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Fuzz testing for various parts of the Lua interpreter, mostly for use as a test-case generator for alternate Lua implementations

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Fuzzing Lua
===========

Fuzz testing for various parts of the Lua interpreter, mostly for use as a test-case generator for alternate Lua implementations (e.g. [Zua](https://github.com/squeek502/zua)). Uses [libFuzzer](https://llvm.org/docs/LibFuzzer.html) via the Clang compiler. A pre-generated corpus and corresponding outputs can be found on the [releases page](https://github.com/squeek502/fuzzing-lua/releases) for quick use as a data set for testing other Lua implementations.

For a writeup of how this was used to test [Zua](https://github.com/squeek502/zua)'s lexer implementation, see:
- [Fuzzing As A Test Case Generator](https://www.ryanliptak.com/blog/fuzzing-as-test-case-generator/)

Currently supports:
- Lua 5.1.5
  + Lexer (llex.c)

## Setup

- Make sure you have [CMake](https://cmake.org), Clang, and Clang++ installed.
- Run `./setup.sh` to generate `build.fuzz`, `build.cov`, and `build.tools` directories via CMake.

## Fuzzing

First, `cd build.fuzz`, then, from within that directory:

- To run the fuzzer: `make fuzz_llex_run`
- To minimize the fuzzer's corpus: `make fuzz_llex_minimize`

## Tools

### fuzz_llex_output

The lexer fuzzer has an accompanying tool to output the tokens generated when lexing each file in the corpus. This set of result files can then be used to test alternate Lua lexer implementations to ensure that they generate the exact same set of tokens.

The format of the output file is a space-separated list of tokens, printed via `luaX_token2str`. For example, if the input file was:

```lua
local hello = 'world'
```

then the output file would be:

```
local <name> = <string> <eof>
```

If there is an error while lexing, then the error appears on the second line of the file. For example, if the last `'` in the `'world'` string in the previous example were omitted, then the output would look like:

```
local <name> =
[string "fuzz"]:3: unfinished string near '<eof>'
```

#### Usage

First, run the fuzzer (see Fuzzing above) to create a comprehensive corpus. Then, to generate the output files for each file in the corpus:

```
cd build.tools
make fuzz_llex_output_run
```

The files will be in `build.tools/tools/output/fuzz_llex/` and the file names will correspond with the file names of the corpus input files.

### fuzz_strings_output

Given a corpus of input files containing valid unparsed string literals, this tool can generate a corresponding set of output files containing the parsed version of the string. These pairs of input/output files can then be used to test the string parsing of alternate Lua implementations.

For example, given the following input file:

```lua
"\\\thello world"
```

the output file would be:

```
\	hello world
```

#### Usage

A comprehensive set of inputs are included in this repository, so generating the outputs is as simple as:

```
cd build.tools
make fuzz_strings_output_run
```

#### Generating an input set

Because the PUC Lua lexer both lexes and parses strings at the same time, it's a bit hard to extract unparsed strings while lexing. Instead, [Zua](https://github.com/squeek502/zua) was used along with a corpus generated by `fuzz_llex` to extract all valid strings from the entire corpus and put each into its own file. Then, that set of strings was minimized by running them back through `fuzz_llex` with `-merge=1`. Specifically, the following was done:

To generate an un-minimized set of inputs with Zua:

1. Clone [zua](https://github.com/squeek502/zua)
2. `zig build fuzz_strings_gen_run`

Once you have a un-minimized set of inputs, you can minimize it using `fuzz_llex` by doing the following:

1. Copy the generated files to `fuzzing-lua/build.fuzz/corpus/fuzz_strings_generated`
2. `cd fuzzing-lua/build.fuzz`
3. `make fuzz_llex`
4. `mkdir corpus/fuzz_strings`
5. `./fuzz_llex corpus/fuzz_strings corpus/fuzz_strings_generated -merge=1`

Your minimized input set will then exist in `fuzzing-lua/build.fuzz/corpus/fuzz_strings`.

## Coverage

This is currently untested. **TODO**: Adapt [this guide](https://github.com/squeek502/d2itemreader/tree/master/tests/fuzz#checking-coverage-of-a-fuzzer).

## Packaging/releasing data sets

```
cd build.tools
make package_data
```

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