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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-useeval(user_input)
# ok: insecure-eval-useeval('print("Hardcoded eval")')
totally_safe_eval = eval# todoruleid: insecure-eval-usetotally_safe_eval(user_input)
# todook: insecure-eval-useeval(safe_get_user_input())

Run the tests with the following:

$ python -m semgrep --quiet --test rules/1 yaml files testedcheck id scoring:--------------------------------------------------------------------------------(TODO: 2) rules/detect-eval.yaml    โœ– 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/1 yaml files testedcheck id scoring:--------------------------------------------------------------------------------(TODO: 2) rules/detect-eval.yaml    โœ” 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 teststestsโ”œโ”€โ”€ rulesโ”‚ย ย  โ””โ”€โ”€ pythonโ”‚ย ย      โ””โ”€โ”€ test.yamlโ””โ”€โ”€ targets    โ””โ”€โ”€ python        โ””โ”€โ”€ test.py
4 directories, 2 files
$ python -m semgrep --quiet --test --config /tmp/tests/rules/ /tmp/tests/targets/1 yaml files testedcheck id scoring:--------------------------------------------------------------------------------(TODO: 0) /tmp/tests/rules/python/test.yaml    โœ” 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.


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