JUnit Integration Implementation

Jazzer‘s JUnit integration starts from the FuzzTest annotation. As mentioned in the annotation’s javadoc, our integration runs in one of two modes: fuzzing and regression. Fuzzing mode will generate new inputs to feed into the tests to find new issues and regression mode will run the tests against previous findings, no fuzzing is done. The main entrypoints for the actual integration code are found in two of the annotations on FuzzTest: @ArgumentsSource(FuzzTestArgumentsProvider.class) and @ExtendsWith(FuzzTestExtensions.class).

Because these same files and functions are involved in two mostly separate sets of functionality, this will look at the flow of the different methods involved in integrating with JUnit in fuzzing mode (when JAZZER_FUZZ is set to any non-empty value) and in regression mode (when JAZZER_FUZZ is not set) separately.

Fuzzing Flow

JUnit will call the following methods for each test marked with FuzzTest.

evaluateExecutionCondition

The first call to this test will determine if the test should be run at all. In fuzzing mode, we only allow one test to be run due to global state in libfuzzer that would mean multiple tests would interfere with each other. Jazzer will accept the first fuzz test that is checked as the test to be run. It will cache which test it has seen first and return that test as enabled.

If this returns that a test is disabled, JUnit will not run the rest of these methods for this test and instead skip to the next one.

provideArguments

This will configure the fuzzing agent to set up code instrumentation, instantiate a FuzzTestExecutor and put it into JUnit's extensionContext, then create a stream of a single empty argument set. As the comment mentions, this is so that JUnit will actually execute the test but the argument will not be used.

evaluateExecutionCondition

This will be called for each argument set for the current test. In fuzzing mode, there will only be the single empty argument set which will be enabled.

interceptTestTemplateMethod

This will call invocation.skip() which prevents invoking the test function with the default set of arguments provideArguments created. It will instead extract the FuzzTestExecutor instance from the extensionContext and then calls FuzzTestExecutor#execute which creates a FuzzTargetRunner to run the actual fuzzing.

Crashes are saved in resources/<package>/<test file name>Inputs/<test method name> and results that are interesting to libfuzzer are saved in .cifuzz-corpus.

Regression Flow

Similar to fuzzing mode, JUnit will call these methods for each test marked with FuzzTest.

evaluateExecutionCondition

This checks if the given test should be run at all. In regression mode, all tests are run so this will always return enabled.

provideArguments

This will configure the fuzzing agent as in fuzzing mode, then gather test cases to run from the following sources:

1. A default argument set of just an empty input
2. A set of arguments from the associated resources directory
3. If a .cifuzz-corpus directory exists, relevant entries from that are added as well

Prior to returning, the stream of test cases is put through adaptInputsForFuzzTest to turn the raw bytes from the files into the actual types to be given to the tested function.

Resources Tests

The tests from the resources directory are gathered by walkInputs. This will look for inputs in two places:

• resources/<package>/<test class name>Inputs - files found directly within this directory will be used as inputs for any tests within this class. This allows for easy sharing of corpus entries. Jazzer does not automatically put entries here, instead a human will need to decide a finding should be shared and manually move it.
• resources/<package>/<test class name>Inputs/<test method name> - files found in this directory and any directory under it are used as inputs for only the test of the same name.

JUnit will use the file's name as the name of the test case for its reporting. It also accepts .jar files where it will search with the given directory in the jar.

CIFuzz Corpus

The corpus kept in .cifuzz-corpus/<test class name>/<test method name> holds any inputs that libfuzzer found worth saving and not necessarily just inputs that caused a crash. Jazzer is able to set the directory but the contents of these directories are managed entirely by libfuzzer. Unlike with the resources test inputs above, this will not look in .cifuzz-corpus/<test class name> for shared test cases. This is a limitation of libfuzzer.

evaluateExecutionCondition

This will run once per argument set returned by provideArguments for this test. All argument sets will return as enabled.

interceptTestTemplateMethod

This will run for each individual test case for each fuzz test and will mostly just allow the test function to proceed with the provided arguments. Prior to the call to the test, it will enable the agent's hooks and then disable them afterward. It will also check for and report any findings from Jazzer to JUnit.

Diagrams

Below are two sequence diagrams for how JUnit calls evaluateExecutionConditions and provideArguments in fuzzing and regression mode. These diagrams ignore interceptTestTemplateMethod for brevity as its behavior and place in the sequence is more clear.

Fuzzing

Regression