Writing Your First Driver

Understanding the Basics of Driver Development

This document will guide you through the process of creating your very first Windows driver. We'll cover the essential components, tools, and a simple example to get you started.

What is a Driver?

A driver is a special type of program that allows the operating system to communicate with a hardware device. Without a driver, Windows wouldn't know how to use your specific hardware, whether it's a graphics card, a network adapter, a USB device, or any other peripheral.

Key Concepts

• Kernel Mode vs. User Mode: Drivers typically run in kernel mode, which grants them direct access to hardware and system resources. This is powerful but requires careful programming to avoid system instability.
• Driver Frameworks: Microsoft provides frameworks like the Windows Driver Model (WDM) and the Windows Driver Foundation (WDF) to simplify driver development. WDF, in particular, offers a more object-oriented and robust approach. We'll be using WDF in our example.
• INF Files: These are configuration files that tell Windows how to install and configure your driver and the associated hardware.

Setting Up Your Development Environment

To begin driver development, you'll need:

1. Windows Driver Kit (WDK): Download and install the latest WDK from the Microsoft website.
2. Visual Studio: Install Visual Studio with the C++ development workload. The WDK integrates with Visual Studio.
3. Target Machine: A separate machine for testing your driver is highly recommended. This prevents potential system crashes on your primary development machine. You'll need to configure driver test signing on this machine.

Your First Driver: A Simple "Hello, World!" Example

Let's create a very basic driver that simply logs a message to the debug output when it's loaded.

Project Creation in Visual Studio

1. Open Visual Studio.
2. Select "Create a new project".
3. Search for "Kernel Mode Driver" or "Driver". Choose the "Kernel Mode Driver (UMDF)" or "Kernel Mode Driver (KMDF)" template. KMDF is often preferred for new development. For this example, we'll use KMDF.
4. Name your project (e.g., `MyFirstDriver`) and click "Create".

Understanding the Generated Code (KMDF Example)

The template provides a basic structure. The core logic usually resides in functions like `DriverEntry` and callbacks. For KMDF, the main entry point often calls a framework initialization function.

#include <ntddk.h>
#include <wdfdriver.h>

// Forward declarations of callback functions
EVT_WDF_DRIVER_DEVICE_ADD MyDriverEvtDeviceAdd;

// DriverEntry is the first function called by the system when the driver is loaded.
NTSTATUS
DriverEntry(
    _In_ PDRIVER_OBJECT  DriverObject,
    _In_ PUNICODE_STRING RegistryPath
    )
{
    NTSTATUS               status;
    WDF_DRIVER_CONFIG      config;

    // Initialize the framework driver object.
    WDF_DRIVER_CONFIG_INIT(&config, MyDriverEvtDeviceAdd);

    // Create a framework driver object.
    status = WdfDriverCreate(DriverObject,
                             RegistryPath,
                             WDF_NO_OBJECT_ATTRIBUTES,
                             &config,
                             WDF_NO_HANDLE); // FrameworkDriver is not returned to the caller.

    if (!NT_SUCCESS(status)) {
        KdPrintEx((DPFLTR_IHVDRIVER_ID, DPFLTR_ERROR_LEVEL, "MyFirstDriver: WdfDriverCreate failed with status 0x%x\n", status));
    } else {
        KdPrintEx((DPFLTR_IHVDRIVER_ID, DPFLTR_INFO_LEVEL, "MyFirstDriver: DriverEntry successful. Driver loaded.\n"));
    }

    return status;
}

// EvtDeviceAdd is called by the framework when a device is detected that
// the driver wants to handle.
EVT_WDF_DRIVER_DEVICE_ADD MyDriverEvtDeviceAdd;
NTSTATUS
MyDriverEvtDeviceAdd(
    _In_ WDFDRIVER       Driver,
    _Inout_ PWDFDEVICE_INIT DeviceInit
    )
{
    UNREFERENCED_PARAMETER(Driver);

    KdPrintEx((DPFLTR_IHVDRIVER_ID, DPFLTR_INFO_LEVEL, "MyFirstDriver: MyDriverEvtDeviceAdd called.\n"));

    // Add device initialization code here.
    // For this simple example, we don't need to do much.
    // A real driver would configure device properties, interfaces, etc.

    // Create a framework device object.
    WDFDEVICE hDevice;
    NTSTATUS status = WdfDeviceCreate(&DeviceInit,
                                      WDF_NO_OBJECT_ATTRIBUTES,
                                      &hDevice);
    if (!NT_SUCCESS(status)) {
        KdPrintEx((DPFLTR_IHVDRIVER_ID, DPFLTR_ERROR_LEVEL, "MyFirstDriver: WdfDeviceCreate failed with status 0x%x\n", status));
        return status;
    }

    KdPrintEx((DPFLTR_IHVDRIVER_ID, DPFLTR_INFO_LEVEL, "MyFirstDriver: Device created successfully.\n"));

    return status;
}
            

Building and Deploying

After writing your code, you'll need to build the driver. Ensure your Visual Studio project is configured to target the correct architecture (x64 for most modern systems). Building will create a `.sys` file (the driver binary) and potentially an `.inf` file.

Deploying to a test machine typically involves:

1. Setting up Debugging: Configure kernel-mode debugging between your development machine and your test machine (usually via a serial port or network).
2. Driver Signing: Drivers must be signed. For testing, you can use test certificates. The WDK provides tools for this.
3. Installation: Copy the driver files (.sys and .inf) to the test machine. Right-click the .inf file and select "Install" or use tools like `pnputil.exe` to add and install the driver.

Viewing Debug Output

The KdPrintEx function logs messages to the kernel debugger. You can view these messages on your development machine using WinDbg (part of the Debugging Tools for Windows) connected to your test machine.

Next Steps

This is a very basic introduction. To create a functional driver, you'll need to learn about:

• Handling I/O Request Packets (IRPs) or WDF Request Objects.
• Interacting with specific hardware registers.
• Managing memory and interrupts.
• Implementing specific driver interfaces (e.g., PnP, Power Management).

Refer to the official Microsoft documentation for detailed information on these advanced topics.