fuzzers: add object parsing fuzzer

Add a simple fuzzer that exercises our object parser code. The fuzzer
is quite trivial in that it simply passes the input data directly to
`git_object__from_raw` for each of the four object types.

fuzzers/corpora/objects/blob

+libgit2 - the Git linkable library
+==================================
+
+[![Travis Build Status](https://secure.travis-ci.org/libgit2/libgit2.svg?branch=master)](http://travis-ci.org/libgit2/libgit2)
+[![AppVeyor Build Status](https://ci.appveyor.com/api/projects/status/xvof5b4t5480a2q3/branch/master?svg=true)](https://ci.appveyor.com/project/libgit2/libgit2/branch/master)
+[![Coverity Scan Build Status](https://scan.coverity.com/projects/639/badge.svg)](https://scan.coverity.com/projects/639)
+
+`libgit2` is a portable, pure C implementation of the Git core methods
+provided as a linkable library with a solid API, allowing to build Git
+functionality into your application.  Language bindings like
+[Rugged](https://github.com/libgit2/rugged) (Ruby),
+[LibGit2Sharp](https://github.com/libgit2/libgit2sharp) (.NET),
+[pygit2](http://www.pygit2.org/) (Python) and
+[NodeGit](http://nodegit.org) (Node) allow you to build Git tooling
+in your favorite language.
+
+`libgit2` is used to power Git GUI clients like
+[GitKraken](https://gitkraken.com/) and [gmaster](https://gmaster.io/)
+and on Git hosting providers like [GitHub](https://github.com/),
+[GitLab](https://gitlab.com/) and
+[Visual Studio Team Services](https://visualstudio.com/team-services/).
+We perform the merge every time you click "merge pull request".
+
+`libgit2` is licensed under a **very permissive license** (GPLv2 with a special
+Linking Exception).  This basically means that you can link it (unmodified)
+with any kind of software without having to release its source code.
+Additionally, the example code has been released to the public domain (see the
+[separate license](examples/COPYING) for more information).
+
+Quick Start
+===========
+
+**Prerequisites** for building libgit2:
+
+1. [CMake](https://cmake.org/), and is recommended to be installed into
+   your `PATH`.
+2. [Python](https://www.python.org) is used by our test framework, and
+   should be installed into your `PATH`.
+3. C compiler: libgit2 is C90 and should compile on most compilers.
+   * Windows: Visual Studio is recommended
+   * Mac: Xcode is recommended
+   * Unix: gcc or clang is recommended.
+
+**Build**
+
+1. Create a build directory beneath the libgit2 source directory, and change
+   into it: `mkdir build && cd build`
+2. Create the cmake build environment: `cmake ..`
+3. Build libgit2: `cmake --build .`
+
+Trouble with these steps?  Read `TROUBLESHOOTING.md`.  More detailed build
+guidance is available below.
+
+Getting Help
+============
+
+**Join us on Slack**
+
+Visit [slack.libgit2.org](http://slack.libgit2.org/) to sign up, then join
+us in `#libgit2`.  If you prefer IRC, you can also point your client to our
+slack channel once you've registered.
+
+**Getting Help**
+
+If you have questions about the library, please be sure to check out the
+[API documentation](http://libgit2.github.com/libgit2/).  If you still have
+questions, reach out to us on Slack or post a question on 
+[StackOverflow](http://stackoverflow.com/questions/tagged/libgit2) (with the `libgit2` tag).
+
+**Reporting Bugs**
+
+Please open a [GitHub Issue](https://github.com/libgit2/libgit2/issues) and
+include as much information as possible.  If possible, provide sample code
+that illustrates the problem you're seeing.  If you're seeing a bug only
+on a specific repository, please provide a link to it if possible.
+
+We ask that you not open a GitHub Issue for help, only for bug reports.
+
+**Reporting Security Issues**
+
+In case you think to have found a security issue with libgit2, please do not
+open a public issue.  Instead, you can report the issue to the private mailing
+list [security@libgit2.com](mailto:security@libgit2.com).
+
+What It Can Do
+==============
+
+libgit2 provides you with the ability to manage Git repositories in the
+programming language of your choice.  It's used in production to power many
+applications including GitHub.com, Plastic SCM and Visual Studio Team Services.
+
+It does not aim to replace the git tool or its user-facing commands. Some APIs
+resemble the plumbing commands as those align closely with the concepts of the
+Git system, but most commands a user would type are out of scope for this
+library to implement directly.
+
+The library provides:
+
+* SHA conversions, formatting and shortening
+* abstracted ODB backend system
+* commit, tag, tree and blob parsing, editing, and write-back
+* tree traversal
