How to modify the data in MDL?

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C++

Windows

driver

WDK

Hi all
I am trying to modify the data in MDL. I would like to change the default Header to my own header. My custom header size is lesser than default header. I don't want to disturb the packet data in Netbuffer. So here my requirement is to shrink NetBufferDataLength. How can I achieve this?
here I used following code: :

for(NetBuffer=NET_BUFFER_LIST_FIRST_NB (NetBufferLists);
             NetBuffer != NULL;
             NetBuffer = NET_BUFFER_NEXT_NB (NetBuffer))
 {
  // here test buffer is Modified data buffer.
 NdisChangeNetBuffer(NetBuffer,testBuffer, Buffersize);
 }

/* I am Trying observe the modified data here in following code:*/
/***** Converted Data ***************/
for(NetBuffer=NET_BUFFER_LIST_FIRST_NB (NetBufferLists);
             NetBuffer != NULL;
             NetBuffer = NET_BUFFER_NEXT_NB (NetBuffer))
 {
dataLength  = NET_BUFFER_DATA_LENGTH (NetBuffer);
DbgPrint("Converted dataLength= %d", dataLength);      // here expecting a modified datalength but not happening
tempData=(PUCHAR)NdisAllocateMemoryWithTagPriority(											 FilterDriverHandle,				(UINT)dataLength,											 Tag,								NormalPoolPriority
												);
NdisQueryNetBuffer(NetBuffer, tempData, &BytesCopied);
DbgPrint("Bytes copied in converted data= %d", BytesCopied);
DbgPrint("Converted Data");
for(count=0;count<BytesCopied;count++)
{ DbgPrint("%d---> 0x%x \t",count, tempData[count]);
}	
 NdisFreeMemory((PVOID)tempData, dataLength, 0);
  tempData = NULL;
 }

/**************************************************************/



NDIS_STATUS
NdisChangeNetBuffer(
	IN  PNET_BUFFER         NetBufferToSend,
	IN  PUCHAR		Modified_data,
	IN  ULONG               BufferLength
	)
  {

   // Here Modified data is a buffer having OwnHeader + Packetdata
   // BufferLength is the whole dataLength of OwnHeader+Packetdata
    NDIS_STATUS     ndisStatus = NDIS_STATUS_SUCCESS;
    PNET_BUFFER     CurrentNetBuffer;
    ULONG           MdlOffset;
    PMDL            Mdl, MdlNext;
    ULONG           dataLength;
	ULONG			exit = 0;
	PMDL            NewMdl = NULL;
	ULONG           ArrayOffset=0;
	SIZE_T MdlByteCount, Remain, CopySize;
   
    do
    {
		CurrentNetBuffer = NetBufferToSend;
        Mdl    = NET_BUFFER_FIRST_MDL (CurrentNetBuffer);
        MdlOffset      = NET_BUFFER_DATA_OFFSET (CurrentNetBuffer);
		
		 //
    // Skip the required offset bytes in the MDL chain.
    //

		while ((Mdl != NULL) && (MdlOffset >= (MdlByteCount = MmGetMdlByteCount(Mdl))))
		{
			MdlOffset -= MdlByteCount;
			Mdl = Mdl->Next;
		}
		for (Remain = BufferLength; Mdl && (Remain > 0); Mdl = Mdl->Next)
		{
            
			PUCHAR SysVa = NULL ;
            MdlNext = Mdl->Next;
			//
			// Skip zero length MDLs.
			//

			MdlByteCount = MmGetMdlByteCount (Mdl);

			if (MdlByteCount == 0)
			{
				continue;
			}

			//
			// Compute the amount to transfer this time.
			//
			ASSERT (MdlOffset < MdlByteCount);

			MdlByteCount -= MdlOffset;
			CopySize = min (Remain, MdlByteCount);
			//
			// Perform the transfer, either directly or using our reserved PTE.
			//
			if(MdlNext!=NULL)
			{
				SysVa = MmGetSystemAddressForMdlSafe (Mdl, NormalPagePriority);
			}
			
			if(MdlNext == NULL)
			{
				NewMdl = NdisAllocateMdl(FilterDriverHandle, (PVOID)(Modified_data+ArrayOffset), CopySize);
			}
			
			else
			{
				RtlCopyMemory ( SysVa + MdlOffset, Modified_data + ArrayOffset, CopySize);
			}
			MdlOffset = 0;

			ArrayOffset += CopySize;
   
			Remain -= CopySize;
			if(MdlNext == NULL)
			{
			   Mdl = NewMdl;
			}
		}
    } while(exit);
	return ndisStatus;

 }

Here I am expecting a new dataLength as lesser value than actual value in converted data. But that is not happing. It is still pointing to original MDL? Why? Where i did mistake?
Thanks