proposal: cmd/go: make fuzzing a first class citizen, like tests or benchmarks

[bradfitz]

Filing a proposal on behalf of @kcc and @dvyukov:

They request that cmd/go support fuzzing natively, just like it does tests and benchmarks and race detection today.

https://github.com/dvyukov/go-fuzz exists but it's not as easy as writing tests and benchmarks and running "go test -race" today.

Should we make this easier?

Motivation
Proposal

[ianlancetaylor]

I think it would be easier to evaluate the idea if it were slightly less abstract.

For example:

• _test.go are permitted to contain functions of the form FuzzXxx(f *testing.F, data []byte)
• these functions are expected to run some test based on the random bytes in data
• errors are reported using the testing.F argument in the usual way
• f.Useful() may be called to indicate useful data, i.e., data that parses correctly
• f.Discard() may be called to indicate that the data should be discarded
• go test -fuzz=. runs the fuzz functions using a regexp like -test and -bench
• naturally go test -fuzz must also rebuild the package in fuzz mode
• the data is cached somewhere under $GOROOT/pkg, but where?

[bradfitz]

@ianlancetaylor, yes, FuzzXxx(f *testing.F, ...) is what this is about. The exact API is probably TBD.

I think the first step before it's designed completely is to determine whether there's interest.

[ianlancetaylor]

As a general concept, I'm in favor.

[dsnet]

I would expect that there would be an additional required flag (when fuzzing) where you specify the corpus directory.

[ianlancetaylor]

Can we just cache the corpus somewhere under $GOROOT/pkg? Are there cases where a typical user would be expected to modify the corpus themselves?

[dsnet]

I think it's wrong to think of the corpus as strictly a cache. The corpus is the save state of the fuzzer and the documentation for go-fuzz even recommends committing them into a version control system. The pkg directory is treated strictly as cache and it is not uncommon for people to recommend clearing out the directory, which will unfortunately delete the fuzzer state.

A specified corpus is not so much for the user modify the corpus themselves, but for them to specify how to persist the corpus data.

[jimmyfrasche]

Could there be some default convention say a _fuzz/xxx directory (where xxx corresponds with FuzzXxx) and a method on the *testing.F object to load a different corpus from the _fuzz/ directory if necessary? It seems like it should just know where the corpus is.

[minux]

I'm in favor. Efficient fuzzing usually requires some help from compiler so it's better to built this into std. (compiler instrumentation will be more efficient than go-fuzz's source code instrumentation. I also want to have cmd/cover built on compiler instrumentation to support branch coverage, but that's off-topic to this issue.) How about add some methods to testing.T (or perhaps invent a new testing.Fuzz to replace testing.T, but see below) in fuzz tests? One of the method could be setting the corpus location (we can recommend it to be saved under testdata). but we probably should also a command line flag to override the test's setting (one compromise is to make both optional: Introduce -test.fuzzdir to hold the corpus path for all fuzz tests. If not provided, default to testdata/fuzz (*testing.T).FuzzDir("parser") // optional call to set corpus directory prefix location for this test, if relative, then relative to the -test.fuzzdir value. To make the feature more useful, I suggest we still use testing.T so that it's quite easy to migrate fuzz found test cases into a (table driven) regular test. Making fuzz tests taking a testing.T will facilitate this.

[cznic]

Quoting @dvyukov

I would appreciate if you drop a line there if you found fuzzing useful and a brief of your success story.

It was very useful for me - found bugs in several lexers.

[mvdan]

I use it regularly on a lexer/parser/formatter for Bash (https://github.com/mvdan/sh).

Having it be a first-class citizen would simplify things for me and for contributors.

[dsnet]

Found a bug in the C decoder for google/brotli by fuzzing a Go implementation of a Brotli decoder.

Also found some divergences in Go bzip2 decoders from the canonical C decoder (this and #18516). All by fuzzing.

[fatih]

My coworker at DigitalOcean was working on a side project to make fuzzing easier. Check his repo out here: https://github.com/tam7t/cautious-pancake Adding it here as I think it would be a valuable piece of information for this discussion.