--- name: semgrep-rule-variant-creator description: Creates language variants of existing Semgrep rules. Use when porting a Semgrep rule to specified target languages. Takes an existing rule and target languages as input, produces independent rule+test directories for each language. allowed-tools: - Bash - Read - Write - Edit - Glob - Grep - WebFetch --- # 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: 1. **Existing Semgrep rule** - YAML file path or YAML rule content 2. **Target languages** - One or more languages to port to (e.g., "Golang and Java") ## Output Specification For each applicable target language, produces: ``` -/ ├── -.yaml # Ported Semgrep rule └── -. # 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:** 1. Does the vulnerability class exist in the target language? 2. Does an equivalent construct exist (function, pattern, library)? 3. 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]({baseDir}/references/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 ```go // 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 1. **Analyze AST**: `semgrep --dump-ast -l test-file` 2. **Translate patterns** to target language syntax 3. **Update metadata**: language key, message, rule ID 4. **Adapt for idioms**: Handle language-specific constructs See [language-syntax-guide.md]({baseDir}/references/language-syntax-guide.md) for translation guidance. ### Phase 4: Validation ```bash # 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: ```bash semgrep --dataflow-traces -f rule.yaml test-file ``` See [workflow.md]({baseDir}/references/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 ` | | 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](https://semgrep.dev/docs/writing-rules/rule-syntax) - YAML structure and operators - [Pattern Syntax](https://semgrep.dev/docs/writing-rules/pattern-syntax) - Pattern matching and metavariables - [Pattern Examples](https://semgrep.dev/docs/writing-rules/pattern-examples) - Per-language pattern references - [Testing Rules](https://semgrep.dev/docs/writing-rules/testing-rules) - Testing annotations - [Trail of Bits Testing Handbook](https://appsec.guide/docs/static-analysis/semgrep/advanced/) - Advanced patterns ## Next Steps - For applicability analysis guidance, see [applicability-analysis.md]({baseDir}/references/applicability-analysis.md) - For language translation guidance, see [language-syntax-guide.md]({baseDir}/references/language-syntax-guide.md) - For detailed workflow and examples, see [workflow.md]({baseDir}/references/workflow.md)

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