• Rule writing

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 searches for path/to/rule.py, path/to/rule.js and similar, 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.

Testing autofix

Semgrep's testing mechanism also provides a way to test the behavior of any fix values defined in the rules.

To define a test for autofix behavior:

1. Create a new autofix test file with the .fixed suffix before the file type extension. For example, name the autofix test file of a rule with test code in path/to/rule.py as path/to/rule.fixed.py.
2. Within the autofix test file, enter the expected result of applied autofix rule to the test code.
3. Run semgrep --test to verify that your autofix test file is correctly detected.

When you use semgrep --test, Semgrep applies the autofix rule to the original test code (path/to/rule.py), and then verifies whether this matches the expected outcome defined in the autofix test file (path/to/rule.fixed.py). If there is a mismatch, the line diffs are printed.

info

Hint: Creating an autofix test for a rule with autofix can take less than a minute with the following flow of commands:

cp rule.py rule.fixed.py
semgrep --config rule.yaml rule.fixed.py --autofix

These commands apply the autofix of the rule to the test code. After Semgrep delivers a fix, inspect whether the outcome of this fix looks as expected (for example using vimdiff rule.py rule.fixed.py).

Example

Consider the following rule:

rules:
- id: insecure-eval-use
  patterns:
  - pattern: eval($VAR)
  - pattern-not: eval("...")
  fix: secure_eval($VAR)
  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, secure_eval

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:

semgrep --test rules/

Which will produce the following output:

1/1: ✓ All tests passed
No tests for fixes found.

Semgrep tests automatically avoid failing on lines marked with # todoruleid or # todook.

Storing rules and test targets in different directories

Creating different directories for rules and tests helps users manage a growing library of custom rules. To store rules and test targets in different directories use the --config option.

For example, in the directory with the following structure:

$ tree tests

tests
├── rules
│   └── python
│       └── insecure-eval-use.yaml
└── targets
    └── python
        └── insecure-eval-use.py

4 directories, 2 files

Use of the following command:

semgrep --test --config tests/rules/ tests/targets/

Produces the same output as in the previous example.

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

To test the autofix behavior, add the autofix test file rules/detect-eval.fixed.py to represent the expected outcome of applying the fix to the test code:

from lib import get_user_input, safe_get_user_input, secure_eval

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

So that the directory structure is printed as the following:

$ tree tests

tests
├── rules
│   └── python
│       └── insecure-eval-use.yaml
└── targets
    └── python
        └── insecure-eval-use.py
        └── insecure-eval-use.fixed.py

4 directories, 2 files

Use of the following command:

semgrep --test --config tests/rules/ tests/targets/

Results in the following outcome:

1/1: ✓ All tests passed
1/1: ✓ All fix tests passed

If the fix does not behave as expected, the output prints a line diff. For example, if we replace secure_eval with safe_eval, we can see that lines 5 and 15 are not rendered as expected.

1/1: ✓ All tests passed
0/1: 1 fix tests did not pass:
--------------------------------------------------------------------------------
	✖ targets/python/detect-eval.fixed.py <> autofix applied to targets/python/detect-eval.py

	---
	+++
	@@ -5 +5 @@
	-safe_eval(user_input)
	+secure_eval(user_input)
	@@ -15 +15 @@
	-safe_eval(safe_get_user_input())
	+secure_eval(safe_get_user_input())

Validating rules

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

You can run semgrep --validate --config [filename] to check the configuration. This command runs a combination of Semgrep rules and OCaml checks against your rules to search for issues such as duplicate patterns and missing fields. All rules submitted to the Semgrep Registry are validated.

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

This feature is still experimental and under active development. Your feedback is welcomed!

Enabling autofix in Semgrep Code

To enable autofix for all projects in your Semgrep AppSec Platform organization, follow these steps:

1. In Semgrep AppSec Platform, go to Settings > General > Code.
2. Click the Autofix toggle to enable this feature.

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