+* revision walking
+* index file (staging area) manipulation
+* reference management (including packed references)
+* config file management
+* high level repository management
+* thread safety and reentrancy
+* descriptive and detailed error messages
+* ...and more (over 175 different API calls)
+
+As libgit2 is purely a consumer of the Git system, we have to
+adjust to changes made upstream. This has two major consequences:
+
+* Some changes may require us to change provided interfaces. While we try to
+  implement functions in a generic way so that no future changes are required,
+  we cannot promise a completely stable API.
+* As we have to keep up with changes in behavior made upstream, we may lag
+  behind in some areas. We usually to document these incompatibilities in our
+  issue tracker with the label "git change".
+
+Optional dependencies
+=====================
+
+While the library provides git functionality without the need for
+dependencies, it can make use of a few libraries to add to it:
+
+- pthreads (non-Windows) to enable threadsafe access as well as multi-threaded pack generation
+- OpenSSL (non-Windows) to talk over HTTPS and provide the SHA-1 functions
+- LibSSH2 to enable the SSH transport
+- iconv (OSX) to handle the HFS+ path encoding peculiarities
+
+Initialization
+===============
+
+The library needs to keep track of some global state. Call
+
+    git_libgit2_init();
+
+before calling any other libgit2 functions. You can call this function many times. A matching number of calls to
+
+    git_libgit2_shutdown();
+
+will free the resources.  Note that if you have worker threads, you should
+call `git_libgit2_shutdown` *after* those threads have exited.  If you
+require assistance coordinating this, simply have the worker threads call
+`git_libgit2_init` at startup and `git_libgit2_shutdown` at shutdown.
+
+Threading
+=========
+
+See [THREADING](THREADING.md) for information
+
+Conventions
+===========
+
+See [CONVENTIONS](CONVENTIONS.md) for an overview of the external
+and internal API/coding conventions we use.
+
+Building libgit2 - Using CMake
+==============================
+
+Building
+--------
+
+`libgit2` builds cleanly on most platforms without any external dependencies.
+Under Unix-like systems, like Linux, \*BSD and Mac OS X, libgit2 expects `pthreads` to be available;
+they should be installed by default on all systems. Under Windows, libgit2 uses the native Windows API
+for threading.
+
+The `libgit2` library is built using [CMake](<https://cmake.org/>) (version 2.8 or newer) on all platforms.
+
+On most systems you can build the library using the following commands
+
+	$ mkdir build && cd build
+	$ cmake ..
+	$ cmake --build .
+
+Alternatively you can point the CMake GUI tool to the CMakeLists.txt file and generate platform specific build project or IDE workspace.
+
+Running Tests
+-------------
+
+Once built, you can run the tests from the `build` directory with the command
+
+	$ ctest -V
+
+Alternatively you can run the test suite directly using,
+
+	$ ./libgit2_clar
+
+Invoking the test suite directly is useful because it allows you to execute
+individual tests, or groups of tests using the `-s` flag.  For example, to
+run the index tests:
+
+    $ ./libgit2_clar -sindex
+
+To run a single test named `index::racy::diff`, which corresponds to the test
+function (`test_index_racy__diff`)[https://github.com/libgit2/libgit2/blob/master/tests/index/racy.c#L23]:
+
+    $ ./libgit2_clar -sindex::racy::diff
+
+The test suite will print a `.` for every passing test, and an `F` for any
+failing test.  An `S` indicates that a test was skipped because it is not
+applicable to your platform or is particularly expensive.
+
+**Note:** There should be _no_ failing tests when you build an unmodified
+source tree from a [release](https://github.com/libgit2/libgit2/releases),
+or from the [master branch](https://github.com/libgit2/libgit2/tree/master).
+Please contact us or [open an issue](https://github.com/libgit2/libgit2/issues)
+if you see test failures.
+
+Installation
+------------
+
+To install the library you can specify the install prefix by setting:
+
+	$ cmake .. -DCMAKE_INSTALL_PREFIX=/install/prefix
+	$ cmake --build . --target install
+
+Advanced Usage
+--------------
+
+For more advanced use or questions about CMake please read <https://cmake.org/Wiki/CMake_FAQ>.
+
+The following CMake variables are declared:
+
+- `BIN_INSTALL_DIR`: Where to install binaries to.
+- `LIB_INSTALL_DIR`: Where to install libraries to.
+- `INCLUDE_INSTALL_DIR`: Where to install headers to.
+- `BUILD_SHARED_LIBS`: Build libgit2 as a Shared Library (defaults to ON)
+- `BUILD_CLAR`: Build [Clar](https://github.com/vmg/clar)-based test suite (defaults to ON)
