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281 lines
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================================
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Fuzzing LLVM libraries and tools
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================================
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.. contents::
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:local:
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:depth: 2
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Introduction
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============
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The LLVM tree includes a number of fuzzers for various components. These are
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built on top of :doc:`LibFuzzer <LibFuzzer>`. In order to build and run these
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fuzzers, see :ref:`building-fuzzers`.
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Available Fuzzers
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=================
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clang-fuzzer
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------------
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A |generic fuzzer| that tries to compile textual input as C++ code. Some of the
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bugs this fuzzer has reported are `on bugzilla`__ and `on OSS Fuzz's
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tracker`__.
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__ https://llvm.org/pr23057
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__ https://bugs.chromium.org/p/oss-fuzz/issues/list?q=proj-llvm+clang-fuzzer
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clang-proto-fuzzer
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------------------
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A |protobuf fuzzer| that compiles valid C++ programs generated from a protobuf
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class that describes a subset of the C++ language.
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This fuzzer accepts clang command line options after `ignore_remaining_args=1`.
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For example, the following command will fuzz clang with a higher optimization
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level:
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.. code-block:: shell
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% bin/clang-proto-fuzzer <corpus-dir> -ignore_remaining_args=1 -O3
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clang-format-fuzzer
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-------------------
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A |generic fuzzer| that runs clang-format_ on C++ text fragments. Some of the
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bugs this fuzzer has reported are `on bugzilla`__
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and `on OSS Fuzz's tracker`__.
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.. _clang-format: https://clang.llvm.org/docs/ClangFormat.html
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__ https://llvm.org/pr23052
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__ https://bugs.chromium.org/p/oss-fuzz/issues/list?q=proj-llvm+clang-format-fuzzer
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llvm-as-fuzzer
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--------------
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A |generic fuzzer| that tries to parse text as :doc:`LLVM assembly <LangRef>`.
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Some of the bugs this fuzzer has reported are `on bugzilla`__.
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__ https://llvm.org/pr24639
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llvm-dwarfdump-fuzzer
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---------------------
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A |generic fuzzer| that interprets inputs as object files and runs
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:doc:`llvm-dwarfdump <CommandGuide/llvm-dwarfdump>` on them. Some of the bugs
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this fuzzer has reported are `on OSS Fuzz's tracker`__
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__ https://bugs.chromium.org/p/oss-fuzz/issues/list?q=proj-llvm+llvm-dwarfdump-fuzzer
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llvm-demangle-fuzzer
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---------------------
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A |generic fuzzer| for the Itanium demangler used in various LLVM tools. We've
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fuzzed __cxa_demangle to death, why not fuzz LLVM's implementation of the same
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function!
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llvm-isel-fuzzer
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----------------
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A |LLVM IR fuzzer| aimed at finding bugs in instruction selection.
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This fuzzer accepts flags after `ignore_remaining_args=1`. The flags match
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those of :doc:`llc <CommandGuide/llc>` and the triple is required. For example,
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the following command would fuzz AArch64 with :doc:`GlobalISel`:
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.. code-block:: shell
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% bin/llvm-isel-fuzzer <corpus-dir> -ignore_remaining_args=1 -mtriple aarch64 -global-isel -O0
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Some flags can also be specified in the binary name itself in order to support
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OSS Fuzz, which has trouble with required arguments. To do this, you can copy
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or move ``llvm-isel-fuzzer`` to ``llvm-isel-fuzzer--x-y-z``, separating options
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from the binary name using "--". The valid options are architecture names
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(``aarch64``, ``x86_64``), optimization levels (``O0``, ``O2``), or specific
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keywords, like ``gisel`` for enabling global instruction selection. In this
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mode, the same example could be run like so:
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.. code-block:: shell
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% bin/llvm-isel-fuzzer--aarch64-O0-gisel <corpus-dir>
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llvm-opt-fuzzer
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---------------
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A |LLVM IR fuzzer| aimed at finding bugs in optimization passes.
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It receives optimzation pipeline and runs it for each fuzzer input.
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Interface of this fuzzer almost directly mirrors ``llvm-isel-fuzzer``. Both
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``mtriple`` and ``passes`` arguments are required. Passes are specified in a
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format suitable for the new pass manager. You can find some documentation about
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this format in the doxygen for ``PassBuilder::parsePassPipeline``.
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.. code-block:: shell
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% bin/llvm-opt-fuzzer <corpus-dir> -ignore_remaining_args=1 -mtriple x86_64 -passes instcombine
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Similarly to the ``llvm-isel-fuzzer`` arguments in some predefined configurations
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might be embedded directly into the binary file name:
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.. code-block:: shell
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% bin/llvm-opt-fuzzer--x86_64-instcombine <corpus-dir>
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llvm-mc-assemble-fuzzer
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-----------------------
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A |generic fuzzer| that fuzzes the MC layer's assemblers by treating inputs as
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target specific assembly.
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Note that this fuzzer has an unusual command line interface which is not fully
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compatible with all of libFuzzer's features. Fuzzer arguments must be passed
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after ``--fuzzer-args``, and any ``llc`` flags must use two dashes. For
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example, to fuzz the AArch64 assembler you might use the following command:
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.. code-block:: console
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llvm-mc-fuzzer --triple=aarch64-linux-gnu --fuzzer-args -max_len=4
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This scheme will likely change in the future.
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llvm-mc-disassemble-fuzzer
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--------------------------
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A |generic fuzzer| that fuzzes the MC layer's disassemblers by treating inputs
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as assembled binary data.
