# Fuzzing Bitcoin Core using libFuzzer

## Quickstart guide

To quickly get started fuzzing Bitcoin Core using [libFuzzer](https://llvm.org/docs/LibFuzzer.html):

```sh
$ git clone https://github.com/bitcoin/bitcoin
$ cd bitcoin/
$ ./autogen.sh
$ CC=clang CXX=clang++ ./configure --enable-fuzz --with-sanitizers=address,fuzzer,undefined
# macOS users: If you have problem with this step then make sure to read "macOS hints for
# libFuzzer" on https://github.com/bitcoin/bitcoin/blob/master/doc/fuzzing.md#macos-hints-for-libfuzzer
$ make
$ src/test/fuzz/process_message
# abort fuzzing using ctrl-c
```

## Fuzzing harnesses, fuzzing output and fuzzing corpora

[`process_message`](https://github.com/bitcoin/bitcoin/blob/master/src/test/fuzz/process_message.cpp) is a fuzzing harness for the [`ProcessMessage(...)` function (`net_processing`)](https://github.com/bitcoin/bitcoin/blob/master/src/net_processing.cpp). The available fuzzing harnesses are found in [`src/test/fuzz/`](https://github.com/bitcoin/bitcoin/tree/master/src/test/fuzz).

The fuzzer will output `NEW` every time it has created a test input that covers new areas of the code under test. For more information on how to interpret the fuzzer output, see the [libFuzzer documentation](https://llvm.org/docs/LibFuzzer.html).

