Fuzzing Interface

The fuzzing interface is glue code living in mozilla-central in order to make it easier for developers and security researchers to test C/C++ code with either libFuzzer or afl-fuzz.

These fuzzing tools, are based on compile-time instrumentation to measure things like branch coverage and more advanced heuristics per fuzzing test. Doing so allows these tools to progress through code with little to no custom logic/knowledge implemented in the fuzzer itself. Usually, the only thing these tools need is a code “shim” that provides the entry point for the fuzzer to the code to be tested. We call this additional code a fuzzing target and the rest of this manual describes how to implement and work with these targets.

As for the tools used with these targets, we currently recommend the use of libFuzzer over afl-fuzz, as the latter is no longer maintained while libFuzzer is being actively developed. Furthermore, libFuzzer has some advanced instrumentation features (e.g. value profiling to deal with complicated comparisons in code), making it overall more effective.

What can be tested?

The interface can be used to test all C/C++ code that either ends up in libxul (more precisely, the gtest version of libxul) or is part of the JS engine.

Note that this is not the right testing approach for testing the full browser as a whole. It is rather meant for component-based testing (especially as some components cannot be easily separated out of the full build).


Note: If you are working on the JS engine (trying to reproduce a bug or seeking to develop a new fuzzing target), then please also read the JS Engine Specifics Section at the end of this documentation, as the JS engine offers additional options for implementing and running fuzzing targets.

Reproducing bugs for existing fuzzing targets

If you are working on a bug that involves an existing fuzzing interface target, you have two options for reproducing the issue:

Using existing builds

We have several fuzzing builds in CI that you can simply download. We recommend using fuzzfetch for this purpose, as it makes downloading and unpacking these builds much easier.

You can install fuzzfetch from Github or via pip.

Afterwards, you can run

$ python -m fuzzfetch -a --fuzzing --gtest -n firefox-fuzzing

to fetch the latest optimized build. Alternatively, we offer non-ASan debug builds which you can download using

$ python -m fuzzfetch -d --fuzzing --gtest -n firefox-fuzzing

In both commands, firefox-fuzzing indicates the name of the directory that will be created for the download.

Afterwards, you can reproduce the bug using

$ FUZZER=TargetName firefox-fuzzing/firefox test.bin

assuming that TargetName is the name of the fuzzing target specified in the bug you are working on and test.bin is the attached testcase.


Note: You should not export the FUZZER variable permanently in your shell, especially if you plan to do local builds. If the FUZZER variable is exported, it will affect the build process.

If the CI builds don’t meet your requirements and you need a local build instead, you can follow the steps below to create one:

Local build requirements and flags

You will need a Linux environment with a recent Clang. Using the Clang downloaded by ./mach bootstrap or a newer version is recommended.

The only build flag required to enable the fuzzing targets is --enable-fuzzing, so adding

ac_add_options --enable-fuzzing

to your .mozconfig is already sufficient for producing a fuzzing build. However, for improved crash handling capabilities and to detect additional errors, it is strongly recommended to combine libFuzzer with AddressSanitizer by adding

ac_add_options --enable-address-sanitizer

at least for optimized builds and bugs requiring ASan to reproduce at all (e.g. you are working on a bug where ASan reports a memory safety violation of some sort).

Once your build is complete, you must additionally run

$ ./mach gtest dontruntests

to force the gtest libxul to be built.


Note: If you modify any code, please ensure that you run both build commands to ensure that the gtest libxul is also rebuilt. It is a common mistake to only run ./mach build and miss the second command.

Once these steps are complete, you can reproduce the bug locally using the same steps as described above for the downloaded builds.

Developing new fuzzing targets

Developing a new fuzzing target using the fuzzing interface only requires a few steps.

Determine if the fuzzing interface is the right tool

The fuzzing interface is not suitable for every kind of testing. In particular if your testing requires the full browser to be running, then you might want to look into other testing methods.

The interface uses the ScopedXPCOM implementation to provide an environment in which XPCOM is available and initialized. You can initialize further subsystems that you might require, but you are responsible yourself for any kind of initialization steps.

There is (in theory) no limit as to how far you can take browser initialization. However, the more subsystems are involved, the more problems might occur due to non-determinism and loss of performance.

If you are unsure if the fuzzing interface is the right approach for you or you require help in evaluating what could be done for your particular task, please don’t hestitate to contact us.

Develop the fuzzing code

Where to put your fuzzing code

The code using the fuzzing interface usually lives in a separate directory called fuzztest that is on the same level as gtests. If your component has no gtests, then a subdirectory either in tests or in your main directory will work. If such a directory does not exist yet in your component, then you need to create one with a suitable moz.build. See the mtransport target for an example

In order to include the new subdirectory into the build process, you will also have to modify the toplevel moz.build file accordingly. For this purpose, you should add your directory to TEST_DIRS only if FUZZING_INTERFACES is set. See again the mtransport target for an example.

