Windows File System Filter Driver Example

This package installs the software AMD USB Filter Driver to enable the following device. Standard OpenHCD USB Host Controller Standard Enhanced PCI to USB Host.

Windows 8 Preview: File System Changes The NT Insider, Vol 18, Issue 3, Sept-Oct 2011 Published: 17-Oct-11 Modified: 17-Oct-11. W ith the recent release of the.

File systems virtualization in windows using mini filter drivers

img class slide_image src 1376326848 alt CSE 524 Project Report

File system virtualization inside Windows system using mini-filter drivers.

Nikhil Pujari

SUNY SB I

img class slide_image src 1376326848 alt The DriverEntry routine is called when the mini-filter is loaded which registers the callbacks for the

respective IRPs and

img class slide_image src 1376326848 alt Sparse files and Reparse points:

I employ the following facilities provided by the Windows NTFS file system for the implem

img class slide_image src 1376326848 alt matched by corresponding FltReleaseContext function to decrease the reference count whenever the

context has been used.

Tw

img class slide_image src 1376326848 alt handle context is allocated and this handle to the original/host file is stored in it. This context is then

attached to th

img class slide_image src 1376326848 alt for an I/O operation. This saves minifilter drivers from having a switch statement to find the position of

these parameter

img class slide_image src 1376326848 alt If it is paging I/O then FltReadFile cannot be used since it can only be used at an IRQL PASSIVE_LEVEL. In

that case the f

img class slide_image src 1376326848 alt IRP_SYNCHRONOUS_PAGING_IO flags, it is added to the faulted list. Subsequent normal

IRP_MJ_READs will be allowed to pass

img class slide_image src 1376326848 alt with the guest file is written to its reparse point using the function FltTagFile and the unique GUID and

Tag values.

An a

img class slide_image src 1376326848 alt Fast I/O handling:

Typically I/O requests are generated by the I/O manager in the form of IRPs. However, if the data is

ca

img class slide_image src 1376326848 alt References:

1 Microsoft Filter Driver Developer Guide

2 MSDN Library and DDK Device Driver Kit.

3 Single Instance Stor

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Guest Article: Writing a File System Minifilter - Pitfalls, Hints and Tips The NT Insider, Vol 12, Issue 4, September-October 2005 Published: 16-Nov-05 Modified.

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Contents

Introduction

This tutorial will show you how to develop a simple file system filter driver. The demo driver will print the names of opening files to the debug output.

The article requires basic Windows driver development and C/C knowledge. However, it may also be interesting to people without Windows driver development experience.

What is a file system filter driver.

A file system filter driver is called on every file system I/O operation create, read, write, rename, and etc., and thus it can modify the file system behavior. File system filter drivers are almost similar to legacy drivers, but they require some special steps to do. Such drivers are used by anti-viruses, security, backup, and snapshot software.

Creating a simple File System Filter Driver

Before starting

To build a driver, you need WDK or the IFS Kit. You can get them from Microsoft s website. Also, you have to set an environment variable WINDDK to the path where you have installed the WDK/IFS Kit.

Be careful: Even a small error in the driver may cause a BSOD or system instability.

Main.c

Driver entry

This is the entry point of any driver. The first thing that we do is to store DriverObject to a global variable we will need it later.

PDRIVER_OBJECT g_fsFilterDriverObject NULL;

NTSTATUS DriverEntry

__inout PDRIVER_OBJECT DriverObject,

__in PUNICODE_STRING RegistryPath

NTSTATUS status STATUS_SUCCESS;

ULONG i 0;

g_fsFilterDriverObject DriverObject;

Set IRP dispatch table

The next step is to populate the IRP dispatch table with function pointers to IRP handlers. In our filter driver, there is a generic pass-through IRP handler which sends the request further. And, we will need a handler for IRP_MJ_CREATE to retrieve the names of the opening files. The implementation of the IRP handlers will be described later.

for i 0; i IRP_MJ_MAXIMUM_FUNCTION; i

DriverObject- MajorFunction i FsFilterDispatchPassThrough;

DriverObject- MajorFunction IRP_MJ_CREATE FsFilterDispatchCreate;

Set Fast-IO dispatch table

A file system filter driver must have the fast-IO dispatch table. If you ve forgot to set up the fast-IO dispatch table, it will lead to system crash. Fast-IO is an alternative way to initiate I/O operations and it s faster than IRP. Fast-IO operations are always synchronous. If the Fast-IO handler returns FALSE, it means the Fast-IO way is impossible and an IRP will be created.

