# Test Flatbuffers library with help of libFuzzer
Test suite of Flatbuffers library has fuzzer section with tests are based on libFuzzer library.

> LibFuzzer is in-process, coverage-guided, evolutionary fuzzing engine.
LibFuzzer is linked with the library under test, and feeds fuzzed inputs to the library via a specific fuzzing entrypoint (aka “target function”);
the fuzzer then tracks which areas of the code are reached, and generates mutations on the corpus of input data in order to maximize the code coverage.
The code coverage information for libFuzzer is provided by LLVM’s SanitizerCoverage instrumentation.

For details about **libFuzzer** see: https://llvm.org/docs/LibFuzzer.html

To build and run these tests LLVM compiler (with clang frontend) and CMake should be installed before.

The fuzzer section include three tests:
- `verifier_fuzzer` checks stability of deserialization engine for `Monster` schema;
- `parser_fuzzer` checks stability of schema and json parser under various inputs;
- `scalar_parser` focused on validation of the parser while parse numeric scalars in schema and/or json files;

## Run tests with a specific locale
The grammar of the Flatbuffers library is based on printable-ASCII characters.
By design, the Flatbuffers library should be independent of the global or thread locales used by an end-user application.
Set environment variable `FLATBUFFERS_TEST_LOCALE` to run a fuzzer with a specific C-locale:
```sh
>FLATBUFFERS_TEST_LOCALE="" ./scalar_parser
>FLATBUFFERS_TEST_LOCALE="ru_RU.CP1251" ./parser_fuzzer
```

## Run fuzzer
These are examples of running a fuzzer.
Flags may vary and depend on a version of the libFuzzer library.
For details, run a fuzzer with `-help` flag: `./parser_fuzzer -help=1`

`./verifier_fuzzer ../.corpus_verifier/ ../.seed_verifier/`

`./parser_fuzzer -only_ascii=1  -max_len=500 -dict=../parser_fbs.dict ../.corpus_parser/ ../.seed_parser/`

`./monster_fuzzer -only_ascii=1 -max_len=500 -dict=../monster_json.dict ../.corpus_monster/ ../.seed_monster/`

`./scalar_fuzzer -use_value_profile=1 -max_len=500 -dict=../scalar_json.dict ../.corpus_scalar/ ../.seed_scalar/`

Flag `-only_ascii=1` is useful for fast number-compatibility checking while run `scalar_fuzzer`.

Run with a specific C-locale:
`FLATBUFFERS_TEST_LOCALE="ru_RU.CP1251" ./scalar_fuzzer -reduce_depth=1 -use_value_profile=1 -shrink=1 -max_len=3000 -timeout=10 -rss_limit_mb=2048 ../.corpus_parser/ ../.seed_parser/`


## Merge (minimize) corpus
The **libFuzzer** allow to filter (minimize) corpus with help of `-merge` flag:
> -merge
    If set to 1, any corpus inputs from the 2nd, 3rd etc. corpus directories that trigger new code coverage will be merged into the first corpus directory.
    Defaults to 0. This flag can be used to minimize a corpus.

Merge several corpuses to a seed directory (a new collected corpus to the seed collection, for example):
`./verifier_fuzzer -merge=1 ../.seed_verifier/ ../.corpus_verifier/`
`./parser_fuzzer -merge=1 ../.seed_parser/ ../.corpus_parser/`
`./monster_fuzzer -merge=1 ../.seed_monster/ ../.corpus_monster/`
`./scalar_fuzzer -merge=1 ../.seed_scalar/ ../.corpus_scalar/`

## Know limitations
- LLVM 7.0 std::regex library has problem with stack overflow, maximum length of input for `scalar_fuzzer` run should be limited to 3000.
  Example: `./scalar_fuzzer -max_len=3000`

# Fuzzing control

## Set timeout or memory limit

`-timeout=10 -rss_limit_mb=2048 -jobs=4 -workers=4`.

## Force stop on first UBSAN error

- `export UBSAN_OPTIONS=halt_on_error=1`
- `export ASAN_OPTIONS=halt_on_error=1`