How your code should look like

In order to define your fuzzing target MyTarget, you only need to implement 2 functions:

  1. A one-time initialization function.

    At startup, the fuzzing interface calls this function once, so this can be used to perform one-time operations like initializing subsystems or parsing extra fuzzing options.

    This function is the equivalent of the LLVMFuzzerInitialize function and has the same signature. However, with our fuzzing interface, it won’t be resolved by its name, so it can be defined static and called whatever you prefer. Note that the function should always return 0 and can (except for the return), remain empty.

    For the sake of this documentation, we assume that you have static int FuzzingInitMyTarget(int* argc, char*** argv);

  2. The fuzzing iteration function.

    This is where the actual fuzzing happens, and this function is the equivalent of LLVMFuzzerTestOneInput. Again, the difference to the fuzzing interface is that the function won’t be resolved by its name. In addition, we offer two different possible signatures for this function, either

    static int FuzzingRunMyTarget(const uint8_t* data, size_t size);


    static int FuzzingRunMyTarget(nsCOMPtr<nsIInputStream> inputStream);

    The latter is just a wrapper around the first one for implementations that usually work with streams. No matter which of the two signatures you choose to work with, the only thing you need to implement inside the function is the use of the provided data with your target implementation. This can mean to simply feed the data to your target, using the data to drive operations on the target API, or a mix of both.

    While doing so, you should avoid altering global state in a permanent way, using additional sources of data/randomness or having code run beyond the lifetime of the iteration function (e.g. on another thread), for one simple reason: Coverage-guided fuzzing tools depend on the deterministic nature of the iteration function. If the same input to this function does not lead to the same execution when run twice (e.g. because the resulting state depends on multiple successive calls or because of additional external influences), then the tool will not be able to reproduce its fuzzing progress and perform badly. Dealing with this restriction can be challenging e.g. when dealing with asynchronous targets that run multi-threaded, but can usually be managed by synchronizing execution on all threads at the end of the iteration function. For implementations accumulating global state, it might be necessary to (re)initialize this global state in each iteration, rather than doing it once in the initialization function, even if this costs additional performance.

    Note that unlike the vanilla libFuzzer approach, you are allowed to return 1 in this function to indicate that an input is “bad”. Doing so will cause libFuzzer to discard the input, no matter if it generated new coverage or not. This is particularly useful if you have means to internally detect and catch bad testcase behavior such as timeouts/excessive resource usage etc. to avoid these tests to end up in your corpus.

Once you have implemented the two functions, the only thing remaining is to register them with the fuzzing interface. For this purpose, we offer two macros, depending on which iteration function signature you used. If you sticked to the classic signature using buffer and size, you can simply use

#include "FuzzingInterface.h"

// Your includes and code

MOZ_FUZZING_INTERFACE_RAW(FuzzingInitMyTarget, FuzzingRunMyTarget, MyTarget);

where MyTarget is the name of the target and will be used later to decide at runtime which target should be used.

If instead you went for the streaming interface, you need a different include, but the macro invocation is quite similar:

#include "FuzzingInterfaceStream.h"

// Your includes and code

MOZ_FUZZING_INTERFACE_STREAM(FuzzingInitMyTarget, FuzzingRunMyTarget, MyTarget);

For a live example, see also the implementation of the STUN fuzzing target.

Add instrumentation to the code being tested

libFuzzer requires that the code you are trying to test is instrumented with special compiler flags. Fortunately, adding these on a per-directory basis can be done just by including the following directive in each moz.build file that builds code under test:

# Add libFuzzer configuration directives

The include already does the appropriate configuration checks to be only active in fuzzing builds, so you don’t have to guard this in any way.


Note: This include modifies CFLAGS and CXXFLAGS accordingly but this only works for source files defined in this particular directory. The flags are not propagated to subdirectories automatically and you have to ensure that each directory that builds source files for your target has the include added to its moz.build file.

By keeping the instrumentation limited to the parts that are actually being tested using this tool, you not only increase the performance but also potentially reduce the amount of noise that libFuzzer sees.

Build your code

See the Build instructions above for instructions how to modify your .mozconfig to create the appropriate build.

Running your code and building a corpus

You need to set the following environment variable to enable running the fuzzing code inside Firefox instead of the regular browser.

  • FUZZER=name

Where name is the name of your fuzzing module that you specified when calling the MOZ_FUZZING_INTERFACE_RAW macro. For the example above, this would be MyTarget or StunParser for the live example.

Now when you invoke the firefox binary in your build directory with the -help=1 parameter, you should see the regular libFuzzer help. On Linux for example:

$ FUZZER=StunParser obj-asan/dist/bin/firefox -help=1

You should see an output similar to this:

Running Fuzzer tests...

