Decompiling a .hex file generated for an ARM Cortex-M0 microcontroller into equivalent C++ code can be a challenging but rewarding process. With the right tools and techniques, it is possible to reverse engineer the machine code in the .hex file back into human-readable C/C++ code that reveals the original program logic and structure. Here is a step-by-step guide on how to decompile a Cortex-M0 .hex file into C++ code.
Overview of the ARM Cortex-M0 Architecture
First, it helps to understand the basic architecture of the Cortex-M0 processor. The Cortex-M0 is a 32-bit RISC processor optimized for low-power embedded applications. It has a simplified 3-stage pipeline, a single cycle multiplier, bit banding, and other features suited for microcontroller usage. The instruction set is a subset of the Thumb-2 instruction set used in other ARM processors. Cortex-M0 supports only the Thumb instruction set, not ARM.
The Cortex-M0 core contains 13 general purpose 32-bit registers R0-R12, with R13 as the stack pointer and R14 as the link register. It has a programmed exception model for interrupts and exceptions. There is built-in support for writing exception handlers and interrupt service routines in C/C++ without assembly. The processor follows the Von Neumann architecture with a unified address space for both code and data.
Understanding these architectural details will help make sense of the disassembled machine code from the .hex file when trying to correlate it to equivalent C++ code during the decompilation process.
Examining the .hex File Contents
The .hex file contains the executable binary image that would be flashed onto the Cortex-M0 microcontroller. It consists of only hexadecimal text characters representing the machine code instructions and data. The file is organized into lines with each line having a start code, byte count, address, record type, data bytes, and checksum.
Before decompiling, we can examine the .hex file contents to get an overview of the program structure. Useful things to look for:
- The memory address range covered by the code and data sections
- Any constant data tables
- Locations of interrupt vectors
- Entry point address of the main program
This information will provide clues on how to reconstruct the C++ code during decompilation later on.
Disassembling the Machine Code
The first major step in decompiling the .hex file is to disassemble the machine code into human readable assembly instructions. This is done using a disassembler tool like objdump or radare2. There are both online and local disassemblers available for ARM Thumb/Thumb-2 instruction set.
For example, to disassemble cortex-m0.hex using radare2: r2 -a arm -b 16 cortex-m0.hex
The -a arm option sets the architecture as ARM and the -b 16 sets the bits as 16-bit Thumb. This will start radare2 in disassembly mode showing the address on the left and the instruction mnemonics on the right. We can now analyze the disassembled code to gain a better understanding of the program structure and logic.
Identifying Functions and Basic Blocks
The disassembled code will consist of blocks of instructions separated by branches, jumps, calls and returns. The next step is to logically group these blocks into probable C functions. Here are some ways to identify functions:
- Blocks ending in a branch to another block are likely function prologues
- Blocks preceded by a branch are likely function epilogues
- Identify branch targets that could be function entry points
- Blocks between paired call and return instructions may be functions
- Lookup addresses of interrupt handlers from .hex file
Within each function, we can further divide the instructions into basic blocks. A basic block is a sequence of instructions with only one entry point and one exit point, with no branches except possibly at the end. Dividing into basic blocks simplifies control flow analysis.
Data Flow Analysis
To convert assembly code into a high level language, we need to understand the data flow. This involves identifying:
- Local variables – registers and stack locations that hold temporary data
- Input/output parameters for functions
- Global variables and constants
- Pointers and references
- Data structures and objects
Data flow analysis examines how data values are propagated through the program by the operations in each basic block. Some useful techniques include:
- Building a def-use chain to see where values are defined and used
- Inferring data types based on instruction operands
- Tracking register and stack pointer usage
- Finding inputs and outputs for function calls
- Looking for address dereferences to infer pointers
- Identifying structures of constants that imply arrays or structs
By thoroughly understanding the data flow, we can start building the variable list, data types, and function prototypes for the final C++ code.
Control Flow Analysis
In addition to data flow, we need to analyze control flow to reconstruct the program logic in C++. This involves:
- Identifying conditional branches and mapping them to if-then-else structures
- Finding loops and switching constructs
- Understanding function calls and returns
- Modeling function side effects
- Handling recursion
- Tracking exception and interrupt control flow
Control flow analysis reveals the higher level code structures such as decisions, loops, and function calls that are needed to generate equivalent C++ code.
Reconstructing C++ Code
With a firm grasp over the data flow and control flow of the disassembled code, we can now start reconstructing equivalent C++ code. Here are some guidelines for generating clean, readable C++ code from the assembly:
- Clearly separate code into functions matching those identified during analysis
- Use proper C++ variable types based on the data flow analysis
- Add comments explaining any aspects that are unclear or ambiguous
- Maintain the control flow structure using if-else, switch, loops etc.
- Break up complex functions into smaller logical pieces
- Give functions and variables meaningful names
- Format the code with proper indentation and spacing
- Test and debug the code to ensure proper decompilation
With these principles, we can produce C++ code that maintains the structure and logic of the original program while being much more readable and maintainable.
Decompiler Tools and Assistance
While a manual decompilation process gives the most flexibility, the process can also be assisted or automated using decompiler tools like:
- Ghidra – NSA developed open-source decompiler with GUI
- RetDec – Online decompiler for multiple platforms
- Hopper – Commercial cross-platform decompiler
- Recaf – Java bytecode decompiler with extensibility
These tools can take a binary or bytecode program and produce a C/C++ codebase that can be further refined manually. They utilize algorithms to analyze code structure, data types, cross-references between functions, and other information to reconstruct source code. However, human assistance is still recommended to improve the readability of their output.
Conclusion
Decompiling a Cortex-M0 .hex file into clean C++ code requires methodically disassembling, analyzing, and reconstructing the program based on its data and control flow. With patience and the right techniques, we can successfully reverse complex machine code back into human readable source code for further study and modification of the original embedded application. Decompiler tools can also assist to automate parts of this process.