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Note that this fuzzer has an unusual command line interface which is not fully
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compatible with all of libFuzzer's features. See the notes above about
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``llvm-mc-assemble-fuzzer`` for details.
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.. |generic fuzzer| replace:: :ref:`generic fuzzer <fuzzing-llvm-generic>`
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.. |protobuf fuzzer|
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replace:: :ref:`libprotobuf-mutator based fuzzer <fuzzing-llvm-protobuf>`
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.. |LLVM IR fuzzer|
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replace:: :ref:`structured LLVM IR fuzzer <fuzzing-llvm-ir>`
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Mutators and Input Generators
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=============================
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The inputs for a fuzz target are generated via random mutations of a
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:ref:`corpus <libfuzzer-corpus>`. There are a few options for the kinds of
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mutations that a fuzzer in LLVM might want.
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.. _fuzzing-llvm-generic:
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Generic Random Fuzzing
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----------------------
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The most basic form of input mutation is to use the built in mutators of
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LibFuzzer. These simply treat the input corpus as a bag of bits and make random
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mutations. This type of fuzzer is good for stressing the surface layers of a
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program, and is good at testing things like lexers, parsers, or binary
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protocols.
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Some of the in-tree fuzzers that use this type of mutator are `clang-fuzzer`_,
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`clang-format-fuzzer`_, `llvm-as-fuzzer`_, `llvm-dwarfdump-fuzzer`_,
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`llvm-mc-assemble-fuzzer`_, and `llvm-mc-disassemble-fuzzer`_.
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.. _fuzzing-llvm-protobuf:
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Structured Fuzzing using ``libprotobuf-mutator``
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------------------------------------------------
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We can use libprotobuf-mutator_ in order to perform structured fuzzing and
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stress deeper layers of programs. This works by defining a protobuf class that
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translates arbitrary data into structurally interesting input. Specifically, we
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use this to work with a subset of the C++ language and perform mutations that
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produce valid C++ programs in order to exercise parts of clang that are more
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interesting than parser error handling.
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To build this kind of fuzzer you need `protobuf`_ and its dependencies
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installed, and you need to specify some extra flags when configuring the build
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with :doc:`CMake <CMake>`. For example, `clang-proto-fuzzer`_ can be enabled by
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adding ``-DCLANG_ENABLE_PROTO_FUZZER=ON`` to the flags described in
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:ref:`building-fuzzers`.
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The only in-tree fuzzer that uses ``libprotobuf-mutator`` today is
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`clang-proto-fuzzer`_.
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.. _libprotobuf-mutator: https://github.com/google/libprotobuf-mutator
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.. _protobuf: https://github.com/google/protobuf
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.. _fuzzing-llvm-ir:
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Structured Fuzzing of LLVM IR
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-----------------------------
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We also use a more direct form of structured fuzzing for fuzzers that take
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:doc:`LLVM IR <LangRef>` as input. This is achieved through the ``FuzzMutate``
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library, which was `discussed at EuroLLVM 2017`_.
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The ``FuzzMutate`` library is used to structurally fuzz backends in
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`llvm-isel-fuzzer`_.
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.. _discussed at EuroLLVM 2017: https://www.youtube.com/watch?v=UBbQ_s6hNgg
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Building and Running
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====================
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.. _building-fuzzers:
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Configuring LLVM to Build Fuzzers
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---------------------------------
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Fuzzers will be built and linked to libFuzzer by default as long as you build
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LLVM with sanitizer coverage enabled. You would typically also enable at least
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one sanitizer to find bugs faster. The most common way to build the fuzzers is
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by adding the following two flags to your CMake invocation:
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``-DLLVM_USE_SANITIZER=Address -DLLVM_USE_SANITIZE_COVERAGE=On``.
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.. note:: If you have ``compiler-rt`` checked out in an LLVM tree when building
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with sanitizers, you'll want to specify ``-DLLVM_BUILD_RUNTIME=Off``
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to avoid building the sanitizers themselves with sanitizers enabled.
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.. note:: You may run into issues if you build with BFD ld, which is the
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default linker on many unix systems. These issues are being tracked
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in https://llvm.org/PR34636.
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Continuously Running and Finding Bugs
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-------------------------------------
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There used to be a public buildbot running LLVM fuzzers continuously, and while
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this did find issues, it didn't have a very good way to report problems in an
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actionable way. Because of this, we're moving towards using `OSS Fuzz`_ more
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instead.
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You can browse the `LLVM project issue list`_ for the bugs found by
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`LLVM on OSS Fuzz`_. These are also mailed to the `llvm-bugs mailing
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list`_.
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.. _OSS Fuzz: https://github.com/google/oss-fuzz
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.. _LLVM project issue list:
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https://bugs.chromium.org/p/oss-fuzz/issues/list?q=Proj-llvm
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.. _LLVM on OSS Fuzz:
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https://github.com/google/oss-fuzz/blob/master/projects/llvm
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.. _llvm-bugs mailing list:
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http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
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Utilities for Writing Fuzzers
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=============================
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There are some utilities available for writing fuzzers in LLVM.
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Some helpers for handling the command line interface are available in
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``include/llvm/FuzzMutate/FuzzerCLI.h``, including functions to parse command
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line options in a consistent way and to implement standalone main functions so
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your fuzzer can be built and tested when not built against libFuzzer.
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There is also some handling of the CMake config for fuzzers, where you should
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use the ``add_llvm_fuzzer`` to set up fuzzer targets. This function works
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similarly to functions such as ``add_llvm_tool``, but they take care of linking
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to LibFuzzer when appropriate and can be passed the ``DUMMY_MAIN`` argument to
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enable standalone testing.
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