+- `THREADSAFE`: Build libgit2 with threading support (defaults to ON)
+- `STDCALL`: Build libgit2 as `stdcall`. Turn off for `cdecl` (Windows; defaults to ON)
+
+Compiler and linker options
+---------------------------
+
+CMake lets you specify a few variables to control the behavior of the
+compiler and linker. These flags are rarely used but can be useful for
+64-bit to 32-bit cross-compilation.
+
+- `CMAKE_C_FLAGS`: Set your own compiler flags
+- `CMAKE_FIND_ROOT_PATH`: Override the search path for libraries
+- `ZLIB_LIBRARY`, `OPENSSL_SSL_LIBRARY` AND `OPENSSL_CRYPTO_LIBRARY`:
+Tell CMake where to find those specific libraries
+
+MacOS X
+-------
+
+If you want to build a universal binary for Mac OS X, CMake sets it
+all up for you if you use `-DCMAKE_OSX_ARCHITECTURES="i386;x86_64"`
+when configuring.
+
+Android
+-------
+
+Extract toolchain from NDK using, `make-standalone-toolchain.sh` script.
+Optionally, crosscompile and install OpenSSL inside of it. Then create CMake
+toolchain file that configures paths to your crosscompiler (substitute `{PATH}`
+with full path to the toolchain):
+
+	SET(CMAKE_SYSTEM_NAME Linux)
+	SET(CMAKE_SYSTEM_VERSION Android)
+
+	SET(CMAKE_C_COMPILER   {PATH}/bin/arm-linux-androideabi-gcc)
+	SET(CMAKE_CXX_COMPILER {PATH}/bin/arm-linux-androideabi-g++)
+	SET(CMAKE_FIND_ROOT_PATH {PATH}/sysroot/)
+
+	SET(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
+	SET(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
+	SET(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
+
+Add `-DCMAKE_TOOLCHAIN_FILE={pathToToolchainFile}` to cmake command
+when configuring.
+
+Language Bindings
+==================================
+
+Here are the bindings to libgit2 that are currently available:
+
+* C++
+    * libqgit2, Qt bindings <https://projects.kde.org/projects/playground/libs/libqgit2/repository/>
+* Chicken Scheme
+    * chicken-git <https://wiki.call-cc.org/egg/git>
+* D
+    * dlibgit <https://github.com/s-ludwig/dlibgit>
+* Delphi
+    * GitForDelphi <https://github.com/libgit2/GitForDelphi>
+* Erlang
+    * Geef <https://github.com/carlosmn/geef>
+* Go
+    * git2go <https://github.com/libgit2/git2go>
+* GObject
+    * libgit2-glib <https://wiki.gnome.org/Projects/Libgit2-glib>
+* Guile
+	* Guile-Git <https://gitlab.com/guile-git/guile-git>
+* Haskell
+    * hgit2 <https://github.com/jwiegley/gitlib>
+* Java
+    * Jagged <https://github.com/ethomson/jagged>
+* Julia
+    * LibGit2.jl <https://github.com/jakebolewski/LibGit2.jl>
+* Lua
+    * luagit2 <https://github.com/libgit2/luagit2>
+* .NET
+    * libgit2sharp <https://github.com/libgit2/libgit2sharp>
+* Node.js
+    * nodegit <https://github.com/nodegit/nodegit>
+* Objective-C
+    * objective-git <https://github.com/libgit2/objective-git>
+* OCaml
+    * ocaml-libgit2 <https://github.com/fxfactorial/ocaml-libgit2>
+* Parrot Virtual Machine
+    * parrot-libgit2 <https://github.com/letolabs/parrot-libgit2>
+* Perl
+    * Git-Raw <https://github.com/jacquesg/p5-Git-Raw>
+* PHP
+    * php-git <https://github.com/libgit2/php-git>
+* PowerShell
+    * PSGit <https://github.com/PoshCode/PSGit>
+* Python
+    * pygit2 <https://github.com/libgit2/pygit2>
+* R
+    * git2r <https://github.com/ropensci/git2r>
+* Ruby
+    * Rugged <https://github.com/libgit2/rugged>
+* Rust
+    * git2-rs <https://github.com/alexcrichton/git2-rs>
+* Swift
+    * SwiftGit2 <https://github.com/SwiftGit2/SwiftGit2>
+* Vala
+    * libgit2.vapi <https://github.com/apmasell/vapis/blob/master/libgit2.vapi>
+
+If you start another language binding to libgit2, please let us know so
+we can add it to the list.
+
+How Can I Contribute?
+==================================
+
+We welcome new contributors!  We have a number of issues marked as
+["up for grabs"](https://github.com/libgit2/libgit2/issues?q=is%3Aissue+is%3Aopen+label%3A%22up+for+grabs%22)
+and
+["easy fix"](https://github.com/libgit2/libgit2/issues?utf8=✓&q=is%3Aissue+is%3Aopen+label%3A%22easy+fix%22)
+that are good places to jump in and get started.  There's much more detailed
+information in our list of [outstanding projects](PROJECTS.md).
+
+Please be sure to check the [contribution guidelines](CONTRIBUTING.md) to
+understand our workflow, and the libgit2 [coding conventions](CONVENTIONS.md).
+
+License
+==================================
+
+`libgit2` is under GPL2 **with linking exception**. This means you can link to
+and use the library from any program, proprietary or open source; paid or
+gratis.  However, if you modify libgit2 itself, you must distribute the
+source to your modified version of libgit2.
+
+See the [COPYING file](COPYING) for the full license text.