If you specify a corpus directory then any new coverage increasing inputs will be saved there:

```sh
$ mkdir -p process_message-seeded-from-thin-air/
$ src/test/fuzz/process_message process_message-seeded-from-thin-air/
INFO: Seed: 840522292
INFO: Loaded 1 modules   (424174 inline 8-bit counters): 424174 [0x55e121ef9ab8, 0x55e121f613a6),
INFO: Loaded 1 PC tables (424174 PCs): 424174 [0x55e121f613a8,0x55e1225da288),
INFO:        0 files found in process_message-seeded-from-thin-air/
INFO: -max_len is not provided; libFuzzer will not generate inputs larger than 4096 bytes
INFO: A corpus is not provided, starting from an empty corpus
#2      INITED cov: 94 ft: 95 corp: 1/1b exec/s: 0 rss: 150Mb
#3      NEW    cov: 95 ft: 96 corp: 2/3b lim: 4 exec/s: 0 rss: 150Mb L: 2/2 MS: 1 InsertByte-
#4      NEW    cov: 96 ft: 98 corp: 3/7b lim: 4 exec/s: 0 rss: 150Mb L: 4/4 MS: 1 CrossOver-
#21     NEW    cov: 96 ft: 100 corp: 4/11b lim: 4 exec/s: 0 rss: 150Mb L: 4/4 MS: 2 ChangeBit-CrossOver-
#324    NEW    cov: 101 ft: 105 corp: 5/12b lim: 6 exec/s: 0 rss: 150Mb L: 6/6 MS: 5 CrossOver-ChangeBit-CopyPart-ChangeBit-ChangeBinInt-
#1239   REDUCE cov: 102 ft: 106 corp: 6/24b lim: 14 exec/s: 0 rss: 150Mb L: 13/13 MS: 5 ChangeBit-CrossOver-EraseBytes-ChangeBit-InsertRepeatedBytes-
#1272   REDUCE cov: 102 ft: 106 corp: 6/23b lim: 14 exec/s: 0 rss: 150Mb L: 12/12 MS: 3 ChangeBinInt-ChangeBit-EraseBytes-
        NEW_FUNC[1/677]: 0x55e11f456690 in std::_Function_base::~_Function_base() /usr/lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/std_function.h:255
        NEW_FUNC[2/677]: 0x55e11f465800 in CDataStream::CDataStream(std::vector<unsigned char, std::allocator<unsigned char> > const&, int, int) src/./streams.h:248
#2125   REDUCE cov: 4820 ft: 4867 corp: 7/29b lim: 21 exec/s: 0 rss: 155Mb L: 6/12 MS: 2 CopyPart-CMP- DE: "block"-
        NEW_FUNC[1/9]: 0x55e11f64d790 in std::_Rb_tree<uint256, std::pair<uint256 const, std::chrono::duration<long, std::ratio<1l, 1000000l> > >, std::_Select1st<std::pair<uint256 const, std::chrono::duration<long, std::ratio<1l, 1000000l> > > >, std::less<uint256>, std::allocator<std::pair<uint256 const, std::chrono::duration<long, std::ratio<1l, 1000000l> > > > >::~_Rb_tree() /usr/lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/stl_tree.h:972
        NEW_FUNC[2/9]: 0x55e11f64d870 in std::_Rb_tree<uint256, std::pair<uint256 const, std::chrono::duration<long, std::ratio<1l, 1000000l> > >, std::_Select1st<std::pair<uint256 const, std::chrono::duration<long, std::ratio<1l, 1000000l> > > >, std::less<uint256>, std::allocator<std::pair<uint256 const, std::chrono::duration<long, std::ratio<1l, 1000000l> > > > >::_M_erase(std::_Rb_tree_node<std::pair<uint256 const, std::chrono::duration<long, std::ratio<1l, 1000000l> > > >*) /usr/lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/stl_tree.h:1875
#2228   NEW    cov: 4898 ft: 4971 corp: 8/35b lim: 21 exec/s: 0 rss: 156Mb L: 6/12 MS: 3 EraseBytes-CopyPart-PersAutoDict- DE: "block"-
        NEW_FUNC[1/5]: 0x55e11f46df70 in std::enable_if<__and_<std::allocator_traits<zero_after_free_allocator<char> >::__construct_helper<char, unsigned char const&>::type>::value, void>::type std::allocator_traits<zero_after_free_allocator<char> >::_S_construct<char, unsigned char const&>(zero_after_free_allocator<char>&, char*, unsigned char const&) /usr/lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/alloc_traits.h:243
        NEW_FUNC[2/5]: 0x55e11f477390 in std::vector<unsigned char, std::allocator<unsigned char> >::data() /usr/lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/stl_vector.h:1056
#2456   NEW    cov: 4933 ft: 5042 corp: 9/55b lim: 21 exec/s: 0 rss: 160Mb L: 20/20 MS: 3 ChangeByte-InsertRepeatedBytes-PersAutoDict- DE: "block"-
#2467   NEW    cov: 4933 ft: 5043 corp: 10/76b lim: 21 exec/s: 0 rss: 161Mb L: 21/21 MS: 1 InsertByte-
#4215   NEW    cov: 4941 ft: 5129 corp: 17/205b lim: 29 exec/s: 4215 rss: 350Mb L: 29/29 MS: 5 InsertByte-ChangeBit-CopyPart-InsertRepeatedBytes-CrossOver-
#4567   REDUCE cov: 4941 ft: 5129 corp: 17/204b lim: 29 exec/s: 4567 rss: 404Mb L: 24/29 MS: 2 ChangeByte-EraseBytes-
#6642   NEW    cov: 4941 ft: 5138 corp: 18/244b lim: 43 exec/s: 2214 rss: 450Mb L: 43/43 MS: 3 CopyPart-CMP-CrossOver- DE: "verack"-
# abort fuzzing using ctrl-c
$ ls process_message-seeded-from-thin-air/
349ac589fc66a09abc0b72bb4ae445a7a19e2cd8 4df479f1f421f2ea64b383cd4919a272604087a7
a640312c98dcc55d6744730c33e41c5168c55f09 b135de16e4709558c0797c15f86046d31c5d86d7
c000f7b41b05139de8b63f4cbf7d1ad4c6e2aa7f fc52cc00ec1eb1c08470e69f809ae4993fa70082
$ cat --show-nonprinting process_message-seeded-from-thin-air/349ac589fc66a09abc0b72bb4ae445a7a19e2cd8
block^@M-^?M-^?M-^?M-^?M-^?nM-^?M-^?
```

In this case the fuzzer managed to create a `block` message which when passed to `ProcessMessage(...)` increased coverage.

The project's collection of seed corpora is found in the [`bitcoin-core/qa-assets`](https://github.com/bitcoin-core/qa-assets) repo.

