Introduction: Unlocking the Secrets of Custom Android Co-Processors

Modern Android devices are complex ecosystems, often featuring specialized co-processors beyond the main ARM or x86 CPU. These custom silicon blocks, ranging from Digital Signal Processors (DSPs) for audio/camera tasks to dedicated security modules (e.g., TrustZone-like implementations or secure elements), frequently employ proprietary instruction sets. Reverse engineering these components is crucial for security analysis, vulnerability research, and even performance optimization. However, standard disassemblers and decompilers often fail to understand these bespoke instruction sets, presenting a significant hurdle.

This tutorial will guide you through writing a custom processor module for Ghidra using its powerful Sleigh language. Sleigh (Semantic Language for Instruction Set Handlers) allows you to describe an instruction set’s syntax and semantics, enabling Ghidra to correctly disassemble and decompile proprietary code. By the end, you’ll have the foundational knowledge to define a custom instruction set and integrate it into your Ghidra analysis workflow.

The Challenge: Reverse Engineering Unknown Architectures

Why do we need Sleigh? Imagine encountering a raw firmware dump from an Android device. After identifying the main processor, you might find sections of code that, when loaded into Ghidra with a standard ARM or x86 language, appear as ‘undefined’ bytes or incorrect instructions. This often signals the presence of a co-processor. Without a definition, Ghidra cannot understand the program flow, register usage, or underlying logic, rendering static analysis almost impossible. Sleigh provides the bridge, translating raw binary patterns into Ghidra’s intermediate representation (P-code), which then drives disassembly, emulation, and decompilation.

Sleigh Language Basics: Building Blocks of Instruction Semantics

Sleigh is a domain-specific language designed to describe processor instruction sets. It focuses on mapping binary instruction patterns to a formal semantic representation. Key concepts include:

• Tokens: Define how raw binary instruction bits are parsed into fields (e.g., opcode, register numbers, immediate values).
• Opcodes: Map specific token patterns to human-readable assembly instructions (disassembly).
• Semantics (sem blocks): Translate assembly instructions into Ghidra’s P-code. This is where the actual behavior of the instruction is defined, such as register writes, memory accesses, and arithmetic operations.
• Spaces: Define memory spaces (e.g., `ram`, `register`).
• Registers: Declare the processor’s registers.

Our goal is to create two main files: a .pspec (processor specification) and a .sla (Sleigh language architecture) file. The .pspec file defines the overall processor characteristics, while the .sla file contains the instruction set definition.

Step-by-Step: Writing Your First Sleigh Module

Let’s assume we’ve identified a hypothetical custom co-processor within an Android firmware. Through painstaking analysis (e.g., examining raw dumps, looking for unique bit patterns, or even educated guesses based on context), we’ve determined it has 8 8-bit general-purpose registers (R0-R7) and a custom 16-bit instruction format for an ADD_CUSTOM instruction. This instruction takes three register operands: Rdest, Rsrc1, Rsrc2, and performs Rdest = Rsrc1 + Rsrc2.

1. Setting Up Your Environment

Ensure you have Ghidra installed. The Sleigh compiler (`sleigh`) is typically bundled with Ghidra and located in its `support` directory.

2. Creating the Processor Specification File (.pspec)

First, we define the overall processor characteristics. Create a file named MyCustomCoProc.pspec:

<?xml version="1.0" encoding="UTF-8"?> <processor_spec> <description>A Custom Android Co-Processor for demonstration</description> <default_memory_block name="ram" start="0" size="0x100000"/> <language id="MyCustomCoProc:LE:16:default" processor="MyCustomCoProc" endian="little" size="16" variant="default" /> <compiler id="default" name="default" /> </processor_spec>

In this file:

• id: Unique identifier for our language.
• endian: Instruction byte order (e.g., little endian).
• size: Default instruction size in bits (our ADD_CUSTOM is 16-bit).

