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Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+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 <https://llvm.org/docs/LibFuzzer.html>`__ or
+`afl-fuzz <http://lcamtuf.coredump.cx/afl/>`__.
+
+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+libFuzzer over afl-fuzz, as the latter is no longer maintained while libFuzzer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::
+
+   **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 :ref:`JS Engine Specifics Section <JS Engine Specifics>` 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 <https://github.com/MozillaSecurity/fuzzfetch>`__ or
+`via pip <https://pypi.org/project/fuzzfetch/>`__.
+
+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::
+
+   **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 :ref:`AddressSanitizer <Address Sanitizer>`
+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::
+
+   **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 :ref:`contact us <Fuzzing#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 transport target
+for an example <https://searchfox.org/mozilla-central/source/dom/media/webrtc/transport/fuzztest/moz.build>`__
+
+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 transport target for an example
+<https://searchfox.org/mozilla-central/rev/de7676288a78b70d2b9927c79493adbf294faad5/media/mtransport/moz.build#18-24>`__.
+
+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 <https://llvm.org/docs/LibFuzzer.html#startup-initialization>`__
+   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 <https://llvm.org/docs/LibFuzzer.html#fuzz-target>`__.
+   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);``
+
+   or
+
+   ``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
+<https://searchfox.org/mozilla-central/source/dom/media/webrtc/transport/fuzztest/stun_parser_libfuzz.cpp>`__.
+
+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
+  include('/tools/fuzzing/libfuzzer-config.mozbuild')
+
+
+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::
+
+   **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 :ref:`Build instructions above <Local build requirements and flags>` 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...
+   Usage:
+
+   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 <https://github.com/MozillaSecurity/FuzzManager/tree/master/misc/afl-libfuzzer>`__.
+
+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;``
+
+   and
+
+   ``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
+<https://searchfox.org/mozilla-central/source/js/src/fuzz-tests/testStructuredCloneReader.cpp>`__.
+
+
+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 <https://searchfox.org/mozilla-central/source/js/src/shell/jsrtfuzzing/jsrtfuzzing-example.js>`__
+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/ <https://searchfox.org/mozilla-central/source/js/src/fuzz-tests/>`__.
+
+Troubleshooting
+~~~~~~~~~~~~~~~
+
+
+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.