semgrep-rule-variant-creator

SKILL.md

Semgrep Rule Variant Creator

Port existing Semgrep rules to new target languages with proper applicability analysis and test-driven validation.

When to Use

Ideal scenarios:

Porting an existing Semgrep rule to one or more target languages

Creating language-specific variants of a universal vulnerability pattern

Expanding rule coverage across a polyglot codebase

Translating rules between languages with equivalent constructs

When NOT to Use

Do NOT use this skill for:

Creating a new Semgrep rule from scratch (use semgrep-rule-creator instead)

Running existing rules against code

Languages where the vulnerability pattern fundamentally doesn't apply

Minor syntax variations within the same language

Input Specification

This skill requires:

Existing Semgrep rule - YAML file path or YAML rule content

Target languages - One or more languages to port to (e.g., "Golang and Java")

Output Specification

For each applicable target language, produces:

<original-rule-id>-<language>/
├── <original-rule-id>-<language>.yaml     # Ported Semgrep rule
└── <original-rule-id>-<language>.<ext>    # Test file with annotations

Example output for porting sql-injection to Go and Java:

sql-injection-golang/
├── sql-injection-golang.yaml
└── sql-injection-golang.go

sql-injection-java/
├── sql-injection-java.yaml
└── sql-injection-java.java

Rationalizations to Reject

When porting Semgrep rules, reject these common shortcuts:

Rationalization
Why It Fails
Correct Approach

"Pattern structure is identical"
Different ASTs across languages
Always dump AST for target language

"Same vulnerability, same detection"
Data flow differs between languages
Analyze target language idioms

"Rule doesn't need tests since original worked"
Language edge cases differ
Write NEW test cases for target

"Skip applicability - it obviously applies"
Some patterns are language-specific
Complete applicability analysis first

"I'll create all variants then test"
Errors compound, hard to debug
Complete full cycle per language

"Library equivalent is close enough"
Surface similarity hides differences
Verify API semantics match

"Just translate the syntax 1:1"
Languages have different idioms
Research target language patterns

Strictness Level

This workflow is strict - do not skip steps:

Applicability analysis is mandatory: Don't assume patterns translate

Each language is independent: Complete full cycle before moving to next

Test-first for each variant: Never write a rule without test cases

100% test pass required: "Most tests pass" is not acceptable

Overview

This skill guides the creation of language-specific variants of existing Semgrep rules. Each target language goes through an independent 4-phase cycle:

FOR EACH target language:
  Phase 1: Applicability Analysis → Verdict
  Phase 2: Test Creation (Test-First)
  Phase 3: Rule Creation
  Phase 4: Validation
  (Complete full cycle before moving to next language)

Foundational Knowledge

The semgrep-rule-creator skill is the authoritative reference for Semgrep rule creation fundamentals. While this skill focuses on porting existing rules to new languages, the core principles of writing quality rules remain the same.

Consult semgrep-rule-creator for guidance on:

When to use taint mode vs pattern matching - Choosing the right approach for the vulnerability type

Test-first methodology - Why tests come before rules and how to write effective test cases

Anti-patterns to avoid - Common mistakes like overly broad or overly specific patterns

Iterating until tests pass - The validation loop and debugging techniques

Rule optimization - Removing redundant patterns after tests pass

When porting a rule, you're applying these same principles in a new language context. If uncertain about rule structure or approach, refer to semgrep-rule-creator first.

Four-Phase Workflow

Phase 1: Applicability Analysis

Before porting, determine if the pattern applies to the target language.

Analysis criteria:

Does the vulnerability class exist in the target language?

Does an equivalent construct exist (function, pattern, library)?

Are the semantics similar enough for meaningful detection?

Verdict options:

APPLICABLE → Proceed with variant creation

APPLICABLE_WITH_ADAPTATION → Proceed but significant changes needed

NOT_APPLICABLE → Skip this language, document why

See applicability-analysis.md for detailed guidance.

Phase 2: Test Creation (Test-First)

Always write tests before the rule.

Create test file with target language idioms:

Minimum 2 vulnerable cases (ruleid:)

Minimum 2 safe cases (ok:)

Include language-specific edge cases

// ruleid: sql-injection-golang
db.Query("SELECT * FROM users WHERE id = " + userInput)

// ok: sql-injection-golang
db.Query("SELECT * FROM users WHERE id = ?", userInput)

Phase 3: Rule Creation

Analyze AST: semgrep --dump-ast -l <lang> test-file

Translate patterns to target language syntax

Update metadata: language key, message, rule ID

Adapt for idioms: Handle language-specific constructs

See language-syntax-guide.md for translation guidance.

Phase 4: Validation

# Validate YAML
semgrep --validate --config rule.yaml

# Run tests
semgrep --test --config rule.yaml test-file

Checkpoint: Output MUST show All tests passed.

For taint rule debugging:

semgrep --dataflow-traces -f rule.yaml test-file

See workflow.md for detailed workflow and troubleshooting.

Quick Reference

Task
Command

Run tests
semgrep --test --config rule.yaml test-file

Validate YAML
semgrep --validate --config rule.yaml

Dump AST
semgrep --dump-ast -l <lang> <file>

Debug taint flow
semgrep --dataflow-traces -f rule.yaml file

Key Differences from Rule Creation

Aspect
semgrep-rule-creator
This skill

Input
Bug pattern description
Existing rule + target languages

Output
Single rule+test
Multiple rule+test directories

Workflow
Single creation cycle
Independent cycle per language

Phase 1
Problem analysis
Applicability analysis per language

Library research
Always relevant
Optional (when original uses libraries)

Documentation

REQUIRED: Before porting rules, read relevant Semgrep documentation:

Rule Syntax - YAML structure and operators

Pattern Syntax - Pattern matching and metavariables

Pattern Examples - Per-language pattern references

Testing Rules - Testing annotations

Trail of Bits Testing Handbook - Advanced patterns

Next Steps

For applicability analysis guidance, see applicability-analysis.md

For language translation guidance, see language-syntax-guide.md

For detailed workflow and examples, see workflow.md