fuzzers/corpora/objects/commit

+tree 3e7ac388cadae960fe7e22175ce0da878afe9d18
+parent 8b89f362a34fcccdf1c6c5f3445895b71d9c6d56
+parent c590b41fe4057a84a9bd31a5605ceef2c309b0f8
+author Patrick Steinhardt <ps@pks.im> 1538760730 +0200
+committer GitHub <noreply@github.com> 1538760730 +0200
+gpgsig -----BEGIN PGP SIGNATURE-----
+ 
+ wsBcBAABCAAQBQJbt6AaCRBK7hj4Ov3rIwAAdHIIAKZGIpS0dAirVRt5NVFj3ZtC
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+ =nGP0
+ -----END PGP SIGNATURE-----
+ 
+
+Merge pull request #4834 from pks-t/pks/v0.27.5
+
+Security release v0.27.5
\ No newline at end of file

fuzzers/corpora/objects/tag

+object a8d447f68076d1520f69649bb52629941be7031f
+type commit
+tag testtag
+tagger Patrick Steinhardt <ps@pks.im> 1539253015 +0200
+
+Tag message

fuzzers/objects_fuzzer.c

+/*
+ * libgit2 packfile fuzzer target.
+ *
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+
+#include "git2.h"
+#include "object.h"
+
+#define UNUSED(x) (void)(x)
+
+int LLVMFuzzerInitialize(int *argc, char ***argv)
+{
+	UNUSED(argc);
+	UNUSED(argv);
+
+	if (git_libgit2_init() < 0)
+		abort();
+
+	return 0;
+}
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
+{
+	const git_otype types[] = {
+		GIT_OBJ_BLOB, GIT_OBJ_TREE, GIT_OBJ_COMMIT, GIT_OBJ_TAG
+	};
+	git_object *object = NULL;
+	size_t i;
+
+	/*
+	 * Brute-force parse this as every object type. We want
+	 * to stress the parsing logic anyway, so this is fine
+	 * to do.
+	 */
+	for (i = 0; i < ARRAY_SIZE(types); i++) {
+		if (git_object__from_raw(&object, (const char *) data, size, types[i]) < 0)
+			continue;
+		git_object_free(object);
+		object = NULL;
+	}
+
+	return 0;
+}