FAST_IO_DISPATCH g_fastIoDispatch

sizeof FAST_IO_DISPATCH,

FsFilterFastIoCheckIfPossible,

;

DriverObject- FastIoDispatch g_fastIoDispatch;

Register a notification for file system changes

We should track the file system being activated/deactivated to perform attaching/detaching of our file system filter driver. How to start tracking file system changes is shown below.

status IoRegisterFsRegistrationChange DriverObject,

FsFilterNotificationCallback ;

if . NT_SUCCESS status

return status;

Set driver unload routine

The last part of the driver initialization sets an unload routine. Setting the driver unload routine makes the driver unloadable, and you can load/unload it multiple times without system restart. However, this driver is made unloadable only for debugging purpose, because file system filters can t be unloaded safely. Never do this in production code.

DriverObject- DriverUnload FsFilterUnload;

return STATUS_SUCCESS;

Driver unload implementation

Driver unload routine is responsible for cleaning up and deallocation of resources. First of all, unregister the notification for file system changes.

VOID FsFilterUnload

__in PDRIVER_OBJECT DriverObject

IoUnregisterFsRegistrationChange DriverObject, FsFilterNotificationCallback ;

Then loop through the devices we created, detach and delete them. Wait for 5 seconds to let all outstanding IRPs to be completed. As it was mentioned before, this is a debug only solution. It works in the majority of cases, but there is no guarantee for all.

for ;;

IoEnumerateDeviceObjectList

DriverObject,

devList,

sizeof devList,

numDevices ;

if 0 numDevices

break;

numDevices min numDevices, RTL_NUMBER_OF devList ;

for i 0; i numDevices; i

FsFilterDetachFromDevice devList i ;

ObDereferenceObject devList i ;

KeDelayExecutionThread KernelMode, FALSE, interval ;

IrpDispatch.c

Dispatch pass-through

This IRP handler does nothing except passing requests further to the next driver. We have the next driver object stored in our device extension.

NTSTATUS FsFilterDispatchPassThrough

__in PDEVICE_OBJECT DeviceObject,

__in PIRP Irp

PFSFILTER_DEVICE_EXTENSION pDevExt

PFSFILTER_DEVICE_EXTENSION DeviceObject- DeviceExtension;

IoSkipCurrentIrpStackLocation Irp ;

return IoCallDriver pDevExt- AttachedToDeviceObject, Irp ;

Dispatch create

This IRP handler is invoked on every create file operation. We will grab a filename from PFILE_OBJECT and print it to the debug output. Then, we call the pass-through handler described above. Pay attention to the fact that a valid file name exists in PFILE_OBJECT only while the create file operation is being performed. Also there are relative opens, and opens by ID. Retrieving file names in those cases is beyond the scope of this article.

NTSTATUS FsFilterDispatchCreate

__in PDEVICE_OBJECT DeviceObject,

PFILE_OBJECT pFileObject IoGetCurrentIrpStackLocation Irp - FileObject;

DbgPrint wZ n, pFileObject- FileName ;

return FsFilterDispatchPassThrough DeviceObject, Irp ;

