Introduction: Unveiling the Hidden Processors

Android devices are marvels of integrated engineering, often housing more than just the primary ARM Application Processor (AP). Deep within, specialized Digital Signal Processors (DSPs) handle critical tasks like audio processing, cellular baseband communications, imaging, and sensor fusion. These DSPs, such as Qualcomm’s Hexagon or various CEVA/Tensilica cores, often run on custom, obscure instruction set architectures (ISAs) with minimal public documentation. This obscurity presents a significant challenge for security researchers and reverse engineers aiming to understand or audit their firmware. Ghidra, with its powerful Sleigh language, offers a robust framework for defining these custom ISAs, allowing us to bring undocumented DSPs into the light of decompilation.

This article will guide you through the process of developing a Ghidra processor module using Sleigh, specifically tailored for reverse engineering an unknown or obscure Android DSP architecture. We’ll cover everything from initial firmware reconnaissance to crafting instruction patterns and P-code semantics.

Understanding the Challenge of DSP Architectures

Unlike the well-documented ARM or x86 architectures, DSPs are often proprietary, designed for specific embedded tasks with efficiency in mind. Their ISAs might feature:

• VLIW (Very Long Instruction Word): Multiple operations packed into a single, wide instruction word, executed in parallel.
• Specialized Registers: Dedicated registers for loop counters, address generation, or SIMD operations.
• Circular Buffers & Hardware Loops: Optimized for signal processing algorithms.
• Custom Addressing Modes: Post-increment, pre-decrement, bit-reversed addressing.

The lack of SDKs, debuggers, or public specifications means we must reverse engineer the ISA from raw binary firmwares – a task perfectly suited for Ghidra’s extensibility.

The Ghidra Sleigh Language: Your Rosetta Stone

Sleigh is Ghidra’s processor specification language. It allows you to describe an ISA’s instruction format, operand parsing, and the underlying P-code semantics (Ghidra’s intermediate representation). A complete Ghidra processor module consists of several files:

• .pspec: Processor specification (endianness, data organization, register definitions).
• .sla: Sleigh language description (instruction patterns, P-code semantics).
• .opinion: Compiler/toolchain-specific settings.

For this tutorial, our primary focus will be on the .sla and relevant parts of the .pspec.

Phase 1: Initial Reconnaissance and Firmware Extraction

Before writing any Sleigh, you need firmware. This typically involves:

1. Extracting Android Firmware: Obtain device firmware (e.g., stock ROM, OTA updates). Tools like `adb pull` for specific partitions or unpacking manufacturer update packages are common.
2. Identifying DSP Blobs: Look for files named `modem.b00`, `dsp.img`, `adsp.mbn`, `qdsp6sw.mbn`, or similar in `/vendor/firmware` or root filesystem images.
3. Initial Binary Analysis: Use command-line tools to peek into the binary.

binwalk -Me firmware.mbn # Extract embedded files/sectionshexdump -C firmware.mbn | head -n 50 # Look for repeating patterns or magic bytesstrings -n 8 firmware.mbn # Identify readable strings, potential function names

Pay close attention to the beginning of the file. Many embedded firmwares have a small header, followed by the actual code. Look for entry points, jump tables, or sequences of what might be NOP instructions (e.g., `00 00 00 00` or `FF FF FF FF`).

Phase 2: Disassembly and Instruction Pattern Identification

Load your DSP firmware into Ghidra as a raw binary. Since you don’t have a Sleigh specification yet, Ghidra will treat it as