vasilito
cb424d7448
verify-patch-sanity.py validates every active recipe .patch has internally- consistent hunk line counts — catching the 'malformed patch at line N' failure at commit/CI/preflight time instead of hours into a cook. This cycle hit that class three times (qtwaylandscanner, sddm, xwayland), each only discovered when cookbook tried to apply the patch. Running it across the repo found 29 latent malformed patches (validated against GNU patch: e.g. relibc/P3-sysv-ipc reproduces 'malformed patch at line 22'). They were harmless only because they sit in vendored recipes (baked, not re- applied) — but would fail on any version-bump re-derivation. --fix recounts the hunk headers (body untouched) and repaired all 29. Wired into build-preflight.sh (Phase 1.0D) and redbear-ci.yml, with a unit test (test-patch-sanity.sh). Skips archived/legacy trees and unvalidatable formats (empty placeholders, bare-@@ git hunks).
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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/index`:
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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 optimization 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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lldb-target-fuzzer
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---------------------
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A |generic fuzzer| that interprets inputs as object files and uses them to
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create a target in lldb.
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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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