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Nimfilt

Nimfilt is a collection of modules and scripts to help with analyzing Nim binaries. It started out as a CLI demangling tool inspired by c++filt. It evolved into a larger set of tools for analyzing Nim, but the original name stuck.

  • nimfilt.py: a Python module that implements demangling for Nim. It can also be run as basic CLI tool.
  • id_nim_binary.yar: a set of YARA rules to identify Nim ELF and PE binaries.
  • nimfilt_ida.py: Nimfilt for IDA, an IDApython script to help reverse-engineers handle Nim binaries.
  • nimfilt_ghidra.py: Nimfilt for Ghidra, a GhidraScript to help reverse-engineers handle Nim binaries.

Context

Nim is compiled to another language (usually C/C++) before being compiled to a native executable. It also doesn't include a large runtime. However, the process still leaves some Nim-specific artefacts and specificites in in the produced binary.

For one, method and module names are mangled using, Nim's own name scheme which is distinct from C++'s. This scheme isn't documented so I relied on the source code of the Nim compiler. Most of this name mangling in implemented in compiler/msgs.nim and compiler/ccgtypes.nim.

Nimfilt for IDA

The IDAPython script can be run as a one-off or installed as a plugin.

If running as a script, simply launch it from the Nimfilt project directory. It is recommended to do so after auto-analysis has completed and you've loaded any additional FLIRT signatures.

Plugin setup using Sark's plugin loader

Add <nimfilt_project_dir>/nimfilt_ida.py to your your plugins.list as per their instruction on installing plugins.

Manual plugin setup

  1. Copy nimfilt.py to a directory that is included in your IDAPython's PYTHONPATH (commonly <IDA_install_dir>/python/ or %APPDATA%/Hex-Rays/IDA Pro/python/3/).
  2. Copy nimfilt_ida.py to your IDAPython plugin directory (usually <IDA_install_dir>/plugins/)

Usage

Note: The current version of Nimfilt for IDA only supports one command which runs all analyses.

Navigate to Edit -> Plugins -> Nimfilt and click on it.

You can set Nimfilt to automatically execute when a loaded file is recognized as a Nim binary. To do so, set the AUTO_RUN global variable to True in nimfilt_ida.py

Nimflit for Ghidra

The GhidraScript is a one-off script.

If running as a script, simply launch it from the Nimfilt project directory. It is recommended to do so after auto-analysis has completed and you've loaded any additional FLIRT signatures.

Manual pluging setup

Copy nimfilt.py and nimfilt_ghidra.py to your ghidra_script directory (usually $HOME/ghidra_scripts)

Usage

Note: The current version of Nimfilt for Ghidra only supports one command which runs all analyses.

Navigate to Window -> Script Manager. Find the nimfilt_ghidra.py script in the list. Select it and click the Run button.

Running tests

Nimfilt uses the unittest package from the Python standard library for unit testing. You can run the test suite using the following command: python -m unittest test/*.py.

Features

Current features include:

  • Identifying if a loaded file is a Nim binary.
  • Demangling Nim function and package names.
  • Demangling Nim package init function names.
  • Organizing functions into directories by package.
  • Identifying, typing and renaming Nim strings.

TODO/Known issues

  • Handle double mangling (C++ and Nim) for executables compiled with cpp
  • Simplify module paths
  • IDA Script: Format Init function' module paths to match regular function format
  • IDA Script: Group packages under root-level directories: Nimble, STD and local/main
  • IDA Script: Use simplified pkg name when renaming functions

Similar and related work

AlphaGolang is a project that fulfills a similar role for Go binaries. While none of AlphaGolang's code was used directly in Nimfilt, it served as a general inspiration and was useful in understanding IDA's folder API.

Nim-IDA-FLIRT-Generator is another project that helps with reverse-engineering Nim binaries. It does so by greatly simplifying the process of creating IDA FLIRT signatures for Nim. It nicely complements Nimfilt for binaries that lack symbols: First generate then apply your FLIRT signatures, then run Nimfilt for the best results.