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Setting up fuzzers for a new library

Fuzzer build configurations are placed into a top-level directory for the library in the oss-fuzz repo on GitHub. For example, fuzzers for the expat library go into https://github.com/google/oss-fuzz/tree/master/expat.

Prerequisites

Install Docker.

Building fuzzers requires building your library with a fresh of Clang compiler and special compiler flags. An easy-to-use Docker image is provided to simplify tool distribution.

If you'd like to get more familiar with how building libFuzzer-style fuzzers work in general, check out this page.

Overview

To add a new OSS library to oss-fuzz, 3 supporting files have to be added to oss-fuzz source code repository:

library_name/Dockerfile - defines an container environment with all the dependencies needed to build the project and the fuzzer.
library_name/build.sh - build script that will be executed inside the container.
library_name/Jenkinsfile - will be needed to integrate fuzzers with ClusterFuzz build and distributed execution system. Specify your library VCS location in it.

To create a new directory for the library and automatically generate these 3 files a python script can be used:

$ cd /path/to/oss-fuzz
$ export LIB_NAME=name_of_the_library
$ python scripts/helper.py generate $LIB_NAME

Create a fuzzer and add it to the library_name/ directory as well.

Dockerfile

This is the Docker image definition that build.sh will be executed in. It is very simple for most libraries:

FROM ossfuzz/base-libfuzzer             # base image with clang toolchain
MAINTAINER YOUR_EMAIL                   # each file should have a maintainer 
RUN apt-get install -y ...              # install required packages to build a project
COPY build.sh /src/                     # install build script for the project.

Expat example: expat/Dockerfile

build.sh

This is where most of the work is done to build fuzzers for your library. The script will be executed within an image built from a Dockerfile.

In general, this script will need to:

Build the library using its build system with correct compiler and its flags provided as environment variables (see below).
Build the fuzzers, linking with the library and libFuzzer. Built fuzzers should be placed in /out.

For expat, this looks like:

#!/bin/bash -eu

cd /src/expat/expat
./buildconf.sh
# configure scripts usually use correct environment variables.
./configure

make clean all

# build the fuzzer, linking with libFuzzer and libexpat.a
$CXX $CXXFLAGS -std=c++11 -Ilib/ \
    /src/oss-fuzz/expat/parse_fuzzer.cc -o /out/expat_parse_fuzzer \
    /work/libfuzzer/*.o .libs/libexpat.a \
    $LDFLAGS

build.sh Script Environment

When build.sh script is executed, the following locations are available within the image:

Path	Description
`/src/$LIB_NAME`	Source code for your library.
`/src/oss-fuzz`	Checked out oss-fuzz source tree.
`/work/libfuzzer/*.o`	Prebuilt libFuzzer object files that need to be linked into all fuzzers.

You must use special compiler flags to build your library and fuzzers. These flags are provided in following environment variables:

Env Variable	Description
`$CC`	The C compiler binary.
`$CXX`, `$CCC`	The C++ compiler binary.
`$CFLAGS`	C compiler flags.
`$CXXFLAGS`	C++ compiler flags.
`$LDFLAGS`	Linker flags for fuzzer binaries.

Many well-crafted build scripts will automatically use these variables. If not, passing them manually to a build tool might be required.

Create Fuzzer Source File

Create a new .cc file, define a LLVMFuzzerTestOneInput function and call your library:

#include <stddef.h>
#include <stdint.h>

extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
  // put your fuzzing code here and use data+size as input.
  return 0;
}

There are lots of examples in this project repository.

Dictionaries and custom libfuzzer options

Any top-level files in the library directory ending with the extension ".dict" or ".options" will be picked up by ClusterFuzz. Files ending with ".dict" are assumed to be libFuzzer compatible dictionaries, and .options files have the format:

[libfuzzer]
dict = dictionary_name.dict
max_len = 9001

This means that -dict=/path/to/dictionary_name.dict and -max_len=9001 will be passed to the fuzzer when it's run.

Others (e.g. fuzzer source)

For some libraries, the upstream repository will contain fuzzers (e.g. freetype2). In other cases, such as expat, we can check in fuzzer source into the oss-fuzz repo.

Jenkinsfile

This file will be largely the same for most libraries, and is used by our build infrastructure. For expat, this is:

// load libFuzzer pipeline definition.
def libfuzzerBuild = fileLoader.fromGit('infra/libfuzzer-pipeline.groovy',
                                        'https://github.com/google/oss-fuzz.git',
                                        'master', null, '')

libfuzzerBuild {
  git = "git://git.code.sf.net/p/expat/code_git"
}

Simply replace the the "git" entry with the correct git url for the library.

Note: only git is supported right now.

Testing locally

Helper script can be used to build images and fuzzers. Non-script version using docker commands directly is documented here.

$ cd /path/to/oss-fuzz
$ python scripts/helper.py build_image $LIB_NAME
$ python scripts/helper.py build_fuzzers $LIB_NAME

This should place the built fuzzers into /path/to/oss-fuzz/build/out/$LIB_NAME on your machine (/out in the container). You can then try to run these fuzzers inside the container to make sure that they work properly:

$ python scripts/helper.py run_fuzzer $LIB_NAME name_of_a_fuzzer

If everything works locally, then it should also work on our automated builders and ClusterFuzz.

Debugging Problems

Debugging document lists ways to debug your build scripts or fuzzers in case you run into problems.

Checking in to oss-fuzz repository

Fork oss-fuzz, commit and push to the fork, and then create a pull request with your change!

Copyright headers

Please include copyright headers for all files checked in to oss-fuzz:

# Copyright 2016 Google Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#      http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#
################################################################################

If porting a fuzzer from Chromium, keep the Chromium license header.

The end

Once your change is merged, the fuzzers should be automatically built and run on ClusterFuzz after a short while!

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