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Jon Ross-Perkins d12583fc08 Do cleanup on explorer fuzzing infrastructure. (#2790 )

I'm partly doing this because the current setup would be difficult to share with the toolchain. e.g., ProtoToCarbon isn't explorer-specific, but the only way to run it via CLI is the explorer's fuzzverter. I want a separate tool.

This change:

- Adds a //common/fuzzing:proto_to_carbon tool.
  - The rest of fuzzverter is now just //explorer/fuzzing:ast_to_proto.
  - The change simplifies overall handling and removes a LLVM CLI dependency.
- Stops allowing unknown fields in the proto.
  - This has mostly led to forgetting to remove fuzzer inputs that were for removed features.
- Moves more non-explorer-specific bits to //common/fuzzing.
- Cleans up remaining pieces in //explorer/fuzzing
  - Merges the //explorer/fuzzing proto tests, which deduplicates AstToString copies.
    - These tests also had duplicate dependencies, etc -- and all complete in ~6s.
  - Updates and fixes regen_corpus which was previously broken by other changes.
  - Updates the README to reflect changes.
- Removes obsolete proto-fuzzer build configuration (AFAICT this is no longer needed).

2023-04-21 08:23:58 -07:00

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Explorer structured fuzzer

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, which is a library to randomly mutate protobuffers.

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 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 normally should not require direct changes, as tests work off of Carbon test files in testdata.

To incorporate AST changes into fuzzing logic:

Add appropriate AST information to carbon.proto. Use existing similar cases as examples.
Modify 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.
Add logic to populate the proto to ast_to_proto.cpp.
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:

bazel test //explorer/fuzzing:explorer_fuzzer

To run in 'fuzzing' mode:

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:

# 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:

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

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