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Contributing to Kern

Thank you for your interest in contributing to Kern! This document outlines the guidelines for contributing to the project.

Getting Started

  1. Fork the repository and clone it locally.
  2. Initialize submodules: git submodule update --init --recursive
  3. Set up ESP-IDF v5.5.3 and source the environment: source ~/esp/esp-idf/export.sh
  4. Build the project: idf.py build (or just build)

Development Guidelines

Architecture

Kern follows a strict layer separation:

  • main/core/ — Bitcoin logic. Must never depend on UI headers. Use callbacks for any user interaction.
  • main/pages/ — UI pages with create/show/hide/destroy lifecycle.
  • main/ui/ — Reusable LVGL UI primitives.
  • main/qr/ — QR scanning, parsing, and encoding.

Do not introduce UI dependencies into core modules. If a core function needs user confirmation, accept a callback parameter.

Code Style

  • Follow the existing style in the file you are modifying.
  • Keep changes focused, one logical change per commit.
  • Format your code before submitting: 
    ./scripts/format.sh
    This runs clang-format (provided by ESP-IDF) on all .c and .h files in main/ and first-party components.

Static Analysis

The project uses two static analysis tools to catch bugs early. Both are available after sourcing ESP-IDF (source ~/esp/esp-idf/export.sh), except cppcheck which is installed separately.

Both tools cover main/ and first-party components (bbqr, cUR, k_quirc, sd_card, video, wave_4b, wave_35, wave_43). libwally-core and wave_5 are excluded (third-party upstream code: libwally-core is the original; wave_5 ships a vendored ST-style HX8394 driver).

clang-tidy (recommended — catches real bugs):

# Requires a build first (for compile_commands.json)
idf.py build

# Run on a single file
clang-tidy -p build/compile_commands.json main/core/wallet.c

# Run on all project source files (excluding libwally-core)
find main components/bbqr components/cUR components/k_quirc \
     components/sd_card components/video \
     components/wave_4b components/wave_35 components/wave_43 \
     -name '*.c' -not -path '*/build/*' 2>/dev/null | \
  xargs -P$(nproc) -I{} clang-tidy -p build/compile_commands.json {}

The project .clang-tidy config enables bug-finding and security checks tuned for embedded C. Warnings about unknown GCC flags (-fno-tree-switch-conversion, -fstrict-volatile-bitfields) are expected and harmless — they come from clang analyzing GCC-compiled code.

cppcheck:

# Install: sudo apt install cppcheck

# Run on project source directories
cppcheck main/ components/bbqr components/cUR components/k_quirc \
  components/sd_card components/video \
  components/wave_4b components/wave_35 components/wave_43 \
  --enable=warning,style,performance \
  --suppress=missingIncludeSystem \
  --suppress=missingInclude \
  -I main/ -I main/core -I main/ui -I main/pages -I main/qr -I main/utils \
  --std=c11

Note: cppcheck does not understand secure memory wipe patterns (zeroing variables before return) and will flag them as dead stores — these are intentional and should be ignored.

Security

Kern is a Bitcoin signing firmware. Security is not optional.

  • Never introduce network or radio functionality. The air-gap is by design.
  • Use secure memory wipe (WIPE macros from utils/) for sensitive data.
  • Use the existing crypto_utils wrappers around mbedtls rather than rolling custom crypto.
  • Be mindful of stack usage on ESP32-P4, especially in deep call chains.

Objectivity

We value focused, purposeful contributions. Every PR should clearly state what problem it solves — this helps reviewers understand your intent and keeps the project lean.

Whenever possible, link to a previously discussed Issue where the problem has been described and agreed upon. If no issue exists yet, consider opening one first to give the community a chance to weigh in. For small, self-evident fixes (typos, obvious bugs), describing the problem directly in the PR is perfectly fine.

Submitting Changes

  1. Create a branch from master with a descriptive name.
  2. Make your changes in focused, well-described commits.
  3. Ensure the project builds cleanly with idf.py build.
  4. Open a Pull Request that:
    • References the related Issue (preferred), or clearly describes the problem being solved.
    • Explains what changed and why.
    • Notes any impact on security, memory usage, or the air-gap model.

Reporting Issues

Open an Issue describing:

  • The problem — what is wrong or missing, and why it matters.
  • Steps to reproduce (for bugs).
  • Hardware and firmware version (if relevant).

License

By contributing, you agree that your contributions will be licensed under the MIT License.