carbon-lang/executable_semantics/fuzzing

Executable semantics 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.

executable_semantics_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. executable_semantics_fuzzer converts the proto to a Carbon source code string, and tries to parse and execute the code using executable_semantics implementation.

Fuzzer data format

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

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 --config=proto-fuzzer --test_output=all //executable_semantics/fuzzing:executable_semantics_fuzzer

To run in 'fuzzing' mode:

bazel build --config=proto-fuzzer //executable_semantics/fuzzing:executable_semantics_fuzzer

bazel-bin/executable_semantics/fuzzing/executable_semantics_fuzzer

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

bazel-bin/executable_semantics/fuzzing/executable_semantics_fuzzer /tmp/crash.textproto

Investigating a crash

To reproduce a crash, run the fuzzer on the crashing input as described above.

A separate tool called fuzzverter can be used for things like converting a crashing input to Carbon source code for running executable_semantics on the code directly.

To convert a Fuzzing::Carbon text proto to Carbon source:

bazel-bin/executable_semantics/fuzzing/fuzzverter --mode proto_to_carbon --input /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-bin/executable_semantics/fuzzing/fuzzverter --mode carbon_to_proto --input /tmp/crash.carbon --output /tmp/crash.textproto