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Setting up New Project

Prerequisites

Overview

To add a new OSS project to OSS-Fuzz, you need a project subdirectory inside the projects/ directory in OSS-Fuzz repository. Example: boringssl project is located in projects/boringssl.

The project directory needs to contain the following three configuration files:

  • projects/<project_name>/Dockerfile - defines the container environment with information on dependencies needed to build the project and its fuzz targets.
  • projects/<project_name>/build.sh - build script that executes inside the container and generates project build.
  • projects/<project_name>/project.yaml - provides metadata about the project.

To automatically create a new directory for your project and generate templated versions of these configuration files, run the following set of commands:

$ cd /path/to/oss-fuzz
$ export PROJECT_NAME=<project_name>
$ python infra/helper.py generate $PROJECT_NAME

It is preferred to keep and maintain fuzz targets in your own source code repository. If this is not possible due to various reasons, you can store them inside the OSS-Fuzz's project directory created above.

Dockerfile

This file defines the Docker image definition. This is where the build.sh script will be executed in. It is very simple for most projects:

FROM ossfuzz/base-libfuzzer               # base image with clang toolchain
MAINTAINER YOUR_EMAIL                     # maintainer for this file
RUN apt-get install -y ...                # install required packages to build your project
RUN git clone <git_url> <checkout_dir>    # checkout all sources needed to build your project
WORKDIR <checkout_dir>                    # current directory for build script
COPY build.sh fuzzer.cc $SRC/             # copy build script and other fuzzer files in src dir

Expat example: expat/Dockerfile

Fuzzer execution environment

This page gives information about the environment that your fuzz targets will run on ClusterFuzz, and the assumptions that you can make.

build.sh

This file describes how to build fuzz targets for your project. The script will be executed within the image built from Dockerfile.

In general, this script will need to:

  1. Build the project using your build system with correct compiler and its flags provided as environment variables (see below).
  2. Build the fuzz targets, linking your project's build and libFuzzer. Resulting fuzz targets should be placed in $OUT.

For expat, this looks like this:

#!/bin/bash -eu

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

make clean
make -j$(nproc) all

$CXX $CXXFLAGS -std=c++11 -Ilib/ \
    $SRC/parse_fuzzer.cc -o $OUT/parse_fuzzer \
    -lfuzzer .libs/libexpat.a

cp $SRC/*.dict $SRC/*.options $OUT/

build.sh Script Environment

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

Path Description
/out/ ($OUT) Directory to store build artifacts (fuzz targets, dictionaries, options files, seed corpus archives).
/src/ ($SRC) Directory to checkout source files.
/work/($WORK) Directory for storing intermediate files
/usr/lib/libfuzzer.a Location of prebuilt libFuzzer library that needs to be linked into all fuzz targets (-lfuzzer).

You must use the special compiler flags needed to build your project and fuzz targets. These flags are provided in the following environment variables:

Env Variable Description
$CC, $CXX, $CCC The C and C++ compiler binaries.
$CFLAGS, $CXXFLAGS C and C++ compiler flags.

Most well-crafted build scripts will automatically use these variables. If not, pass them manually to the build tool.

See Provided Environment Variables section in base-libfuzzer image documentation for more details.

Testing locally

Use the helper script build docker image and fuzz targets.

$ cd /path/to/oss-fuzz
$ python infra/helper.py build_image $PROJECT_NAME
$ python infra/helper.py build_fuzzers $PROJECT_NAME

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

$ python infra/helper.py run_fuzzer $PROJECT_NAME <fuzz_target>

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

It's recommended to look at code coverage as a sanity check to make sure that fuzz target gets to the code you expect.

$ python infra/helper.py coverage $PROJECT_NAME <fuzz_target>

Debugging Problems

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

Custom libFuzzer options for ClusterFuzz

By default, ClusterFuzz will run your fuzzer without any options. You can specify custom options by creating a my_fuzzer.options file next to a my_fuzzer executable in $OUT:

[libfuzzer]
max_len = 1024

List of available options. Use of max_len is highly recommended.

For out of tree fuzz targets, you will likely add options file using docker's COPY directive and will copy it into output in build script. (example: woff2).

Seed Corpus

OSS-Fuzz uses evolutionary fuzzing algorithms. Supplying seed corpus consisting of good sample inputs is one of the best ways to improve fuzz target's coverage.

To provide a corpus for my_fuzzer, put my_fuzzer_seed_corpus.zip file next to the fuzz target binary in $OUT during the build. Individual files in this archive will be used as starting inputs for mutations. You can store the corpus next to source files, generate during build or fetch it using curl or any other tool of your choice. (example: boringssl).

Seed corpus files will be used for cross-mutations and portions of them might appear in bug reports or be used for further security research. It is important that corpus has an appropriate and consistent license.

Dictionaries

Dictionaries hugely improve fuzz target's effectiveness for inputs with lots of similar sequences of bytes. libFuzzer documentation

Put your dict file in $OUT and specify in .options file:

[libfuzzer]
dict = dictionary_name.dict

It is common for several fuzz targets to reuse the same dictionary if they are fuzzing very similar inputs. (example: expat).

project.yaml

This file stores the metadata about your project. This includes things like project's homepage, list of sanitizers used, list of ccs on newly filed bugs, etc. (example: expat).

Checking in to OSS-Fuzz repository

Fork OSS-Fuzz, commit and push to the fork, and then create a pull request with your change! Follow the Forking Project guide if you are new to contributing via GitHub.

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 you are porting a fuzz target from Chromium, keep the original Chromium license header.

The end

Once your change is merged, your project and fuzz targets should be automatically built and run on ClusterFuzz after a short while!

Check your project's build status here.