FastIo.c

To test the Fast-IO dispatch table validity for the next driver, we will use the following helper macro not all of the Fast-IO routines must be implemented by the underlying file system, so we have to be sure about that :

define VALID_FAST_IO_DISPATCH_HANDLER _FastIoDispatchPtr, _FieldName

_FastIoDispatchPtr . NULL

_FastIoDispatchPtr - SizeOfFastIoDispatch

FIELD_OFFSET FAST_IO_DISPATCH, _FieldName sizeof void

_FastIoDispatchPtr - _FieldName. NULL

Fast-IO pass-through

Passing through Fast-IO requests requires writing a lot of code in contrast to passing through IRP requests because each Fast-IO function has its own set of parameters. A typical pass-through function is shown below:

BOOLEAN FsFilterFastIoQueryBasicInfo

__in PFILE_OBJECT FileObject,

__in BOOLEAN Wait,

__out PFILE_BASIC_INFORMATION Buffer,

__out PIO_STATUS_BLOCK IoStatus,

__in PDEVICE_OBJECT DeviceObject

PDEVICE_OBJECT nextDeviceObject

PFSFILTER_DEVICE_EXTENSION DeviceObject- DeviceExtension - AttachedToDeviceObject;

PFAST_IO_DISPATCH fastIoDispatch nextDeviceObject- DriverObject- FastIoDispatch;

if VALID_FAST_IO_DISPATCH_HANDLER fastIoDispatch, FastIoQueryBasicInfo

return fastIoDispatch- FastIoQueryBasicInfo

FileObject,

Wait,

Buffer,

IoStatus,

nextDeviceObject ;

return FALSE;

Fast-IO detach device

This is a special Fast-IO request which we have to handle ourselves and not call the next driver. We have to detach our filter device from the file system device stack and delete our device. That can be done easily by the following code:

VOID FsFilterFastIoDetachDevice

__in PDEVICE_OBJECT SourceDevice,

__in PDEVICE_OBJECT TargetDevice

IoDetachDevice TargetDevice ;

IoDeleteDevice SourceDevice ;

Notification.c

A typical file system consists of a control device and volume devices. A volume device is attached to the storage device stack. The control device is registered as a file system.

Figure 1 - Devices of the typical file system

We have a callback which is invoked for all active file systems and whenever a file system has either registered or unregistered itself as an active one. This is a good place to attach/detach our filter device. When a file system activates itself, we attach to its control device only if we are not already attached, enumerate its volume devices, and attach to them too. On file system deactivation, we examine the file system control device stack, find our device, and detach it. Detaching from the file system volume devices is performed in the FsFilterFastIoDetachDevice routine described earlier.

VOID FsFilterNotificationCallback

__in BOOLEAN FsActive

if FsActive

FsFilterAttachToFileSystemDevice DeviceObject ;

else

FsFilterDetachFromFileSystemDevice DeviceObject ;

AttachDetach.c

This file contains helper routines for attaching, detaching, and checking whether our filter is already attached.

Attaching

To perform attaching, we create a new device object with the device extension call IoCreateDevice and the propagate device object flags from the device object we are trying to attach to DO_BUFFERED_IO, DO_DIRECT_IO, FILE_DEVICE_SECURE_OPEN. Then, we call IoAttachDeviceToDeviceStackSafe in a loop with a delay in the case of failure. It is possible for this attachment request to fail because the device object has not finished initialization. This situation can occur if we try to mount the filter that was loaded as the volume only. When attaching is finished, we save the attached to device object to the device extension and clear the DO_DEVICE_INITIALIZING flag. The device extension is shown below:

typedef struct _FSFILTER_DEVICE_EXTENSION

PDEVICE_OBJECT AttachedToDeviceObject;

FSFILTER_DEVICE_EXTENSION, PFSFILTER_DEVICE_EXTENSION;

Detaching

Detaching is quite simple. Get the attached to device object from the device extension and call IoDetachDevice and IoDeleteDevice.

void FsFilterDetachFromDevice

IoDetachDevice pDevExt- AttachedToDeviceObject ;

IoDeleteDevice DeviceObject ;

Checking whether our device is attached

To check whether we are attached to a device, we have to iterate through the device stack using IoGetAttachedDeviceReference and IoGetLowerDeviceObject and search for our device there. We distinguish our devices by comparing the device driver object with our driver object g_fsFilterDriverObject.

