串口过滤的简单问题

我写了一个过滤驱动，可是在Attach的时候出现问题。代码如下:

估计在ObDereferenceObject(pTargetFileObj);出现问题因为这个调用好像没起作用，如果这里不调用，在UnLoad中调用就好用，不过在
UnLoad之前Com口就无法打开。如果先ObReferenceObject(pTargetDevice)在UnLoad事就重启，别的代码好像也是如此，可我怎么不行，急死了，各位帮忙。
NTSTATUS FilterAttach(PDRIVER_OBJECT pDriverObject,
 PUNICODE_STRING pTargetDeviceName,
 ACCESS_MASK DesiredAccess,
 PDEVICE_OBJECT* ppNewDeviceObject)
{
NTSTATUS status=STATUS_SUCCESS;
PDEVICE_OBJECT pDevObj=NULL;
PDEVICE_OBJECT pLowerDevice=NULL;
PDEVICE_OBJECT pTargetDevice=NULL;
    PFILE_OBJECT pTargetFileObj=NULL;
PDEVICE_EXTENSION pDevExt=NULL;

status=IoGetDeviceObjectPointer(
pTargetDeviceName,
DesiredAccess,//FILE_ALL_ACCESS,
&pTargetFileObj,
&pTargetDevice
);
if(!NT_SUCCESS(status))
{
return status;
}

status=IoCreateDevice(
pDriverObject,
sizeof(DEVICE_EXTENSION),
NULL,
pTargetDevice->DeviceType,//FILE_DEVICE_UNKNOWN,
pTargetDevice->Characteristics,//0,
FALSE,
&pDevObj
);
if(!NT_SUCCESS(status))
{
*ppNewDeviceObject=NULL;
ObDereferenceObject(pTargetFileObj);
return status;
}
*ppNewDeviceObject=pDevObj;
pDevObj->Flags&=~DO_DEVICE_INITIALIZING;

//Initialize the device extension
pDevExt=pDevObj->DeviceExtension;
RtlZeroMemory(pDevExt,sizeof(DEVICE_EXTENSION));
// Fill device extension
pDevExt->Num=0;
pDevExt->bFirstTime=TRUE;
//
pLowerDevice=IoAttachDeviceToDeviceStack(pDevObj,pTargetDevice);
if(!pLowerDevice)
{
ExDeleteResourceLite(&pDevExt->eres);
ObDereferenceObject(pTargetFileObj);
IoDeleteDevice(pDevObj);
return STATUS_DEVICE_REMOVED;//STATUS_IO_DEVICE_ERROR
}
ObDereferenceObject(pTargetFileObj);
pTargetFileObj=NULL;

// Fill device extension
pDevExt->pDevce=pDevObj;
pDevExt->DeviceType=VIRTUALPORT_DEVICE_FILTER_TYPE;
pDevExt->pLowerDevice=pLowerDevice;
pDevExt->pTargetDevice=pTargetDevice;
pDevExt->pTargetFileObj=pTargetFileObj;

return status;
}

ObDereferenceObject不能在这调用
估计问题出在DesiredAccess上，一般应该是FILE_READ_ATTRIBUTES，而不是FILE_ALL_ACCESS

自己解决了，代码有问题，很低级的错误。