A Comprehensive Guide to Windows File System Filter Driver Development93

IntroductionA file system filter driver is a type of kernel-mode driver that intercepts and modifies file system operations. They are commonly used for security, data protection, and performance optimization purposes. In this detailed guide, we will explore the basics of Windows file system filter driver development and provide a step-by-step approach to creating your own driver.

PrerequisitesBefore you begin, ensure you have the following prerequisites:
* Windows Development Environment: Visual Studio with the Windows Driver Kit (WDK) installed
* Windows 10 or later: As the operating system that will host your driver
* Basic C/C++ programming knowledge: Understanding of data structures, memory management, and kernel programming concepts

Understanding File System Filter DriversA file system filter driver acts as an intermediary between applications and the underlying file system. When an application performs a file system operation, such as reading or writing a file, the request is intercepted by the filter driver. The driver can then perform custom processing, modify the request, or block the operation if necessary.

Developing a File System Filter DriverLet's create a simple file system filter driver that demonstrates the core principles:

1. Create a New Driver Project

* Open Visual Studio and create a new Windows Driver Project.
* Select File System Filter Driver (KMDF) as the project type.
* Name and create the project.

2. Define your Filter Function

* In the DriverEntry routine, register your filter callback function:
```c
NTSTATUS DriverEntry(PDRIVER_OBJECT DriverObject, PUNICODE_STRING RegistryPath)
{
...
IoRegisterFsRegistrationChange(DriverObject, FSREG_MOUNT_POINT_CHANGE_CALLBACK, FsRegistrationCallback, NULL);
...
}
```
* Implement the FsRegistrationCallback function to receive mount point change notifications.
* In the callback, hook a filter to the target volume using FltRegisterFilter:
```c
NTSTATUS FsRegistrationCallback(PFLT_CALLBACK_DATA Data, PCFLT_RELATED_OBJECTS FltObjects, PVOID Context)
{
...
if (Data-> == FLTFL_FS_FILTER_MOUNT_POINT_CHANGE_GAINED)
{
NTSTATUS status = FltRegisterFilter(FltObjects->Volume, &g_Filter);
if (!NT_SUCCESS(status))
{
...
}
}
...
}
```

3. Implement the File System Filter

* Implement the callbacks that correspond to the file system operations you want to intercept, such as PreCreate, PostCreate, PreRead, etc.
* For each callback, perform any necessary processing or modifications to the file system operation:
```c
NTSTATUS PreCreateCallback(PFLT_CALLBACK_DATA Data, PCFLT_RELATED_OBJECTS FltObjects, PVOID Context)
{
...
// Get the file name from the request
PFLT_REQUEST_PARAMETER_BLOCK request = FltGetRequestParameterBlock(Data);
UNICODE_STRING fileName;
FltGetFileNameFromRequest(request, &fileName);
...
}
```

4. Unregister Your Driver

* In the DriverUnload routine, unregister the file system filter and perform any necessary cleanup:
```c
NTSTATUS DriverUnload(PDRIVER_OBJECT DriverObject)
{
...
FltUnregisterFilter(g_Filter);
...
}
```

ConclusionDeveloping Windows file system filter drivers requires a solid understanding of kernel programming and file system internals. By following the steps outlined in this guide, you can create and deploy your own custom drivers to enhance the functionality and security of your system.

2025-02-02

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