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+================================
+Fuzzing LLVM libraries and tools
+================================
+
+.. contents::
+   :local:
+   :depth: 2
+
+Introduction
+============
+
+The LLVM tree includes a number of fuzzers for various components. These are
+built on top of :doc:`LibFuzzer <LibFuzzer>`.
+
+
+Available Fuzzers
+=================
+
+clang-fuzzer
+------------
+
+A |generic fuzzer| that tries to compile textual input as C++ code. Some of the
+bugs this fuzzer has reported are `on bugzilla`__ and `on OSS Fuzz's
+tracker`__.
+
+__ https://llvm.org/pr23057
+__ https://bugs.chromium.org/p/oss-fuzz/issues/list?q=proj-llvm+clang-fuzzer
+
+clang-proto-fuzzer
+------------------
+
+A |protobuf fuzzer| that compiles valid C++ programs generated from a protobuf
+class that describes a subset of the C++ language.
+
+This fuzzer accepts clang command line options after `ignore_remaining_args=1`.
+For example, the following command will fuzz clang with a higher optimization
+level:
+
+.. code-block:: shell
+
+   % bin/clang-proto-fuzzer <corpus-dir> -ignore_remaining_args=1 -O3
+
+clang-format-fuzzer
+-------------------
+
+A |generic fuzzer| that runs clang-format_ on C++ text fragments. Some of the
+bugs this fuzzer has reported are `on bugzilla`__
+and `on OSS Fuzz's tracker`__.
+
+.. _clang-format: https://clang.llvm.org/docs/ClangFormat.html
+__ https://llvm.org/pr23052
+__ https://bugs.chromium.org/p/oss-fuzz/issues/list?q=proj-llvm+clang-format-fuzzer
+
+llvm-as-fuzzer
+--------------
+
+A |generic fuzzer| that tries to parse text as :doc:`LLVM assembly <LangRef>`.
+Some of the bugs this fuzzer has reported are `on bugzilla`__.
+
+__ https://llvm.org/pr24639
+
+llvm-dwarfdump-fuzzer
+---------------------
+
+A |generic fuzzer| that interprets inputs as object files and runs
+:doc:`llvm-dwarfdump <CommandGuide/llvm-dwarfdump>` on them. Some of the bugs
+this fuzzer has reported are `on OSS Fuzz's tracker`__
+
+__ https://bugs.chromium.org/p/oss-fuzz/issues/list?q=proj-llvm+llvm-dwarfdump-fuzzer
+
+llvm-demangle-fuzzer
+---------------------
+
+A |generic fuzzer| for the Itanium demangler used in various LLVM tools. We've
+fuzzed __cxa_demangle to death, why not fuzz LLVM's implementation of the same
+function!
+
+llvm-isel-fuzzer
+----------------
+
+A |LLVM IR fuzzer| aimed at finding bugs in instruction selection.
+
+This fuzzer accepts flags after `ignore_remaining_args=1`. The flags match
+those of :doc:`llc <CommandGuide/llc>` and the triple is required. For example,
+the following command would fuzz AArch64 with :doc:`GlobalISel`:
+
+.. code-block:: shell
+
+   % bin/llvm-isel-fuzzer <corpus-dir> -ignore_remaining_args=1 -mtriple aarch64 -global-isel -O0
+
+Some flags can also be specified in the binary name itself in order to support
+OSS Fuzz, which has trouble with required arguments. To do this, you can copy
+or move ``llvm-isel-fuzzer`` to ``llvm-isel-fuzzer--x-y-z``, separating options
+from the binary name using "--". The valid options are architecture names
+(``aarch64``, ``x86_64``), optimization levels (``O0``, ``O2``), or specific
+keywords, like ``gisel`` for enabling global instruction selection. In this
+mode, the same example could be run like so:
+
+.. code-block:: shell
+
+   % bin/llvm-isel-fuzzer--aarch64-O0-gisel <corpus-dir>
+
+llvm-mc-assemble-fuzzer
+-----------------------
+
+A |generic fuzzer| that fuzzes the MC layer's assemblers by treating inputs as
+target specific assembly.
+
+Note that this fuzzer has an unusual command line interface which is not fully
+compatible with all of libFuzzer's features. Fuzzer arguments must be passed
+after ``--fuzzer-args``, and any ``llc`` flags must use two dashes. For
+example, to fuzz the AArch64 assembler you might use the following command:
+
+.. code-block:: console
+
+  llvm-mc-fuzzer --triple=aarch64-linux-gnu --fuzzer-args -max_len=4
+
+This scheme will likely change in the future.
+
+llvm-mc-disassemble-fuzzer
+--------------------------
+
+A |generic fuzzer| that fuzzes the MC layer's disassemblers by treating inputs
+as assembled binary data.
+
+Note that this fuzzer has an unusual command line interface which is not fully
+compatible with all of libFuzzer's features. See the notes above about
+``llvm-mc-assemble-fuzzer`` for details.
