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Dec. 8, 2017
Oct. 11, 2016

RamFuzz: Combining Unit Tests, Fuzzing, and AI

RamFuzz is a fuzzer for individual method parameters in unit tests. A unit test can use RamFuzz to generate random parameter values for methods under test. The values are logged, and the log can be replayed to repeat the exact same test scenario. But random parameter values aren't limited to just fundamental types: RamFuzz can also automatically produce random objects of any class from the user's code. This allows the user to fuzz methods that accept class parameters. For example, if a method takes an int, a struct S, and a class C as parameters, RamFuzz can randomly generate them all! The test writer needn't worry about the technicalities of creating S and C objects; RamFuzz will do it automatically.

To accomplish this, RamFuzz includes a code generator that reads the C++ source code and generates from it some test code. This test code creates a class instance via a randomly chosen constructor, then proceeds to invoke a random sequence of instance methods with randomly generated arguments. The result is a random class object that can be fed to any method taking that class as a paremeter.

Of course, this produces superficial tests that likely aren't very useful on average -- most methods don't take completely random parameters but constrain them in some way. The intent is that these constraints can be inferred automatically from the logs of many RamFuzz test runs. Because tests are randomized, each run is a different scenario that adds coverage of the code under test. If the quality of randomness is good, then running tests repeatedly for a long time will cover a wide range of possible parameter values, likely including some perfectly valid tests. These logs can then be used, for example, to train an AI to recognize which parameter values are valid. Alternatively, fuzzing strategies can be used to steer the evolution of parameter-value generation towards valid tests.

RamFuzz provides some Python tools to make it easy to train AI on the logs it generates -- see the ai directory and the overview paper. It is possible, for instance, to train a neural network to accurately predict the outcome of a test run based on its RamFuzz log. This is possible because RamFuzz manages to provide test runs sufficiently varied to be useful for discerning what succeeds and what does not.

Note, however, that RamFuzz doesn't automatically generate test assertions on the results of method invocations. Other projects will be created to accomplish that automatically, but RamFuzz will make it possible by conveniently handling parameter fuzzing and logging.

RamFuzz is provided under the Apache 2.0 license. It currently supports only C++ on input (please see "Known Limitations" below), and it requires C++11 support for compiling its output code and runtime.

How to Use

The bin/ramfuzz executable (in LLVM build, see "How to Build" below) generates test code from C++ headers declaring the classes under test. Say we have a header file a.hpp with the following contents:

class Base {
  unsigned sum = 0;
  void bump(unsigned delta) { sum += delta; }
  virtual char id() const { return 'B'; }

class Sub : public Base {
  char id() const override { return 'S'; }

We feed this header to bin/ramfuzz like this:

~/src/llvmbuild/bin/ramfuzz a.hpp -- -std=c++11

The result is two files named fuzz.hpp and fuzz.cpp. These, together with the RamFuzz runtime, contain code that can generate random objects of class Base. The interface is simple: just invoke runtime::gen::make<Base>(), and you get an object of type Base. Here is a program to demonstrate it:

#include <iostream>
#include "fuzz.hpp"

int main(int argc, char *argv[]) {
  ramfuzz::runtime::gen g(argc, argv);
  for (;;) {
    const auto b = g.make<Base>(g.or_subclass);
    std::cout << b->id() << b->sum << std::endl;

unsigned ramfuzz::runtime::spinlimit = 2;

Including fuzz.hpp brings in all the required declarations, including the RamFuzz runtime. The runtime::gen object keeps RNG state, manages logging, and provides the make() method for creating random values of any type. It is documented in runtime/ramfuzz-rt.hpp. The above program simply generates random Base objects and prints them out in an infinite loop.

Say the above code is in a file named main.cpp in the same directory as fuzz.* and ramfuzz-rt.*. Then we can compile it like this:

 c++ -std=c++11 main.cpp fuzz.cpp ramfuzz-rt.cpp 

Here's an excerpt from the resulting executable's output:


Note that make<Base>(or_subclass) sometimes produces a B object and sometimes an S one. The created objects will have their methods (including bump()) invoked a random number of times in random order with random arguments -- this is how a random Base is created.

As the executable runs, it logs the random numbers generated into a file named fuzzlog. And this log can be replayed by running the executable again with fuzzlog as the command-line argument -- that will execute the same code paths and print the same output again.

You can see more examples in the test directory, where each .hpp file is processed by bin/ramfuzz and the result linked with the eponymous .cpp file during testing.

Known Limitations

C++ is a huge language, so the code generator is a work in progress. Although improvements are made constantly, it currently can't handle the following important categories:

  • template parameters with default values
  • variadic templates
  • STL containers other than vector and string
  • array parameters
  • function pointers
  • parameter values that must equal some method's return value

These limitations will typically manifest themselves as ill-formed C++ on the output.

How to Build

  1. Get Clang: the RamFuzz code generator is a Clang tool, so first get and build Clang using these instructions. RamFuzz is known to work with Clang/LLVM version 4.0.0 and has been successfully tested with version 3.8.1.

  2. Drop RamFuzz into Clang: RamFuzz source is intended to go under clang/tools/extra and build from there (as described in this Clang tutorial). Drop the top-level RamFuzz directory into clang/tools/extra and add it (using add_subdirectory) to clang/tools/extra/CMakeLists.txt.

  3. Rebuild Clang: Now the standard LLVM build procedure should produce a bin/ramfuzz executable.

  4. Run Tests: There are some end-to-end tests in the test directory -- see there. There are also unit tests in the unittests directory. RamFuzz adds a new build target check-ramfuzz, which executes all unit- and end-to-end tests. The end-to-end tests depend on bin/ramfuzz, so bin/ramfuzz will be rebuilt before testing if it's out of date.

How to Contribute

RamFuzz welcomes contributions by the community. Each contributor will retain copyright over their code but must sign the developer certificate in the CONTRIBUTORS file by adding their name/contact to the list and using the -s flag in all commits.

RamFuzz code relies on extensive Doxygen-style comments to provide guidance to the reader. Each directory also contains a README file that briefly summarizes what's in it and where to start reading.

To avoid any hurdles to contribution, there is no formal coding style. Don't worry about formalities, just write good code. Look it over and ask yourself: is it simple to use, simple to read, and simple to modify? If so, it's a welcome contribution to this project.