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Contributing to Trail of Bits Semgrep Rules

Thank you for your interest in contributing to ToB semgrep-rules!

The information below will help you set up a local development environment, as well as performing common development tasks.

Requirements

semgrep-rules's only development environment requirement should be Python 3.7 or newer. Development and testing is actively performed on macOS and Linux, but Windows and other supported platforms that are supported by Python should also work.

Development steps

First, clone this repository:

git clone https://github.com/trailofbits/semgrep-rules
cd semgrep-rules

Then install semgrep CLI, and you are good to start development.

Linting

First, install prettier, or use brew to do so.

Use the following command to check rule files for formatting errors:

prettier --check '**/*.{yaml,yml}'

Any issues can be automatically fixed with the following command:

prettier --write '**/*.{yaml,yml}'

Testing

You can run tests locally with:

semgrep --test --test-ignore-todo --metrics=off

To test a specific file:

semgrep --test --test-ignore-todo --metrics=off --config ./go/iterate-over-empty-map.yaml ./go/iterate-over-empty-map.go

Development practices

Before publishing a new rule, or updating an existing one, make sure to review the checklist below:

  • Check if the rule does not already exists. Review this repository and Semgrep registry. If there already is a rule that finds the vulnerability your new rule is targeting, consider making updates to this rule instead of creating a new one.

  • Add metadata. Semgrep defines which metadata fields are required

    • Add a non-standard metadata.description field. It will be used as a description in the semgrep-rules README table.
    • For metadata.references provide a link to official documentation, Trail of Bits blogpost, GitHub issue, or some reputable website. Avoid linking to websites that may disappear in the future.

  • Validate metadata against the official schema

    • Download python validation script wget https://raw.githubusercontent.com/returntocorp/semgrep-rules/develop/.github/scripts/validate-metadata.py
    • Download rules schema wget https://raw.githubusercontent.com/returntocorp/semgrep-rules/develop/metadata-schema.yaml.schm
    • Run python ./validate-metadata.py -s ./metadata-schema.yaml.schm -f .

  • Add tests

    • At least one true positive (ruleid: comment)
    • At least one true negative (ok: comment)
    • Tests are allowed to crash when running them directly or to be meaningless
    • However, try writing tests that can be compiled or parsed by the language interpreter
    • The first few test cases should be easy to understand, the later should be more complex or check for edge-cases
    • Make sure all tests pass, run semgrep --test --test-ignore-todo --metrics=off

  • Run official semgrep lints with semgrep --validate --metrics=off --config ./<new-rule>.yaml

  • Review style of the rules

    • Use 2 spaces for indentation
    • Use >- for multiline messages
    • Use backticks in messages e.g., $VAR, $FUNC, some.method()
    • The languages field in [go, java] format are preferable (not - go \n -java)
    • Run prettier (see Linting)

  • Check amount of false-positives on some large public repositories

  • Check performance - take a look at r2c methodology

  • Add the new rules to the README

    • Run python ./rules_table_generator.py to re-generate the table
    • Manually check if the table was correctly generated

Documentation

We don't provide any documentation for the rules. All information that you need to understand a rule is inside it. Semgrep documentation can be found here.

Releasing

NOTE: If you're a non-maintaining contributor, you don't need the steps here! They're documented for completeness and for onboarding future maintainers.

We don't have a release cycle yet.

All changes to the repository's main branch are automatically pushed to the semgrep registry (with a GitHub action).

Modifying rule's filename, path, or ID will result in duplication of the rule in the registry. This is a known issue, r2c team still works on resolving it.