+
+
+.. |generic fuzzer| replace:: :ref:`generic fuzzer <fuzzing-llvm-generic>`
+.. |protobuf fuzzer|
+   replace:: :ref:`libprotobuf-mutator based fuzzer <fuzzing-llvm-protobuf>`
+.. |LLVM IR fuzzer|
+   replace:: :ref:`structured LLVM IR fuzzer <fuzzing-llvm-ir>`
+
+
+Mutators and Input Generators
+=============================
+
+The inputs for a fuzz target are generated via random mutations of a
+:ref:`corpus <libfuzzer-corpus>`. There are a few options for the kinds of
+mutations that a fuzzer in LLVM might want.
+
+.. _fuzzing-llvm-generic:
+
+Generic Random Fuzzing
+----------------------
+
+The most basic form of input mutation is to use the built in mutators of
+LibFuzzer. These simply treat the input corpus as a bag of bits and make random
+mutations. This type of fuzzer is good for stressing the surface layers of a
+program, and is good at testing things like lexers, parsers, or binary
+protocols.
+
+Some of the in-tree fuzzers that use this type of mutator are `clang-fuzzer`_,
+`clang-format-fuzzer`_, `llvm-as-fuzzer`_, `llvm-dwarfdump-fuzzer`_,
+`llvm-mc-assemble-fuzzer`_, and `llvm-mc-disassemble-fuzzer`_.
+
+.. _fuzzing-llvm-protobuf:
+
+Structured Fuzzing using ``libprotobuf-mutator``
+------------------------------------------------
+
+We can use libprotobuf-mutator_ in order to perform structured fuzzing and
+stress deeper layers of programs. This works by defining a protobuf class that
+translates arbitrary data into structurally interesting input. Specifically, we
+use this to work with a subset of the C++ language and perform mutations that
+produce valid C++ programs in order to exercise parts of clang that are more
+interesting than parser error handling.
+
+To build this kind of fuzzer you need `protobuf`_ and its dependencies
+installed, and you need to specify some extra flags when configuring the build
+with :doc:`CMake <CMake>`. For example, `clang-proto-fuzzer`_ can be enabled by
+adding ``-DCLANG_ENABLE_PROTO_FUZZER=ON`` to the flags described in
+:ref:`building-fuzzers`.
+
+The only in-tree fuzzer that uses ``libprotobuf-mutator`` today is
+`clang-proto-fuzzer`_.
+
+.. _libprotobuf-mutator: https://github.com/google/libprotobuf-mutator
+.. _protobuf: https://github.com/google/protobuf
+
+.. _fuzzing-llvm-ir:
+
+Structured Fuzzing of LLVM IR
+-----------------------------
+
+We also use a more direct form of structured fuzzing for fuzzers that take
+:doc:`LLVM IR <LangRef>` as input. This is achieved through the ``FuzzMutate``
+library, which was `discussed at EuroLLVM 2017`_.
+
+The ``FuzzMutate`` library is used to structurally fuzz backends in
+`llvm-isel-fuzzer`_.
+
+.. _discussed at EuroLLVM 2017: https://www.youtube.com/watch?v=UBbQ_s6hNgg
+
+
+Building and Running
+====================
+
+.. _building-fuzzers:
+
+Configuring LLVM to Build Fuzzers
+---------------------------------
+
+Fuzzers will be built and linked to libFuzzer by default as long as you build
+LLVM with sanitizer coverage enabled. You would typically also enable at least
+one sanitizer to find bugs faster. The most common way to build the fuzzers is
+by adding the following two flags to your CMake invocation:
+``-DLLVM_USE_SANITIZER=Address -DLLVM_USE_SANITIZE_COVERAGE=On``.
+
+.. note:: If you have ``compiler-rt`` checked out in an LLVM tree when building
+          with sanitizers, you'll want to specify ``-DLLVM_BUILD_RUNTIME=Off``
+          to avoid building the sanitizers themselves with sanitizers enabled.
+
+Continuously Running and Finding Bugs
+-------------------------------------
+
+There used to be a public buildbot running LLVM fuzzers continuously, and while
+this did find issues, it didn't have a very good way to report problems in an
+actionable way. Because of this, we're moving towards using `OSS Fuzz`_ more
+instead.
+
+You can browse the `LLVM project issue list`_ for the bugs found by
+`LLVM on OSS Fuzz`_. These are also mailed to the `llvm-bugs mailing
+list`_.
+
+.. _OSS Fuzz: https://github.com/google/oss-fuzz
+.. _LLVM project issue list:
+   https://bugs.chromium.org/p/oss-fuzz/issues/list?q=Proj-llvm
+.. _LLVM on OSS Fuzz:
+   https://github.com/google/oss-fuzz/blob/master/projects/llvm
+.. _llvm-bugs mailing list:
+   http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
+
+
+Utilities for Writing Fuzzers
+=============================
+
+There are some utilities available for writing fuzzers in LLVM.
+
+Some helpers for handling the command line interface are available in
+``include/llvm/FuzzMutate/FuzzerCLI.h``, including functions to parse command
+line options in a consistent way and to implement standalone main functions so
+your fuzzer can be built and tested when not built against libFuzzer.
+
+There is also some handling of the CMake config for fuzzers, where you should
+use the ``add_llvm_fuzzer`` to set up fuzzer targets. This function works
+similarly to functions such as ``add_llvm_tool``, but they take care of linking
+to LibFuzzer when appropriate and can be passed the ``DUMMY_MAIN`` argument to
+enable standalone testing.