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ReStructuredText
500 строки
20 KiB
ReStructuredText
Fuzzing Interface
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=================
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The fuzzing interface is glue code living in mozilla-central in order to
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make it easier for developers and security researchers to test C/C++
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code with either `libFuzzer <https://llvm.org/docs/LibFuzzer.html>`__ or
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`afl-fuzz <http://lcamtuf.coredump.cx/afl/>`__.
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These fuzzing tools, are based on *compile-time instrumentation* to measure
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things like branch coverage and more advanced heuristics per fuzzing test.
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Doing so allows these tools to progress through code with little to no custom
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logic/knowledge implemented in the fuzzer itself. Usually, the only thing
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these tools need is a code "shim" that provides the entry point for the fuzzer
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to the code to be tested. We call this additional code a *fuzzing target* and
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the rest of this manual describes how to implement and work with these targets.
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As for the tools used with these targets, we currently recommend the use of
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libFuzzer over afl-fuzz, as the latter is no longer maintained while libFuzzer
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is being actively developed. Furthermore, libFuzzer has some advanced
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instrumentation features (e.g. value profiling to deal with complicated
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comparisons in code), making it overall more effective.
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What can be tested?
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~~~~~~~~~~~~~~~~~~~
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The interface can be used to test all C/C++ code that either ends up in
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``libxul`` (more precisely, the gtest version of ``libxul``) **or** is
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part of the JS engine.
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Note that this is not the right testing approach for testing the full
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browser as a whole. It is rather meant for component-based testing
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(especially as some components cannot be easily separated out of the
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full build).
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.. note::
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**Note:** If you are working on the JS engine (trying to reproduce a
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bug or seeking to develop a new fuzzing target), then please also read
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the :ref:`JS Engine Specifics Section <JS Engine Specifics>` at the end
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of this documentation, as the JS engine offers additional options for
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implementing and running fuzzing targets.
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Reproducing bugs for existing fuzzing targets
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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If you are working on a bug that involves an existing fuzzing interface target,
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you have two options for reproducing the issue:
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Using existing builds
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^^^^^^^^^^^^^^^^^^^^^
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We have several fuzzing builds in CI that you can simply download. We recommend
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using ``fuzzfetch`` for this purpose, as it makes downloading and unpacking
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these builds much easier.
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You can install ``fuzzfetch`` from
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`Github <https://github.com/MozillaSecurity/fuzzfetch>`__ or
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`via pip <https://pypi.org/project/fuzzfetch/>`__.
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Afterwards, you can run
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::
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$ python -m fuzzfetch -a --fuzzing --gtest -n firefox-fuzzing
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to fetch the latest optimized build. Alternatively, we offer non-ASan debug builds
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which you can download using
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::
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$ python -m fuzzfetch -d --fuzzing --gtest -n firefox-fuzzing
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In both commands, ``firefox-fuzzing`` indicates the name of the directory that
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will be created for the download.
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Afterwards, you can reproduce the bug using
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::
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$ FUZZER=TargetName firefox-fuzzing/firefox test.bin
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assuming that ``TargetName`` is the name of the fuzzing target specified in the
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bug you are working on and ``test.bin`` is the attached testcase.
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.. note::
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**Note:** You should not export the ``FUZZER`` variable permanently
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in your shell, especially if you plan to do local builds. If the ``FUZZER``
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variable is exported, it will affect the build process.
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If the CI builds don't meet your requirements and you need a local build instead,
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you can follow the steps below to create one:
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Local build requirements and flags
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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You will need a Linux environment with a recent Clang. Using the Clang downloaded
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by ``./mach bootstrap`` or a newer version is recommended.
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The only build flag required to enable the fuzzing targets is ``--enable-fuzzing``,
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so adding
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::
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ac_add_options --enable-fuzzing
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to your ``.mozconfig`` is already sufficient for producing a fuzzing build.
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However, for improved crash handling capabilities and to detect additional errors,
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it is strongly recommended to combine libFuzzer with :ref:`AddressSanitizer <Address Sanitizer>`
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by adding
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::
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ac_add_options --enable-address-sanitizer
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at least for optimized builds and bugs requiring ASan to reproduce at all
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(e.g. you are working on a bug where ASan reports a memory safety violation
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of some sort).
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Once your build is complete, you **must** additionally run
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::
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$ ./mach gtest dontruntests
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to force the gtest libxul to be built.
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.. note::
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**Note:** If you modify any code, please ensure that you run **both** build
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commands to ensure that the gtest libxul is also rebuilt. It is a common mistake
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to only run ``./mach build`` and miss the second command.
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Once these steps are complete, you can reproduce the bug locally using the same
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steps as described above for the downloaded builds.
