Introduction: The Unseen Architectures of Android

When we talk about reverse engineering Android applications or firmware, the immediate assumption is often ARM. Indeed, the vast majority of Android devices run on ARM-based System-on-Chips (SoCs). However, the Android ecosystem is far richer and more complex than a monolithic ARM landscape. Deep within many Android devices, especially those with specialized functions, you might encounter co-processors, Digital Signal Processors (DSPs), microcontrollers, or even legacy/niche SoCs that operate on instruction set architectures (ISAs) completely different from ARM. These ‘forgotten’ processors often handle critical, performance-sensitive tasks like audio processing, image signal processing, security functions, or communication protocols.

Standard reverse engineering tools, including Ghidra, offer robust support for common architectures. But what happens when you encounter a raw binary blob from an unfamiliar processor? This is where Ghidra’s powerful Sleigh language comes into play. Sleigh allows you to define custom processor modules, enabling Ghidra to parse, disassemble, and decompile code for virtually any ISA, bringing these forgotten processors into the light.

The Challenge of Non-ARM Android Processors

Identifying and analyzing non-ARM code within an Android device presents several hurdles:

• Unknown ISA: Without a known instruction set, disassemblers cannot correctly interpret byte sequences into meaningful instructions.
• Lack of Tooling: Most debugging and analysis tools are tailored for popular architectures, leaving custom or niche processors unsupported.
• Limited Documentation: Datasheets or programming manuals for these specialized components are often proprietary or non-existent.
• Inter-processor Communication: Understanding how the main ARM processor interacts with these auxiliary units is crucial but complex without proper disassembly.

These challenges highlight the necessity for a flexible and extensible framework capable of adapting to arbitrary ISAs. Ghidra Sleigh provides exactly this.

Introducing Ghidra Sleigh: The Processor Specification Language

Sleigh (Specification Language for Engineering Interface for Ghidra’s Hardware) is a declarative language used by Ghidra to describe processor instruction sets. It allows reverse engineers to define:

• Instruction Formats: How instructions are encoded in raw bytes.
• Registers: All general-purpose, special-purpose, and segment registers.
• Memory Spaces: Different addressable regions (e.g., RAM, ROM, I/O).
• P-code Semantics: The low-level, architecture-independent representation of what each instruction does. This is critical for Ghidra’s decompiler.
• Context: Processor modes, flags, and other state variables.

By defining these elements, Sleigh essentially teaches Ghidra how to ‘think’ like a specific CPU, enabling accurate disassembly and decompilation even for highly obscure or proprietary architectures.

Identifying Your Target Architecture

Before writing Sleigh, you need to gather as much information as possible about the unknown architecture. This often involves:

• Firmware Dumps: Obtain a full firmware image, preferably including the bootloader and various partitions.
• String Analysis: Look for readable strings in the binary that might hint at the processor type, compiler, or libraries used.
• Entropy Analysis: High entropy usually indicates compressed or encrypted data, while lower entropy might point to code or structured data.
• Pattern Recognition: Even without knowing the ISA, you might spot repetitive byte sequences that suggest function prologues/epilogues, common instructions (like NOPs), or data structures.
• External Research: Search for information about the device’s specific SoC, other components, or related products that might share the same auxiliary processors.
• JTAG/SWD/UART Access: If hardware access is possible, these interfaces can often provide valuable boot logs or allow memory dumps.

For example, you might encounter a section of a firmware dump that the `file` command reports as ‘data’, and initial string analysis yields nothing. This is your prime candidate for Sleigh analysis.

$ file unknown_coprocessor.bin Unknown_coprocessor.bin: data

Sleigh Basics: A Minimal Example

Let’s create a hypothetical, ultra-simple 8-bit processor. Imagine it has two 8-bit registers, `R0` and `R1`, and a Program Counter `PC`. It has only two instructions: `MOV R0, imm` (move immediate to R0) and `ADD R0, R1` (add R1 to R0).

A `.slaspec` file might look like this:

attach names[PC] [R0, R1]define endian=lsb; # Little-endian processordefine alignment=1; # Byte-aligned instructionsdefine space ram type=ram_space size=1; # Main memory spacedefine register R0 size=1 offset=0x0; # 8-bit Register 0define register R1 size=1 offset=0x1; # 8-bit Register 1define register PC size=2 offset=0x2; # Program Counter (16-bit for addressability)define register SP size=2 offset=0x4; # Stack Pointerdefine token instruction (1) # All instructions are 1 byte { op = (0,0); } # First 2 bits for opcode: (MSB, LSB) with 0-index. instruction: MOV_IMM is op=0b00 (1) { # MOV R0,     operand: imm (8) signed=no { value = instruction[0,7]; }    : instruction[0,7] & 0b11 = 0b00; # Check the last 2 bits of the instruction byte. Wait, instruction format is 1 byte, so it should be the full byte    # Corrected:    : instruction[0,7] == 0x00 && (instruction[0,7] >> 6) == 0b00; # The 'op' is defined as 2 bits.    # Let's redefine 'instruction' token bits for simplicity or better, specify fields.    # Re-evaluating: A 1-byte instruction, 2 bits for opcode, 6 bits for immediate.    op = (0,1); # 2 bits, from bit 0 to 1 (LSB to MSB)    immval = (2,7); # 6 bits, from bit 2 to 7 (LSB to MSB) } {    print

        
        
        

            

                
            
            

                
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