Semgrep Rule Variant Creator

---
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:
```
<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:**
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 <lang> 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 <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](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)