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Developing new fuzzing targets
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Developing a new fuzzing target using the fuzzing interface only requires a few steps.
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Determine if the fuzzing interface is the right tool
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The fuzzing interface is not suitable for every kind of testing. In particular
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if your testing requires the full browser to be running, then you might want to
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look into other testing methods.
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The interface uses the ``ScopedXPCOM`` implementation to provide an environment
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in which XPCOM is available and initialized. You can initialize further subsystems
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that you might require, but you are responsible yourself for any kind of
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initialization steps.
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There is (in theory) no limit as to how far you can take browser initialization.
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However, the more subsystems are involved, the more problems might occur due to
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non-determinism and loss of performance.
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If you are unsure if the fuzzing interface is the right approach for you or you
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require help in evaluating what could be done for your particular task, please
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don't hestitate to :ref:`contact us <Fuzzing#contact-us>`.
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Develop the fuzzing code
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^^^^^^^^^^^^^^^^^^^^^^^^
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Where to put your fuzzing code
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''''''''''''''''''''''''''''''
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The code using the fuzzing interface usually lives in a separate directory
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called ``fuzztest`` that is on the same level as gtests. If your component
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has no gtests, then a subdirectory either in tests or in your main directory
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will work. If such a directory does not exist yet in your component, then you
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need to create one with a suitable ``moz.build``. See `the transport target
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for an example <https://searchfox.org/mozilla-central/source/dom/media/webrtc/transport/fuzztest/moz.build>`__
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In order to include the new subdirectory into the build process, you will
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also have to modify the toplevel ``moz.build`` file accordingly. For this
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purpose, you should add your directory to ``TEST_DIRS`` only if ``FUZZING_INTERFACES``
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is set. See again `the transport target for an example
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<https://searchfox.org/mozilla-central/rev/de7676288a78b70d2b9927c79493adbf294faad5/media/mtransport/moz.build#18-24>`__.
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How your code should look like
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''''''''''''''''''''''''''''''
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In order to define your fuzzing target ``MyTarget``, you only need to implement 2 functions:
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1. A one-time initialization function.
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At startup, the fuzzing interface calls this function **once**, so this can
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be used to perform one-time operations like initializing subsystems or parsing
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extra fuzzing options.
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This function is the equivalent of the `LLVMFuzzerInitialize <https://llvm.org/docs/LibFuzzer.html#startup-initialization>`__
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function and has the same signature. However, with our fuzzing interface,
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it won't be resolved by its name, so it can be defined ``static`` and called
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whatever you prefer. Note that the function should always ``return 0`` and
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can (except for the return), remain empty.
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For the sake of this documentation, we assume that you have ``static int FuzzingInitMyTarget(int* argc, char*** argv);``
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2. The fuzzing iteration function.
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This is where the actual fuzzing happens, and this function is the equivalent
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of `LLVMFuzzerTestOneInput <https://llvm.org/docs/LibFuzzer.html#fuzz-target>`__.
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Again, the difference to the fuzzing interface is that the function won't be
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resolved by its name. In addition, we offer two different possible signatures
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for this function, either
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``static int FuzzingRunMyTarget(const uint8_t* data, size_t size);``
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or
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``static int FuzzingRunMyTarget(nsCOMPtr<nsIInputStream> inputStream);``
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The latter is just a wrapper around the first one for implementations that
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usually work with streams. No matter which of the two signatures you choose
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to work with, the only thing you need to implement inside the function
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is the use of the provided data with your target implementation. This can
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mean to simply feed the data to your target, using the data to drive operations
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on the target API, or a mix of both.
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While doing so, you should avoid altering global state in a permanent way,
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using additional sources of data/randomness or having code run beyond the
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lifetime of the iteration function (e.g. on another thread), for one simple
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reason: Coverage-guided fuzzing tools depend on the **deterministic** nature
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of the iteration function. If the same input to this function does not lead
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to the same execution when run twice (e.g. because the resulting state depends
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on multiple successive calls or because of additional external influences),
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then the tool will not be able to reproduce its fuzzing progress and perform
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badly. Dealing with this restriction can be challenging e.g. when dealing
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with asynchronous targets that run multi-threaded, but can usually be managed
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by synchronizing execution on all threads at the end of the iteration function.
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For implementations accumulating global state, it might be necessary to
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(re)initialize this global state in each iteration, rather than doing it once
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in the initialization function, even if this costs additional performance.
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Note that unlike the vanilla libFuzzer approach, you are allowed to ``return 1``
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in this function to indicate that an input is "bad". Doing so will cause
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libFuzzer to discard the input, no matter if it generated new coverage or not.
