Testing rules

Semgrep provides a convenient testing mechanism for your rules. You can simply write code and provide a few annotations to let Semgrep know where you are or aren't expecting findings. Semgrep provides the following annotations:

• ruleid: <rule-id>, for protecting against false negatives
• ok: <rule-id> for protecting against false positives
• todoruleid: <rule-id> for future "positive" rule improvements
• todook: <rule-id> for future "negative" rule improvements

Other than annotations there are three things to remember when creating tests:

1. The --test flag tells Semgrep to run tests in the specified directory.
2. Annotations are specified as a comment above the offending line.
3. Semgrep looks for tests based on the rule filename and the languages specified in the rule. In other words, path/to/rule.yaml will look for path/to/rule.py, path/to/rule.js, etc., based on the languages specified in the rule.

info

The .test.yaml file extension can also be used for test files. This is necessary when testing YAML language rules.

Example

Consider the following rule:

rules:
- id: insecure-eval-use
  patterns:
  - pattern: eval(...)
  - pattern-not: eval("...")
  message: Calling 'eval' with user input
  languages: [python]
  severity: WARNING

Given the above is named rules/detect-eval.yaml, you can create rules/detect-eval.py:

from lib import get_user_input, safe_get_user_input

user_input = get_user_input()
# ruleid: insecure-eval-use
eval(user_input)

# ok: insecure-eval-use
eval('print("Hardcoded eval")')

totally_safe_eval = eval
# todoruleid: insecure-eval-use
totally_safe_eval(user_input)

# todook: insecure-eval-use
eval(safe_get_user_input())

Run the tests with the following:

python -m semgrep --quiet --test rules/

Which will produce the following output:

1 yaml files tested
check id scoring:
--------------------------------------------------------------------------------
(TODO: 2) rules/detect-eval.yaml
    ✖ insecure-eval-use                                  TP: 1 TN: 2 FP: 1 FN: 1
    test: rules/detect-eval.py, expected lines: [5, 12], reported lines: [5, 15]
--------------------------------------------------------------------------------
final confusion matrix: TP: 1 TN: 2 FP: 1 FN: 1
--------------------------------------------------------------------------------

• True positives (TP) correspond to ruleid
• True negatives (TN) correspond to ok
• False positives (FP) correspond to todook
• False negatives (FN) correspond to todoruleid

To avoid failing on TODOs you can specify --test-ignore-todo:

python -m semgrep --quiet --test --test-ignore-todo rules/

This will produce the following output:

1 yaml files tested
check id scoring:
--------------------------------------------------------------------------------
(TODO: 2) rules/detect-eval.yaml
    ✔ insecure-eval-use                                  TP: 1 TN: 1 FP: 0 FN: 0
--------------------------------------------------------------------------------
final confusion matrix: TP: 1 TN: 1 FP: 0 FN: 0
--------------------------------------------------------------------------------

To store rules and test targets in different directories you can specify --config:

tree tests

will produce the following output:

tests
├── rules
│   └── python
│       └── test.yaml
└── targets
    └── python
        └── test.py

4 directories, 2 files

python -m semgrep --quiet --test --config /tmp/tests/rules/ /tmp/tests/targets/

will produce the following output:

1 yaml files tested
check id scoring:
--------------------------------------------------------------------------------
(TODO: 0) /tmp/tests/rules/python/test.yaml
    ✔ eqeq-is-bad                                        TP: 1 TN: 0 FP: 0 FN: 0
--------------------------------------------------------------------------------
final confusion matrix: TP: 1 TN: 0 FP: 0 FN: 0
--------------------------------------------------------------------------------

The subdirectory structure of these two directories must be the same for Semgrep to correctly find the associated files.

Validating rules

At r2c, we believe in checking the code we write, and that includes rules.

You can run semgrep --validate --config [file] to check the given config. This will run a combination of Semgrep rules and OCaml checks against your rules to search for things like duplicate patterns and missing fields. All rules submitted to the semgrep-rules repository are validated.

The semgrep rules are pulled from p/semgrep-rule-lints.

This feature is still experimental and under active development. Feedback is welcome!

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