proposal: testing: custom mutator support for fuzzing

[s3nt3]

As the official fuzzer implementation provided by golang, the native fuzzer should be well suited for various usage scenarios. However, currently native fuzzers only support general mutation algorithms for built-in types. Therefore, in many cases, the native fuzzer cannot efficiently generate test inputs. For example, when testing a DSL parser, the mutator will generate a large amount of output that cannot pass the syntax or semantic check. A possible solution is to provide support for custom mutators, so that users can implement custom mutators for various fuzz targets and reuse other parts of the native fuzzer.

I tried on the existing code and designed the following interface:

 type CustomMutator interface {
     Marshal() ([]byte, error)
     Unmarshal([]byte) error
     Mutate() error
 }

The object that implements the above interface can be passed as an argument to the testing.F.Fuzz method, and it will call the Mutate method to use the custom mutation algorithm. The Marshal and Unmarshal methods ensure that it can be imported/exported to a corpus file.

I think supporting custom mutator will bring the following benefits:

• the usage scenarios of native fuzzer are expanded
• it is more convenient for the community to test and optimize mutation algorithms
• the interface of custom mutator is very similar to the struct mutation interface mentioned in the draft proposal, so implementing a custom mutator may be a way to support struct mutation

In addition, custom mutator will also bring some side effects, such as the custom mutator code will be instrumented, which may affect performance and the accuracy of coverage statistics.

/cc @jayconrod

[jayconrod]

cc @golang/fuzzing

[s3nt3]

A friendly ping: any update?

[rolandshoemaker]

This is unlikely to be implemented for 1.18, but might be considered for 1.19.

[s3nt3]

Thanks for your reply, I will maintain an implementation in my local branch(https://github.com/s3nt3/go/tree/dev.fuzz.custom_mutator). Looking forward to supporting this feature in the new development cycle.

[icholy]

The interface should probably satisfy encoding.{TextMarshaler,TextUnmarshaler}.

 type CustomMutator interface {
     Mutate() error
     MarshalText() ([]byte, error)
     UnmarshalText([]byte) error
 }

[nevkontakte]

I believe Go 1.20 development cycle is currently open, and implementation in https://github.com/s3nt3/go/tree/dev.fuzz.custom_mutator looks like a very compact change. Any chance of this being included in 1.20? I could definitely use this capability in some of my projects :)

[rhansen]

Additional background: Structure-Aware Fuzzing with libFuzzer

[rhansen]

The interface should probably satisfy encoding.{TextMarshaler,TextUnmarshaler}.

Users might be fuzzing binary data such as images or protobufs. Wouldn't binary marshaler/unmarshaler be a better choice?

type CustomMutator interface {
	encoding.BinaryMarshaler
	encoding.BinaryUnmarshaler
	Mutate() error
}

Also, would it be useful to supply an int64 seed, rand.Source, or rand.Rand to help with reproducibility? For example:

type CustomMutator interface {
	encoding.BinaryMarshaler
	encoding.BinaryUnmarshaler
	Mutate(seed int64) error
}

[icholy]

@rhansen the corpus data is stored in a text format