFile OPTIONAL,
         IN ULONG Flags OPTIONAL, 
         IN PUNICODE_STRING ModuleFileName, 
         OUT PHANDLE ModuleHandle 
    ); 





/**************************************************************************
  *      RtlInitUnicodeString   (NTDLL.@)
  *
  * Initializes a buffered unicode string.
  *
  * RETURNS
  *  Nothing.
  *
  * NOTES
  *  Assigns source to target->Buffer. The length of source is assigned to
  *  target->Length and target->MaximumLength. If source is NULL the length
  *  of source is assumed to be 0.
  */
 void WINAPI RtlInitUnicodeString(
     PUNICODE_STRING target, /* [I/O] Buffered unicode string to be initialized */
     PCWSTR source)          /* [I]   '\0' terminated unicode string used to initialize target */
 {
     if ((target->Buffer = (PWSTR) source))
     {
         unsigned int length = lstrlenW(source) * sizeof(WCHAR);
         if (length > 0xfffc)
             length = 0xfffc;
         target->Length = length;
         target->MaximumLength = target->Length + sizeof(WCHAR);
     }
     else target->Length = target->MaximumLength = 0;
 }



NTSTATUS LoadDll( LPCSTR lpFileName)
{
  HMODULE hmodule = GetModuleHandleA("ntdll.dll");
  fLdrLoadDll   _LdrLoadDll = (fLdrLoadDll) GetProcAddress ( hmodule, "LdrLoadDll" );

  int AnsiLen = lstrlenA(lpFileName);
  BSTR WideStr = SysAllocStringLen(NULL, AnsiLen);
  ::MultiByteToWideChar(CP_ACP, 0, lpFileName, AnsiLen, WideStr, AnsiLen);

  UNICODE_STRING usDllFile;
  RtlInitUnicodeString(&usDllFile, WideStr); //Initialize UNICODE_STRING for LdrLoadDll function
  ::SysFreeString(WideStr);

  NTSTATUS result = _LdrLoadDll(NULL, LOAD_WITH_ALTERED_SEARCH_PATH, &usDllFile,0); //Error on this line!

  return result;
}



void main()
{
  LoadDll("Kernel32.dll");
}

Upvotes: 1

Views: 9059

Answers (2)

user3409863
user3409863

Reputation: 43

you can't call SysFreeString before you call _LdrLoadDll, since the usDllFile.buffer parameter points to this string

Upvotes: -1

pepek
pepek

Reputation: 31

in _LdrLoadDll(NULL, LOAD_WITH_ALTERED_SEARCH_PATH, &usDllFile,0); last parameter can't be zero

Upvotes: 3

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