# Explorer structured fuzzer

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## Overview

Fuzz testing is based on generating a large amount of random inputs for a
software component in order to trigger bugs and unexpected behavior. Basic
fuzzing uses randomly generated arrays of bytes as inputs, which works great for
some applications but is problematic for testing the logic that operates on
highly structured data, as most random inputs are immediately rejected as
invalid before any interesting parts of the code get a chance to run.

Structured fuzzing addresses this issue by ensuring the randomly generated data
is itself structured, and as such has a high chance of presenting a valid input.

`explorer_fuzzer` is a structured fuzzer based on
[libprotobuf-mutator](https://github.com/google/libprotobuf-mutator), which is a
library to randomly mutate
[protobuffers](https://github.com/protocolbuffers/protobuf).

The input to the fuzzer is an instance of `Carbon::Fuzzing::Carbon` proto
randomly generated by the `libprotobuf-mutator` framework. `explorer_fuzzer`
converts the proto to a Carbon source code string, and tries to parse and
execute the code using `explorer` implementation.

## Fuzzer data format

`libprotobuf-mutator` supports fuzzer inputs in either text or binary protocol
buffer format. `explorer_fuzzer` uses text proto format with `Carbon` proto
message definition in `common/fuzzing/carbon.proto`.

## Incorporating AST changes into the fuzzer

Fuzzer AST representation in [carbon.proto](/common/fuzzing/carbon.proto) needs
to be updated when changes are made to the AST, like adding a new AST node
classes or changing relevant data members of existing nodes.

[ast_to_proto_test](ast_to_proto_test.cpp) normally should not require direct
changes, as tests work off of Carbon test files in
[testdata](/explorer/testdata).

To incorporate AST changes into fuzzing logic:

1. Add appropriate AST information to
   [carbon.proto](/common/fuzzing/carbon.proto). Use existing similar cases as
   examples.

2. Modify [proto_to_carbon.cpp](/common/fuzzing/proto_to_carbon.cpp) which
   handles printing of a Carbon proto instance as a Carbon source string. For
   example, add code to print newly introduced proto fields.

3. Add logic to populate the proto to
   [ast_to_proto.cpp](/explorer/fuzzing/ast_to_proto.cpp).

4. Make sure `bazel test //explorer/fuzzing:ast_to_proto_test` passes with the
   new changes.

## Running the fuzzer

The fuzzer can be run in 'unit test' mode, where the fuzzer executes on each
input file from the `fuzzer_corpus/` folder, or in 'fuzzing' mode, where the
fuzzer will keep generating random inputs and executing the logic on them until
a crash is triggered, or forever in a bug-free program ;).

To run in 'unit test' mode:

```bash
bazel test //explorer/fuzzing:explorer_fuzzer
```

To run in 'fuzzing' mode:

```bash
bazel build --config=fuzzer //explorer/fuzzing:explorer_fuzzer.full_corpus

bazel-bin/explorer/fuzzing/explorer_fuzzer.full_corpus
```

## Investigating a crash

Typically it's going to be easiest to run explorer on directly. You can do this
with:

````bash
# Convert the crash to a source file.
bazel run //common/fuzzing:proto_to_carbon -- /tmp/crash.textproto > crash.carbon

# Run explorer on the crash.
bazel run //explorer -- crash.carbon
```

It's also possible to run the fuzzer on a single input:

```bash
bazel-bin/explorer/fuzzing/explorer_fuzzer.full_corpus /tmp/crash.textproto
````

## Generating new fuzzer corpus entries

The ability of the fuzzing framework to generate 'interesting' inputs can be
improved by providing 'seed' inputs known as the fuzzer corpus. The inputs need
to be a `Fuzzing::Carbon` text proto.

To generate a text proto from Carbon source:

```bash
bazel run //explorer/fuzzing:ast_to_proto -- /tmp/crash.carbon > crash.textproto
```