Reverse Engineering Existing VHAL Implementations for Custom Property Enhancements Lab

Introduction: Unlocking the Android Automotive VHAL Potential

The Vehicle Hardware Abstraction Layer (VHAL) is a cornerstone of Android Automotive OS, providing a standardized interface between vehicle hardware and the Android framework. It enables car manufacturers and developers to access and control various vehicle properties, from basic speed and gear status to complex climate control and advanced driver-assistance systems (ADAS) parameters. While VHAL offers a rich set of predefined properties, many automotive OEMs extend it with custom, vendor-specific properties to expose unique hardware capabilities. This article delves into the expert-level methodology of reverse engineering existing VHAL implementations to understand these custom properties, paving the way for advanced integrations and enhancements.

Understanding an OEM’s custom VHAL properties is crucial for several scenarios:

Developing custom Android Automotive applications: Accessing unique vehicle features.
Integrating with aftermarket hardware: Bridging communication gaps.
Troubleshooting and debugging: Pinpointing issues in hardware-software interaction.
Extending VHAL functionality: Building new services that leverage or enhance existing custom properties.

Our goal is to equip you with the knowledge and steps to perform static and dynamic analysis on VHAL implementations, ultimately enabling you to interact with or even extend these proprietary interfaces.

VHAL Architecture Overview

Before diving into reverse engineering, a brief understanding of VHAL’s architecture is essential. The VHAL consists of several key components:

VHAL Client: An Android application or service that requests vehicle properties (e.g., CarPropertyManager).
VHAL Service: A system service ([email protected] or similar) that implements the IVehicle AIDL interface.
VHAL HAL Implementation: A shared library (e.g., vehicle.default.so or a vendor-specific variant like vehicle.oem.so) loaded by the VHAL service. This library directly interfaces with the vehicle’s underlying hardware via various protocols (CAN bus, Ethernet, etc.).

Our focus will be on analyzing the VHAL HAL Implementation library, as this is where the proprietary logic and custom property definitions reside.

The Reverse Engineering Toolkit

To embark on this journey, you’ll need a set of specialized tools:

Android Debug Bridge (ADB): For interacting with the target Android Automotive device.
Decompiler/Disassembler (Ghidra or IDA Pro): Essential for static analysis of compiled binaries. Ghidra is an excellent open-source choice.
Android Open Source Project (AOSP) Source Code: As a reference for standard VHAL interfaces and property definitions.
Text Editor/IDE: For code analysis and development (e.g., VS Code, Android Studio).

Step 1: Identifying and Extracting the Target HAL Implementation

First, we need to locate the VHAL service and its corresponding HAL implementation library on the target device.

Locating the VHAL Service Process

adb shell 'ps -ef | grep android.hardware.automotive.vehicle'

You’ll typically see an output similar to this, confirming the service is running:

system    2135  1     334232 40044   0 S com.android.car.vehiclehal.VehicleHalService

Identifying the VHAL HAL Shared Library

The VHAL service loads its implementation from a shared library. Common paths include /vendor/lib64/hw/ or /vendor/lib/hw/. Look for files named vehicle.<vendor>.so or vehicle.default.so.

adb shell 'ls -l /vendor/lib64/hw/'

Identify the relevant .so file, for example, vehicle.oem.so or vehicle.qcom.so if running on a Qualcomm platform.

Pulling the Shared Library

Once identified, pull the library to your local machine for static analysis:

adb pull /vendor/lib64/hw/vehicle.oem.so .

Step 2: Static Analysis with Ghidra

With the .so file in hand, load it into Ghidra (or IDA Pro). Perform an initial auto-analysis. Our goal is to locate structures related to VHAL properties and their handling logic.

Searching for Key VHAL Structures and Functions

In Ghidra’s Symbol Tree or Function Filter, search for common VHAL patterns:

IVehicle methods: Look for functions like get, set, subscribe, oneway_set. These are the entry points for VHAL property operations.
VehiclePropConfig: This structure defines each property’s characteristics (ID, type, access, change mode, area, config array). Search for references to this structure.
Property IDs: Standard VHAL properties have IDs like VEHICLE_PROPERTY_HVAC_FAN_SPEED. Vendor properties typically start from VEHICLE_PROPERTY_VENDOR_EXTENSION_START (0x10000000). Search for this constant value.

Analyzing Property Initialization Functions

Most VHAL implementations have a function responsible for populating the list of supported properties, often named something like fillPropConfigList, getHwPropertyConfig, or similar. Navigate to the cross-references of VehiclePropConfig or the IVehicle methods, and you’ll likely find these initialization routines.

Inside these functions, you’ll see arrays or lists of VehiclePropConfig structures being defined. A typical C++ representation might look like this:

// Example of a custom property definition (pseudo-C++)
VehiclePropConfig customPropConfig = {
    .prop = 0x12345678, // Custom Property ID (above VENDOR_EXTENSION_START)
    .access = VehiclePropertyAccess::READ_WRITE,
    .changeMode = VehiclePropertyChangeMode::ON_CHANGE,
    .valueType = VehiclePropertyType::INT32,
    .areaType = VehicleArea::VEHICLE_AREA_TYPE_GLOBAL,
    .configArray = {0, 0, 0, 0, 0, 0},
    .configString = nullptr,
    .minSampleRate = 0.0f,
    .maxSampleRate = 0.0f,
    .supportAreas = 0,
    .areaConfigs = nullptr
};
supportedProperties.push_back(customPropConfig);

By analyzing these definitions, you can deduce:

Property ID: The unique identifier for the custom property.
Data Type: valueType (e.g., INT32, FLOAT, STRING, BYTES, INT32_VEC).
Access Mode: access (READ, WRITE, READ_WRITE).
Change Mode: changeMode (STATIC, ON_CHANGE, CONTINUOUS).
Area Type: areaType (e.g., GLOBAL, HVAC, SEAT).
configArray and configString: These fields often hold critical metadata for vendor properties, such as unit types, range limits, or even command codes for underlying hardware interfaces. Deeper analysis of how these values are used in associated functions is required.

Tracing Property Handling Logic

Once you identify a custom property ID, trace its cross-references. You’ll find functions that are called when this property is requested (in get) or set (in set). These functions will often interact with low-level hardware drivers or proprietary IPC mechanisms. This is where you might uncover:

Specific memory addresses being read/written.
Calls to internal OEM libraries.
CAN bus message constructions or parsers.
Proprietary serial communication protocols.

This deep dive allows you to understand *what* the property controls and *how* it communicates with the vehicle’s hardware.

Step 3: Dynamic Observation (Optional but Recommended)

While static analysis provides the blueprint, dynamic observation confirms your hypotheses and reveals real-time property values.

Monitoring VHAL Logs

The VHAL service often logs its operations. Use adb logcat to capture these:

adb logcat -s VehicleHal:D -s CarPropertyManager:D

Perform actions on the vehicle (e.g., change settings via the infotainment system) and observe corresponding log entries. Look for lines indicating property reads/writes and their associated IDs and values. This can help confirm property types and ranges.

Advanced Dynamic Analysis with Frida

For more granular control, tools like Frida can be used to hook into VHAL functions at runtime, allowing you to intercept arguments, return values, and even modify behavior. This is an advanced technique and requires setting up Frida on the target device.

// Example Frida script snippet to hook a getProperty function (conceptual)
Interceptor.attach(Module.findExportByName(
        
        
        
            
                
            
            
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