Writing Your First Driver

Creating a device driver for Windows can be an exciting yet daunting task. However, with the right approach and guidance, you can write your first simple device driver that interacts with the Windows operating system. In this article, we will walk you through the steps of writing a basic Windows driver, complete with code examples.

Prerequisites

Before diving into writing your driver, ensure you have the following prerequisites:

Development Environment: Use Windows 10 or later with the Windows Driver Kit (WDK) installed. You can download the WDK from the Microsoft website.
Visual Studio: It’s recommended to use Visual Studio (preferably 2019 or later) for developing your driver.
Basic Knowledge of C: Familiarity with C programming language is essential, as Windows drivers are primarily written in C.
Testing Setup: Having a virtual machine or secondary physical device to test your driver is ideal to prevent potential system crashes on your main machine.

Step 1: Set Up Your Development Environment

Install the Windows Driver Kit (WDK): The WDK provides all the necessary tools and libraries for driver development. Make sure to choose the version that matches your version of Visual Studio.
Create a Driver Project:
- Open Visual Studio.
- Select Create a new project.
- From the “Create a new project” dialogue, search for “Driver”.
- Choose a suitable template (e.g., Kernel Mode Driver (KMDF) or User Mode Driver (UMDF)).
- Name your project (e.g., "MyFirstDriver") and click Create.

Step 2: Understanding the Project Structure

Upon creating your driver project, Visual Studio generates a basic structure that includes several important files:

Driver.c: Contains the core functions of your driver.
MyFirstDriver.h: This header file is where you can define your constants, data structures, and function prototypes.
MyFirstDriver.def: If you have specific entry points or exports, they should be defined here.

Take a moment to familiarize yourself with these files. The most important of these for now is Driver.c.

Step 3: Writing Your First Driver Code

Open Driver.c in Visual Studio. You will see some boilerplate code. Let’s start by modifying it to create a simple driver that handles simple device open and close functions.

Code Example

Here’s a simple implementation for a basic driver:

#include <ntddk.h>

VOID MyDriverUnload(_In_ PDRIVER_OBJECT DriverObject) {
    KdPrint(("MyDriver unloaded\n"));
}

NTSTATUS MyDriverCreateClose(
    _In_ PDEVICE_OBJECT DeviceObject,
    _Inout_ PIRP Irp) {
    
    // Using the IRP to complete the request
    Irp->IoStatus.Status = STATUS_SUCCESS;
    Irp->IoStatus.Information = 0;
    IoCompleteRequest(Irp, IO_NO_INCREMENT);
    KdPrint(("Device opened/closed\n"));
    return STATUS_SUCCESS;
}

NTSTATUS DriverEntry(
    _In_ PDRIVER_OBJECT DriverObject,
    _In_ PUNICODE_STRING RegistryPath) {

    UNREFERENCED_PARAMETER(RegistryPath);

    KdPrint(("MyDriver loaded\n"));

    DriverObject->DriverUnload = MyDriverUnload;
    
    // Create a device object
    PDEVICE_OBJECT DeviceObject;
    UNICODE_STRING deviceName = RTL_CONSTANT_STRING(L"\\Device\\MyFirstDevice");
    NTSTATUS status = IoCreateDevice(
        DriverObject,
        0,
        &deviceName,
        FILE_DEVICE_UNKNOWN,
        FILE_DEVICE_SECURE_OPEN,
        FALSE,
        &DeviceObject);
        
    if (!NT_SUCCESS(status)) {
        KdPrint(("Failed to create device\n"));
        return status;
    }

    // Set up dispatch points
    DriverObject->MajorFunction[IRP_MJ_CREATE] = MyDriverCreateClose;
    DriverObject->MajorFunction[IRP_MJ_CLEANUP] = MyDriverCreateClose;
    DriverObject->MajorFunction[IRP_MJ_CLOSE] = MyDriverCreateClose;

    // For cleanup
    return STATUS_SUCCESS;
}

Explanation of the Code

DriverEntry: This is the entry point for your driver. It sets up the driver object and defines how to handle various IRPs (I/O Request Packets).
MyDriverCreateClose: This function handles open and close operations for the device. It simply completes the IRP.
MyDriverUnload: This function is called when the driver is unloaded. It can be used to free resources.

Step 4: Building the Driver

Build the Driver:
- Click on Build -> Build Solution.
- Ensure there are no errors.
Signing the Driver: Windows requires drivers to be digitally signed. For testing, you can use a self-signed certificate, but know that on production systems, you will need a certificate from a trusted Certificate Authority.

Step 5: Testing the Driver

To test the driver effectively:

Set Up a Testing Environment: It is safer to test drivers in a virtual machine. Use Hyper-V or similar virtualization software for this purpose.
Install the Driver:
- You can use a tool like pnputil to install the driver.
- Open Command Prompt with administrative rights and execute:
```
pnputil /add-driver C:\Path\To\Your\Driver.sys /install
```
Check Driver Status:
- You can verify installation via Device Manager. Look for "MyFirstDevice" under the appropriate device category.
Test the Driver’s Functions: Open and close the device multiple times through scripting or manually to ensure the MyDriverCreateClose function works as intended.

Step 6: Debugging Your Driver

Debugging drivers can be challenging. Here are some tips:

Use Windbg: It’s a powerful debugging tool that can connect to your VM/sandbox for debugging.
Use DbgPrint for logging messages from your driver. Make use of KdPrint to log useful information to the kernel debugger.

KdPrint(("This is a debug message\n"));

Conclusion

Congratulations! You have successfully written and tested your first simple device driver for Windows. While this is just a basic example, you can expand on this foundation by exploring more complex functionalities, implementing hardware interfaces, or diving deeper into driver frameworks like KMDF and UMDF.

Remember that driver development requires attention to detail and stability, as a poorly coded driver can lead to system crashes. Always test in a safe environment, follow best practices, and keep learning!

Networking & Infrastructure - Windows Driver Development