To fuzz `process_message` using the [`bitcoin-core/qa-assets`](https://github.com/bitcoin-core/qa-assets) seed corpus:

```sh
$ git clone https://github.com/bitcoin-core/qa-assets
$ src/test/fuzz/process_message qa-assets/fuzz_seed_corpus/process_message/
INFO: Seed: 1346407872
INFO: Loaded 1 modules   (424174 inline 8-bit counters): 424174 [0x55d8a9004ab8, 0x55d8a906c3a6),
INFO: Loaded 1 PC tables (424174 PCs): 424174 [0x55d8a906c3a8,0x55d8a96e5288),
INFO:      991 files found in qa-assets/fuzz_seed_corpus/process_message/
INFO: -max_len is not provided; libFuzzer will not generate inputs larger than 4096 bytes
INFO: seed corpus: files: 991 min: 1b max: 1858b total: 288291b rss: 150Mb
#993    INITED cov: 7063 ft: 8236 corp: 25/3821b exec/s: 0 rss: 181Mb
…
```

If you find coverage increasing inputs when fuzzing you are highly encouraged to submit them for inclusion in the [`bitcoin-core/qa-assets`](https://github.com/bitcoin-core/qa-assets) repo.

Every single pull request submitted against the Bitcoin Core repo is automatically tested against all inputs in the [`bitcoin-core/qa-assets`](https://github.com/bitcoin-core/qa-assets) repo. Contributing new coverage increasing inputs is an easy way to help make Bitcoin Core more robust.

## macOS hints for libFuzzer

The default Clang/LLVM version supplied by Apple on macOS does not include
fuzzing libraries, so macOS users will need to install a full version, for
example using `brew install llvm`.

Should you run into problems with the address sanitizer, it is possible you
may need to run `./configure` with `--disable-asm` to avoid errors
with certain assembly code from Bitcoin Core's code. See [developer notes on sanitizers](https://github.com/bitcoin/bitcoin/blob/master/doc/developer-notes.md#sanitizers)
for more information.

You may also need to take care of giving the correct path for `clang` and
`clang++`, like `CC=/path/to/clang CXX=/path/to/clang++` if the non-systems
`clang` does not come first in your path.

Full configure that was tested on macOS Catalina with `brew` installed `llvm`:

```sh
./configure --enable-fuzz --with-sanitizers=fuzzer,address,undefined CC=/usr/local/opt/llvm/bin/clang CXX=/usr/local/opt/llvm/bin/clang++ --disable-asm
```

Read the [libFuzzer documentation](https://llvm.org/docs/LibFuzzer.html) for more information. This [libFuzzer tutorial](https://github.com/google/fuzzing/blob/master/tutorial/libFuzzerTutorial.md) might also be of interest.

# Fuzzing Bitcoin Core using american fuzzy lop (`afl-fuzz`)

## Quickstart guide

To quickly get started fuzzing Bitcoin Core using [`afl-fuzz`](https://github.com/google/afl):

```sh
$ git clone https://github.com/bitcoin/bitcoin
$ cd bitcoin/
$ git clone https://github.com/google/afl
$ make -C afl/
$ make -C afl/llvm_mode/
$ ./autogen.sh
# It is possible to compile with afl-gcc and afl-g++ instead of afl-clang. However, running afl-fuzz
# may require more memory via the -m flag.
$ CC=$(pwd)/afl/afl-clang-fast CXX=$(pwd)/afl/afl-clang-fast++ ./configure --enable-fuzz
$ make
# For macOS you may need to ignore x86 compilation checks when running "make". If so,
# try compiling using: AFL_NO_X86=1 make
$ mkdir -p inputs/ outputs/
$ echo A > inputs/thin-air-input
$ afl/afl-fuzz -i inputs/ -o outputs/ -- src/test/fuzz/bech32
# You may have to change a few kernel parameters to test optimally - afl-fuzz
# will print an error and suggestion if so.
```

Read the [`afl-fuzz` documentation](https://github.com/google/afl) for more information.

# Fuzzing Bitcoin Core using Honggfuzz

## Quickstart guide

To quickly get started fuzzing Bitcoin Core using [Honggfuzz](https://github.com/google/honggfuzz):

```sh
$ git clone https://github.com/bitcoin/bitcoin
$ cd bitcoin/
$ ./autogen.sh
$ git clone https://github.com/google/honggfuzz
$ cd honggfuzz/
$ make
$ cd ..
$ CC=$(pwd)/honggfuzz/hfuzz_cc/hfuzz-clang CXX=$(pwd)/honggfuzz/hfuzz_cc/hfuzz-clang++ ./configure --enable-fuzz --with-sanitizers=address,undefined
$ make
$ mkdir -p inputs/
$ honggfuzz/honggfuzz -i inputs/ -- src/test/fuzz/process_message
```

Read the [Honggfuzz documentation](https://github.com/google/honggfuzz/blob/master/docs/USAGE.md) for more information.
