Gh0stBlade / mgs_reversing

Metal Gear Solid PSX reimpl repo

What, what is this?

This project aims to completely reverse engineer Metal Gear Solid for PlayStation back to C source code which when compiled produces the same assembly code.

How to build

1. Install Python 3 if you haven't and make sure it's in your PATH.
2. Clone down the PsyQ SDK repo from: https://github.com/FoxdieTeam/psyq_sdk.git
3. Clone down this repo.
4. Open a terminal and cd into the build directory.
5. Issue the command pip install -r requirements.txt.
6. Issue the command python build.py --psyq_path=YourPath where YourPath is the location of your cloned psyq_sdk depot.
   • Alternatively you can add PSYQ_SDK to your environment variables before invoking python build.py.
7. At the end you should see a message confirming that the built binary's hash matches the original game's binary's hash. If your code caused the compiler to emit warnings, try to fix them without breaking the match.

How to decompile a function

Using IDA or Ghidra (with the ghidra_psx_ldr extension) disassemble the original game binary (SLPM-86247), or use one that you compiled yourself provided that the output was OK. Now choose a .s file from the asm directory where that function isn’t part of the mts system and also isn’t part of psyq.

Given the address of the function go to this location in your reversing tool. Delete the .s file and search for a .c file which has a #pragma INCLUDE_ASM() directive pointing to the former .s file; if none exists, create a .c file with the name of the function and open it. Now write an empty C function that has the same name as the former assembly function as well as a suitable signature; when you re-execute python build.py, the build will not be OK as your empty function will no longer build a matching binary.

Now comes the hard part: implement the function such that it matches the functionality of the assembly and build again. Repeat this until your build is OK – ie your C code is functionally the same and produces exactly the same assembly as the original function.

Iterative building is currently unreliable and it is highly recommended to run make clean && python build.py to be certain that your binary is truly a match.

Help, I am totally stuck?

Join our Discord and ask for help in #metal_gear_dev.

There are various Ghidra scripts in build/ghidra_scripts/ to help with decompilation:

• import_map.py: when you have produced a matching build, this imports the symbols from the map file into Ghidra;
• update_data.py: make sure to read the instructions to this script, which updates data in accordance with declared data types provided they have been imported from header files;
• update_functions.py: updates function return types and parameters according to the declarations of matched functions.

It is highly recommended to re-run auto-analysis whenever you have executed these scripts.

We make extensive use of decomp.me, which has a Metal Gear Solid preset, to help match functions; before working on a function, search for it on the website and if you don't find it, go to the build folder and run python decompme_asm.py [path to .s file] to have the assembly instructions in your clipboard ready to paste into a new decomp.me scratch.