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This is particularly useful if you have means to internally detect and catch
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bad testcase behavior such as timeouts/excessive resource usage etc. to avoid
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these tests to end up in your corpus.
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Once you have implemented the two functions, the only thing remaining is to
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register them with the fuzzing interface. For this purpose, we offer two
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macros, depending on which iteration function signature you used. If you
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sticked to the classic signature using buffer and size, you can simply use
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::
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#include "FuzzingInterface.h"
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// Your includes and code
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MOZ_FUZZING_INTERFACE_RAW(FuzzingInitMyTarget, FuzzingRunMyTarget, MyTarget);
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where ``MyTarget`` is the name of the target and will be used later to decide
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at runtime which target should be used.
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If instead you went for the streaming interface, you need a different include,
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but the macro invocation is quite similar:
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::
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#include "FuzzingInterfaceStream.h"
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// Your includes and code
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MOZ_FUZZING_INTERFACE_STREAM(FuzzingInitMyTarget, FuzzingRunMyTarget, MyTarget);
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For a live example, see also the `implementation of the STUN fuzzing target
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<https://searchfox.org/mozilla-central/source/dom/media/webrtc/transport/fuzztest/stun_parser_libfuzz.cpp>`__.
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Add instrumentation to the code being tested
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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libFuzzer requires that the code you are trying to test is instrumented
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with special compiler flags. Fortunately, adding these on a per-directory basis
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can be done just by including the following directive in each ``moz.build``
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file that builds code under test:
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::
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# Add libFuzzer configuration directives
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include('/tools/fuzzing/libfuzzer-config.mozbuild')
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The include already does the appropriate configuration checks to be only
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active in fuzzing builds, so you don't have to guard this in any way.
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.. note::
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**Note:** This include modifies `CFLAGS` and `CXXFLAGS` accordingly
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but this only works for source files defined in this particular
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directory. The flags are **not** propagated to subdirectories automatically
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and you have to ensure that each directory that builds source files
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for your target has the include added to its ``moz.build`` file.
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By keeping the instrumentation limited to the parts that are actually being
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tested using this tool, you not only increase the performance but also potentially
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reduce the amount of noise that libFuzzer sees.
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Build your code
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^^^^^^^^^^^^^^^
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See the :ref:`Build instructions above <Local build requirements and flags>` for instructions
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how to modify your ``.mozconfig`` to create the appropriate build.
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Running your code and building a corpus
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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You need to set the following environment variable to enable running the
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fuzzing code inside Firefox instead of the regular browser.
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- ``FUZZER=name``
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Where ``name`` is the name of your fuzzing module that you specified
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when calling the ``MOZ_FUZZING_INTERFACE_RAW`` macro. For the example
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above, this would be ``MyTarget`` or ``StunParser`` for the live example.
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Now when you invoke the firefox binary in your build directory with the
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``-help=1`` parameter, you should see the regular libFuzzer help. On
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Linux for example:
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::
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$ FUZZER=StunParser obj-asan/dist/bin/firefox -help=1
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You should see an output similar to this:
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::
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Running Fuzzer tests...
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Usage:
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To run fuzzing pass 0 or more directories.
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obj-asan/dist/bin/firefox [-flag1=val1 [-flag2=val2 ...] ] [dir1 [dir2 ...] ]
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To run individual tests without fuzzing pass 1 or more files:
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obj-asan/dist/bin/firefox [-flag1=val1 [-flag2=val2 ...] ] file1 [file2 ...]
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Flags: (strictly in form -flag=value)
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verbosity 1 Verbosity level.
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seed 0 Random seed. If 0, seed is generated.
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runs -1 Number of individual test runs (-1 for infinite runs).
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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.
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...
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Reproducing a Crash
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'''''''''''''''''''
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In order to reproduce a crash from a given test file, simply put the
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file as the only argument on the command line, e.g.
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::
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$ FUZZER=StunParser obj-asan/dist/bin/firefox test.bin
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This should reproduce the given problem.
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FuzzManager and libFuzzer
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'''''''''''''''''''''''''
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Our FuzzManager project comes with a harness for running libFuzzer with
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an optional connection to a FuzzManager server instance. Note that this
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connection is not mandatory, even without a server you can make use of
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the local harness.
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You can find the harness
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`here <https://github.com/MozillaSecurity/FuzzManager/tree/master/misc/afl-libfuzzer>`__.