BOOLEAN FsFilterIsMyDeviceObject

return DeviceObject- DriverObject g_fsFilterDriverObject;

Sources and makefile

Sources and makefile files are used by the build utility to build the driver. It contains project settings and source file names.

Sources file contents:

TARGETNAME FsFilter

TARGETPATH obj

TARGETTYPE DRIVER

DRIVERTYPE FS

SOURCES

Main.c

IrpDispatch.c

AttachDetach.c

Notification.c

FastIo.c

The makefile is standard:

.include NTMAKEENV makefile.def

The MSVC makefile project build command line is:

call WINDDK bin setenv.bat WINDDK chk wxp

cd /d ProjectDir

build.exe –I

How to install a driver

SC.EXE overview

We will use sc.exe sc – service control to manage our driver. It is a command-line utility that can be used to query or modify the database of installed services. It is shipped with Windows XP and higher, or you can find it in the Windows SDK/DDK.

Install

To install the driver, call:

sc create FsFilter type filesys binPath c: FSFilter.sys

A new service entry will be created with the name FsFilter; the service type will be file system, and the binary path, c: FsFilter.sys.

Start

To start the driver, call:

sc start FsFilter

This starts a service named FsFilter.

Stop

To stop the driver, call:

sc stop FsFilter

This stop the service named FsFilter.

Uninstall

And to uninstall, call:

sc delete FsFilter

This instructs the service manager to delete the service entry with the name FsFilter.

Resulting script

All those commands are put into a single batch file to make driver testing easier. Here is the listing of the Install.cmd command file:

sc create FsFilter type filesys binPath c: FsFilter.sys

pause

Running a sample

This is the most interesting part. To demonstrate the file system filter work, we will use Sysinternals DebugView for Windows to monitor the debug output, and OSR Device Tree to see the devices and drivers.

So, build the driver. Then, copy the build output file FsFilter.sys and the install the script Install.cmd to the root of the disk C.

Figure 2 - The driver and the install script on the disk.

Run Install.cmd. It will install and start the driver and then wait for user input.

Figure 3 - The driver is successfully installed and started.

Now start the DebugView utility.

Figure 4 - Monitoring the debug output.

We can see what files are being opened. Our filter works. Now, run the device tree utility and locate our driver there.

Figure 5 - Our filter driver in the device tree.

We can see numerous devices created by our driver. Now let s open the NTFS driver and look at the device tree:

Figure 6 - Our filter is attached to NTFS.

We are attached. Let s take a look at the other file systems.

Figure 7 - Our filter is attached to the other file systems too.

And finally, press any key to move our install script forward. It will stop and uninstall the driver.

Figure 8 - The driver is stopped and uninstalled.

Refresh the device tree list by pressing F5:

Figure 9 - Our filter device isnot in the device tree.

Our filter is gone. The system is running as before.

Improvements

The sample driver lacks a commonly required functionality of attaching to the newly arrived volumes. It is done so to make the driver as easy to understand as possible. You can write a IRP_MJ_FILE_SYSTEM_CONTROL handler of your own to track the newly arrived volumes.

Conclusion

This tutorial showed how to create a simple file system filter driver, and how to install, start, stop, and uninstall it from a command line. Also, some file system filter driver aspects were discussed. We saw the file system device stack with the attached filters, and learned how to monitor the debug output from the driver. You may use the provided sources as a skeleton for your own file system filter driver or modify its behavior.

Useful references

Read more Driver Development tips at Apriorit Development Blog.

An application or service that uses a file system filter driver may experience function failure on a computer that is running Windows Vista, Windows Server 2003, or.

Aug 12, 2013  CSE 524 Project Report File system virtualization inside Windows system using mini-filter drivers. Nikhil Pujari SUNY SB ID: 106667284 Email.