3. Defining the Sleigh Architecture (.sla)

This is where the core logic resides. Create a file named MyCustomCoProc.sla:

@define processor MyCustomCoProc @include "base.sinc" define endian=little; define alignment=1; define attach=0; define space ram; define space register [size=1]; define register r0 [size=1 offset=0]; define register r1 [size=1 offset=1]; define register r2 [size=1 offset=2]; define register r3 [size=1 offset=3]; define register r4 [size=1 offset=4]; define register r5 [size=1 offset=5]; define register r6 [size=1 offset=6]; define register r7 [size=1 offset=7]; define register sp [size=1 offset=8]; { } @define token instr(16) [ opcode = (15,12) rdest = (11,9) rsrc1 = (8,6) rsrc2 = (5,3) ] { } @segment MyCustomCoProc; @export const * [ MyCustomCoProc:LE:16:default ] = MyCustomCoProc; opcode ADD_CUSTOM:1010 rdest rsrc1 rsrc2 is rdest = r[rsrc1] + r[rsrc2]; { r[rdest] = r[rsrc1] + r[rsrc2]; }

Let’s break down the MyCustomCoProc.sla file:

• @define processor MyCustomCoProc: Declares the processor name.
• @include "base.sinc": Includes common Sleigh definitions.
• define endian=little; define alignment=1; define attach=0;: Basic architectural properties.
• define space ram; define space register [size=1];: Declares memory and register spaces. We’re using 1-byte (8-bit) registers.
• define register ...;: Defines our 8 general-purpose registers (R0-R7) and a stack pointer (SP). The `offset` is crucial for Ghidra to map registers correctly.
• @define token instr(16) [...]: This defines our 16-bit instruction format.
  • opcode = (15,12): Bits 15 down to 12 form the opcode.
  • rdest = (11,9): Bits 11 down to 9 specify the destination register.
  • rsrc1 = (8,6): Bits 8 down to 6 specify the first source register.
  • rsrc2 = (5,3): Bits 5 down to 3 specify the second source register.
• opcode ADD_CUSTOM:1010 rdest rsrc1 rsrc2 is ...: This is the core instruction definition.
  • ADD_CUSTOM:1010: This part matches the instruction when the opcode bits (15-12) are 0b1010. The 1010 is a binary pattern.
  • rdest rsrc1 rsrc2: These are the operands to be displayed in disassembly, corresponding to the token fields.
  • is rdest = r[rsrc1] + r[rsrc2];: This is the Sleigh *disassembly* syntax description. It helps Ghidra understand how to present the instruction.
  • { r[rdest] = r[rsrc1] + r[rsrc2]; }: This is the semantic block. It describes the instruction’s effect using Ghidra’s P-code syntax. Here, it signifies that the value of `rsrc1` is added to `rsrc2`, and the result is stored in `rdest`. `r[]` is how Sleigh refers to registers.

4. Compiling Your Sleigh Module

Open a terminal and navigate to the directory where you saved your `.pspec` and `.sla` files. Use the `sleigh` compiler from Ghidra’s `support` directory. For example, on Linux:

/path/to/ghidra_install_dir/support/sleigh -a MyCustomCoProc.sla -p MyCustomCoProc.pspec

If successful, this command will generate MyCustomCoProc.sla (the compiled version, usually an empty file or placeholder, but it’s the output of the compilation process, alongside the `.pspec` for language registration) and potentially `MyCustomCoProc.sinc` if you defined any macros. The key output is that it validates your Sleigh code. Any errors will be reported here, guiding you to correct syntax or semantic issues.

5. Integrating with Ghidra

To use your new processor module in Ghidra:

1. Create a new directory structure within your Ghidra installation. Navigate to `GHIDRA_INSTALL_DIR/Ghidra/Processors/`.
2. Create a new folder here named `MyCustomCoProc`.
3. Inside `MyCustomCoProc`, create another folder named `data`.
4. Inside `data`, create another folder named `languages`.
5. Copy your compiled `MyCustomCoProc.sla` file into `GHIDRA_INSTALL_DIR/Ghidra/Processors/MyCustomCoProc/data/languages/`.
6. Copy your `MyCustomCoProc.pspec` file into `GHIDRA_INSTALL_DIR/Ghidra/Processors/MyCustomCoProc/language/`. (Note: sometimes the `language` directory is directly under `MyCustomCoProc`, depending on Ghidra version, so check existing processor directories like `ARM` for guidance).
7. Restart Ghidra.

Now, when you import a binary, you should see