# Fuzzing

This directory contains code and test data for fuzz testing h2o with LLVM's [LibFuzzer](https://llvm.org/docs/LibFuzzer.html). 

## Building the test drivers

To build the fuzz test drivers, pass `-DBUILD_FUZZER=ON` to `cmake`. This this will instrument `libh2o`, and build fuzz test driver programs for both HTTP/1 and HTTP/2: `h2o-fuzzer-http1` and
  `h2o-fuzzer-http2`, respectively. Note that you must have a version of LLVM Clang installed that supports LibFuzzer in order for this build to succeed.

## Test corpus information

The test corpuses included in this were initially created by first patching h2o with the included `fuzz/gather-data.patch` and then running the unit tests. The corpuses were then broadened via fuzzing with this fuzzer and minimized. See the [LibFuzzer docs](https://llvm.org/docs/LibFuzzer.html) for more information on how minimization works.

## Submitting new seed files

This project welcomes seed files that exercise new paths in the target programs. Before submitting new seed files, please ensure they add coverage to the existing corpus via the driver `-merge` flag. For example:

```
$ ./h2o-fuzzer-http2 -max_len=$((16 * 1024 )) -merge=1 ./fuzz/http2-corpus ./fuzz/my-new-seeds
```

Over time, various factors may affect the coverage that a given seed file exercises so it may also be worthwhile to re-minimize the resulting corpus periodically:

```
$ ./h2o-fuzzer-http2 -max_len=$((16 * 1024 )) -merge=1 ./fuzz/http2-corpus.fresh ./fuzz/http2-corpus
$ cp ./fuzz/http2-corpus.fresh/* ./fuzz/http2-corpus
```

## Running the fuzzers

You will likely want to tailor fuzzer options to your execution environment, but here are basic examples of running each fuzzer:

HTTP/1: `ASAN_OPTIONS=detect_leaks=0 ./h2o-fuzzer-http1 -max_len=$((16 * 1024 )) -dict=fuzz/http.dict fuzz/http1-corpus`

HTTP/2: `ASAN_OPTIONS=detect_leaks=0 ./h2o-fuzzer-http2 -max_len=$((16 * 1024 )) -dict=fuzz/http.dict fuzz/http2-corpus`

The fuzzer looks at `H2O_FUZZER_CLIENT_TIMEOUT` in order to configure the
client thread event loop timeout in milli-seconds. It defaults to 10 ms.
