mirror of
https://github.com/capstone-engine/llvm-capstone.git
synced 2024-11-30 17:21:10 +00:00
61efe14e21
This patch adds a generic fuzzer that interprets inputs as object files and uses them to create a target in lldb. It is very similar to the llvm-dwarfdump fuzzer which found a bunch of issues in libObject. Differential revision: https://reviews.llvm.org/D122461
286 lines
10 KiB
ReStructuredText
286 lines
10 KiB
ReStructuredText
================================
|
|
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>`. In order to build and run these
|
|
fuzzers, see :ref:`building-fuzzers`.
|
|
|
|
|
|
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/index`:
|
|
|
|
.. 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-opt-fuzzer
|
|
---------------
|
|
|
|
A |LLVM IR fuzzer| aimed at finding bugs in optimization passes.
|
|
|
|
It receives optimization pipeline and runs it for each fuzzer input.
|
|
|
|
Interface of this fuzzer almost directly mirrors ``llvm-isel-fuzzer``. Both
|
|
``mtriple`` and ``passes`` arguments are required. Passes are specified in a
|
|
format suitable for the new pass manager. You can find some documentation about
|
|
this format in the doxygen for ``PassBuilder::parsePassPipeline``.
|
|
|
|
.. code-block:: shell
|
|
|
|
% bin/llvm-opt-fuzzer <corpus-dir> -ignore_remaining_args=1 -mtriple x86_64 -passes instcombine
|
|
|
|
Similarly to the ``llvm-isel-fuzzer`` arguments in some predefined configurations
|
|
might be embedded directly into the binary file name:
|
|
|
|
.. code-block:: shell
|
|
|
|
% bin/llvm-opt-fuzzer--x86_64-instcombine <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>`
|
|
|
|
lldb-target-fuzzer
|
|
---------------------
|
|
|
|
A |generic fuzzer| that interprets inputs as object files and uses them to
|
|
create a target in lldb.
|
|
|
|
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.
|
|
|
|
.. note:: You may run into issues if you build with BFD ld, which is the
|
|
default linker on many unix systems. These issues are being tracked
|
|
in https://llvm.org/PR34636.
|
|
|
|
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.
|