To run fuzzing pass 0 or more directories.
obj-asan/dist/bin/firefox [-flag1=val1 [-flag2=val2 ...] ] [dir1 [dir2 ...] ]

To run individual tests without fuzzing pass 1 or more files:
obj-asan/dist/bin/firefox [-flag1=val1 [-flag2=val2 ...] ] file1 [file2 ...]

Flags: (strictly in form -flag=value)
 verbosity                      1       Verbosity level.
 seed                           0       Random seed. If 0, seed is generated.
 runs                           -1      Number of individual test runs (-1 for infinite runs).
 max_len                        0       Maximum length of the test input. If 0, libFuzzer tries to guess a good value based on the corpus and reports it.

Reproducing a Crash

In order to reproduce a crash from a given test file, simply put the file as the only argument on the command line, e.g.

$ FUZZER=StunParser obj-asan/dist/bin/firefox test.bin

This should reproduce the given problem.

FuzzManager and libFuzzer

Our FuzzManager project comes with a harness for running libFuzzer with an optional connection to a FuzzManager server instance. Note that this connection is not mandatory, even without a server you can make use of the local harness.

You can find the harness here.

An example invocation for the harness to use with StunParser could look like this:

FUZZER=StunParser python /path/to/afl-libfuzzer-daemon.py --fuzzmanager \
    --stats libfuzzer-stunparser.stats --libfuzzer-auto-reduce-min 500 --libfuzzer-auto-reduce 30 \
    --tool libfuzzer-stunparser --libfuzzer --libfuzzer-instances 6 obj-asan/dist/bin/firefox \
    -max_len=256 -use_value_profile=1 -rss_limit_mb=3000 corpus-stunparser

What this does is

  • run libFuzzer on the StunParser target with 6 parallel instances using the corpus in the corpus-stunparser directory (with the specified libFuzzer options such as -max_len and -use_value_profile)

  • automatically reduce the corpus and restart if it grew by 30% (and has at least 500 files)

  • use FuzzManager (need a local .fuzzmanagerconf and a firefox.fuzzmanagerconf binary configuration as described in the FuzzManager manual) and submit crashes as libfuzzer-stunparser tool

  • write statistics to the libfuzzer-stunparser.stats file

JS Engine Specifics

The fuzzing interface can also be used for testing the JS engine, in fact there are two separate options to implement and run fuzzing targets:

Implementing in C++

Similar to the fuzzing interface in Firefox, you can implement your target in entirely C++ with very similar interfaces compared to what was described before.

There are a few minor differences though:

  1. All of the fuzzing targets live in js/src/fuzz-tests.

  2. All of the code is linked into a separate binary called fuzz-tests, similar to how all JSAPI tests end up in jsapi-tests. In order for this binary to be built, you must build a JS shell with --enable-fuzzing and --enable-tests. Again, this can and should be combined with AddressSanitizer for maximum effectiveness. This also means that there is no need to (re)build gtests when dealing with a JS fuzzing target and using a shell as part of a full browser build.

  3. The harness around the JS implementation already provides you with an initialized JSContext and global object. You can access these in your target by declaring

    extern JS::PersistentRootedObject gGlobal;


    extern JSContext* gCx;

    but there is no obligation for you to use these.

For a live example, see also the implementation of the StructuredCloneReader target.

Implementing in JS

In addition to the C++ targets, you can also implement targets in JavaScript using the JavaScript Runtime (JSRT) fuzzing approach. Using this approach is not only much simpler (since you don’t need to know anything about the JSAPI or engine internals), but it also gives you full access to everything defined in the JS shell, including handy functions such as timeout().

Of course, this approach also comes with disadvantages: Calling into JS and performing the fuzzing operations there costs performance. Also, there is more chance for causing global side-effects or non-determinism compared to a fairly isolated C++ target.

As a rule of thumb, you should implement the target in JS if

  • you don’t know C++ and/or how to use the JSAPI (after all, a JS fuzzing target is better than none),

  • your target is expected to have lots of hangs/timeouts (you can catch these internally),

  • or your target is not isolated enough for a C++ target and/or you need specific JS shell functions.

There is an example target in-tree that shows roughly how to implement such a fuzzing target.

To run such a target, you must run the js (shell) binary instead of the fuzz-tests binary and point the FUZZER variable to the file containing your fuzzing target, e.g.

$ FUZZER=/path/to/jsrtfuzzing-example.js obj-asan/dist/bin/js --fuzzing-safe --no-threads -- <libFuzzer options here>

More elaborate targets can be found in js/src/fuzz-tests/.


Fuzzing Interface: Error: No testing callback found

This error means that the fuzzing callback with the name you specified using the FUZZER environment variable could not be found. Reasons for are typically either a misspelled name or that your code wasn’t built (check your moz.build file and build log).

mach build doesn’t seem to update my fuzzing code

Keep in mind you always need to run both the mach build and mach gtest dontruntests commands in order to update your fuzzing code. The latter rebuilds the gtest version of libxul, containing your code.