Go: Fuzzing Is Beta Ready

Katie Hockman and Jay Conrod
3 June 2021

We are excited to announce that native fuzzing is ready for beta testing in its development branch, dev.fuzz!

Fuzzing is a type of automated testing which continuously manipulates inputs to a program to find issues such as panics or bugs. These semi-random data mutations can discover new code coverage that existing unit tests may miss, and uncover edge case bugs which would otherwise go unnoticed. Since fuzzing can reach these edge cases, fuzz testing is particularly valuable for finding security exploits and vulnerabilities.

See golang.org/s/draft-fuzzing-design for more details about this feature.

Getting started

To get started, you may run the following

$ go get golang.org/dl/gotip
$ gotip download dev.fuzz

This builds the Go toolchain from the dev.fuzz development branch, and won’t be needed once the code is merged to the master branch in the future. After running this, gotip can act as a drop-in replacement for the go command. You can now run commands like

$ gotip test -fuzz=FuzzFoo

There will be ongoing development and bug fixes in the dev.fuzz branch, so you should regularly run gotip download dev.fuzz to use the latest code.

For compatibility with released versions of Go, use the gofuzzbeta build tag when committing source files containing fuzz targets to your repository. This tag is enabled by default at build-time in the dev.fuzz branch. See the go command documentation about build tags if you have questions about how to use them.

// +build gofuzzbeta

Writing a fuzz target

A fuzz target must be in a *_test.go file as a function in the form FuzzXxx. This function must be passed a *testing.F argument, much like a *testing.T argument is passed to a TestXxx function.

Below is an example of a fuzz target that’s testing the behavior of the net/url package.

// +build gofuzzbeta

package fuzz

import (
    "net/url"
    "reflect"
    "testing"
)

func FuzzParseQuery(f *testing.F) {
    f.Add("x=1&y=2")
    f.Fuzz(func(t *testing.T, queryStr string) {
        query, err := url.ParseQuery(queryStr)
        if err != nil {
            t.Skip()
        }
        queryStr2 := query.Encode()
        query2, err := url.ParseQuery(queryStr2)
        if err != nil {
            t.Fatalf("ParseQuery failed to decode a valid encoded query %s: %v", queryStr2, err)
        }
        if !reflect.DeepEqual(query, query2) {
            t.Errorf("ParseQuery gave different query after being encoded\nbefore: %v\nafter: %v", query, query2)
        }
    })
}

You can read more about the fuzzing APIs with go doc

gotip doc testing
gotip doc testing.F
gotip doc testing.F.Add
gotip doc testing.F.Fuzz

Expectations

This is a beta release in a development branch, so you should expect some bugs and an incomplete feature set. Check the issue tracker for issues labelled “fuzz” to stay up-to-date on existing bugs and missing features.

Please be aware that fuzzing can consume a lot of memory and may impact your machine’s performance while it runs. go test -fuzz defaults to running fuzzing in $GOMAXPROCS processes in parallel. You may lower the number of processes used while fuzzing by explicitly setting the -parallel flag with go test. Read the documentation for the go test command by running gotip help testflag if you want more information.

Also be aware that the fuzzing engine writes values that expand test coverage to a fuzz cache directory within $GOCACHE/fuzz while it runs. There is currently no limit to the number of files or total bytes that may be written to the fuzz cache, so it may occupy a large amount of storage (ie. several GBs). You can clear the fuzz cache by running gotip clean -fuzzcache.

What’s next?

This feature will not be available in the upcoming Go release (1.17), but there are plans to land this in a future Go release. We hope that this working prototype will allow Go developers to start writing fuzz targets and provide helpful feedback about the design in preparation for a merge to master.

If you experience any problems or have an idea for a feature request, please file an issue.

For discussion and general feedback about the feature, you can also participate in the #fuzzing channel in Gophers Slack.

Happy fuzzing!

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