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An example invocation for the harness to use with StunParser could look
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like this:
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::
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FUZZER=StunParser python /path/to/afl-libfuzzer-daemon.py --fuzzmanager \
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--stats libfuzzer-stunparser.stats --libfuzzer-auto-reduce-min 500 --libfuzzer-auto-reduce 30 \
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--tool libfuzzer-stunparser --libfuzzer --libfuzzer-instances 6 obj-asan/dist/bin/firefox \
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-max_len=256 -use_value_profile=1 -rss_limit_mb=3000 corpus-stunparser
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What this does is
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- run libFuzzer on the ``StunParser`` target with 6 parallel instances
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using the corpus in the ``corpus-stunparser`` directory (with the
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specified libFuzzer options such as ``-max_len`` and
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``-use_value_profile``)
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- automatically reduce the corpus and restart if it grew by 30% (and
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has at least 500 files)
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- use FuzzManager (need a local ``.fuzzmanagerconf`` and a
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``firefox.fuzzmanagerconf`` binary configuration as described in the
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FuzzManager manual) and submit crashes as ``libfuzzer-stunparser``
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tool
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- write statistics to the ``libfuzzer-stunparser.stats`` file
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JS Engine Specifics
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~~~~~~~~~~~~~~~~~~~
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The fuzzing interface can also be used for testing the JS engine, in fact there
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are two separate options to implement and run fuzzing targets:
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Implementing in C++
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^^^^^^^^^^^^^^^^^^^
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Similar to the fuzzing interface in Firefox, you can implement your target in
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entirely C++ with very similar interfaces compared to what was described before.
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There are a few minor differences though:
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1. All of the fuzzing targets live in `js/src/fuzz-tests`.
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2. All of the code is linked into a separate binary called `fuzz-tests`,
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similar to how all JSAPI tests end up in `jsapi-tests`. In order for this
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binary to be built, you must build a JS shell with ``--enable-fuzzing``
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**and** ``--enable-tests``. Again, this can and should be combined with
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AddressSanitizer for maximum effectiveness. This also means that there is no
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need to (re)build gtests when dealing with a JS fuzzing target and using
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a shell as part of a full browser build.
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3. The harness around the JS implementation already provides you with an
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initialized ``JSContext`` and global object. You can access these in
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your target by declaring
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``extern JS::PersistentRootedObject gGlobal;``
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and
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``extern JSContext* gCx;``
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but there is no obligation for you to use these.
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For a live example, see also the `implementation of the StructuredCloneReader target
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<https://searchfox.org/mozilla-central/source/js/src/fuzz-tests/testStructuredCloneReader.cpp>`__.
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Implementing in JS
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^^^^^^^^^^^^^^^^^^
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In addition to the C++ targets, you can also implement targets in JavaScript
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using the JavaScript Runtime (JSRT) fuzzing approach. Using this approach is
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not only much simpler (since you don't need to know anything about the
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JSAPI or engine internals), but it also gives you full access to everything
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defined in the JS shell, including handy functions such as ``timeout()``.
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Of course, this approach also comes with disadvantages: Calling into JS and
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performing the fuzzing operations there costs performance. Also, there is more
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chance for causing global side-effects or non-determinism compared to a
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fairly isolated C++ target.
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As a rule of thumb, you should implement the target in JS if
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* you don't know C++ and/or how to use the JSAPI (after all, a JS fuzzing target is better than none),
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* your target is expected to have lots of hangs/timeouts (you can catch these internally),
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* or your target is not isolated enough for a C++ target and/or you need specific JS shell functions.
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There is an `example target <https://searchfox.org/mozilla-central/source/js/src/shell/jsrtfuzzing/jsrtfuzzing-example.js>`__
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in-tree that shows roughly how to implement such a fuzzing target.
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To run such a target, you must run the ``js`` (shell) binary instead of the
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``fuzz-tests`` binary and point the ``FUZZER`` variable to the file containing
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your fuzzing target, e.g.
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::
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$ FUZZER=/path/to/jsrtfuzzing-example.js obj-asan/dist/bin/js --fuzzing-safe --no-threads -- <libFuzzer options here>
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More elaborate targets can be found in `js/src/fuzz-tests/ <https://searchfox.org/mozilla-central/source/js/src/fuzz-tests/>`__.
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Troubleshooting
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~~~~~~~~~~~~~~~
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Fuzzing Interface: Error: No testing callback found
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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This error means that the fuzzing callback with the name you specified
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using the ``FUZZER`` environment variable could not be found. Reasons
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for are typically either a misspelled name or that your code wasn't
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built (check your ``moz.build`` file and build log).
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``mach build`` doesn't seem to update my fuzzing code
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Keep in mind you always need to run both the ``mach build`` and
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``mach gtest dontruntests`` commands in order to update your fuzzing
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code. The latter rebuilds the gtest version of ``libxul``, containing
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your code.
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