Add support to fuzz from go-fuzz input (#35)

* Add NewFromGoFuzz

Add a helper function that enables using gofuzz (this project) with
go-fuzz (https://github.com/dvyukov/go-fuzz) for continuose
fuzzing. Essentially, it enables translating the fuzzing bytes from
go-fuzz to any Go object using this library.

This change will enable using this project with fuzzing websites such as fuzzit.dev
or fuzzbuzz.io.

The underlying implementation was an idea of lavalamp, by which a random source
is created that generates random numbers from the input bytes slice. In this way
changes in the source result in logical changes in the random data, which enables
the fuzzer to efficiently search the space of inputs.

Fixes #33

README.md

@@ -68,4 +68,22 @@ f.Fuzz(&myObject) // Type will correspond to whether A or B info is set.
 
 See more examples in ```example_test.go```.
 
+You can use this library for easier [go-fuzz](https://github.com/dvyukov/go-fuzz)ing.
+go-fuzz provides the user a byte-slice, which should be converted to different inputs
+for the tested function. This library can help convert the byte slice. Consider for
+example a fuzz test for a the function `mypackage.MyFunc` that takes an int arguments:
+```go
+// +build gofuzz
+package mypackage
+
+import "github.com/google/go-fuzz"
+
+func Fuzz(data []byte) int {
+        var i int
+        fuzz.NewFromGoFuzz(data).Fuzz(&i)
+        MyFunc(i)
+        return 0
+}
+```
+
 Happy testing!

bytesource/bytesource.go

@@ -0,0 +1,81 @@
+/*
+Copyright 2014 Google Inc. All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Package bytesource provides a rand.Source64 that is determined by a slice of bytes.
+package bytesource
+
+import (
+	"bytes"
+	"encoding/binary"
+	"io"
+	"math/rand"
+)
+
+// ByteSource implements rand.Source64 determined by a slice of bytes. The random numbers are
+// generated from each 8 bytes in the slice, until the last bytes are consumed, from which a
+// fallback pseudo random source is created in case more random numbers are required.
+// It also exposes a `bytes.Reader` API, which lets callers consume the bytes directly.
+type ByteSource struct {
+	*bytes.Reader
+	fallback rand.Source
+}
+
+// New returns a new ByteSource from a given slice of bytes.
+func New(input []byte) *ByteSource {
+	s := &ByteSource{
+		Reader:   bytes.NewReader(input),
+		fallback: rand.NewSource(0),
+	}
+	if len(input) > 0 {
+		s.fallback = rand.NewSource(int64(s.consumeUint64()))
+	}
+	return s
+}
+
+func (s *ByteSource) Uint64() uint64 {
+	// Return from input if it was not exhausted.
+	if s.Len() > 0 {
+		return s.consumeUint64()
+	}
+
+	// Input was exhausted, return random number from fallback (in this case fallback should not be
+	// nil). Try first having a Uint64 output (Should work in current rand implementation),
+	// otherwise return a conversion of Int63.
+	if s64, ok := s.fallback.(rand.Source64); ok {
+		return s64.Uint64()
+	}
+	return uint64(s.fallback.Int63())
+}
+
+func (s *ByteSource) Int63() int64 {
+	return int64(s.Uint64() >> 1)
+}
+
+func (s *ByteSource) Seed(seed int64) {
+	s.fallback = rand.NewSource(seed)
+	s.Reader = bytes.NewReader(nil)
+}
+
+// consumeUint64 reads 8 bytes from the input and convert them to a uint64. It assumes that the the
+// bytes reader is not empty.
+func (s *ByteSource) consumeUint64() uint64 {
+	var bytes [8]byte
+	_, err := s.Read(bytes[:])
+	if err != nil && err != io.EOF {
+		panic("failed reading source") // Should not happen.
+	}
+	return binary.BigEndian.Uint64(bytes[:])
+}

bytesource/bytesource_test.go

@@ -0,0 +1,109 @@
+/*
+Copyright 2014 Google Inc. All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+package bytesource
+
+import (
+	"math/rand"
+	"testing"
+)
+
+func TestByteSource(t *testing.T) {
+	t.Parallel()
+
+	randFromSource := func(s ...byte) *rand.Rand {
+		return rand.New(New(s))
+	}
+
+	randFromSeed := func(seed int64) *rand.Rand {
+		var s1 ByteSource
+		s1.Seed(seed)
+		return rand.New(&s1)
+	}
+
+	t.Run("Inputs with identical 8 byte prefix", func(t *testing.T) {
+		rand1 := randFromSource(1, 2, 3, 4, 5, 6, 7, 8, 9)
+		rand2 := randFromSource(1, 2, 3, 4, 5, 6, 7, 8, 9)
+		if rand1.Int() != rand2.Int() {
+			t.Errorf("Inputs with identical 9 byte prefix result in different 1st output.")
+		}
+		if rand1.Int() != rand2.Int() {
+			t.Errorf("Inputs with identical 9 byte prefix result in different 2nd output.")
+		}
+	})
+
+	t.Run("Inputs with different 8 byte prefix", func(t *testing.T) {
+		rand2 := randFromSource(1, 2, 3, 4, 5, 6, 7, 1, 9)
+		rand1 := randFromSource(1, 2, 3, 4, 5, 6, 7, 8, 9)
+		if rand1.Int() != rand2.Int() {
+			t.Errorf("Inputs with identical 9th byte prefix result in different 1st output.")
+		}
+		if rand1.Int() == rand2.Int() {
+			t.Errorf("Inputs with different 8 bytes prefix result in identical 2nd output.")
+		}
+	})
+
+	t.Run("Multiple invocation", func(t *testing.T) {
+		// First random from input byte, second from random source.
+		r := randFromSource(1, 2, 3, 4, 6, 7, 8, 9)
+		if r.Int() == r.Int() {
+			t.Errorf("Two random numbers are identical.")
+		}
+		// First and second numbers from random source.
+		r = randFromSource(1)
+		if r.Int() == r.Int() {
+			t.Errorf("Two random numbers are identical.")
+		}
+	})
+
+	t.Run("Seed", func(t *testing.T) {
+		if randFromSeed(42).Int() != randFromSeed(42).Int() {
+			t.Error("Two random numbers from the same seed differ.")
+		}
+		if randFromSeed(42).Int() == randFromSeed(43).Int() {
+			t.Error("Two random numbers from different seeds are identical.")
+		}
+	})
+}
+
+func TestByteSourceValues(t *testing.T) {
+	t.Parallel()
+
+	// Data in chunks of 8 bytes.
+	data := []byte{
+		99, 12, 23, 12, 65, 34, 12, 12,
+		99, 12, 23, 12, 25, 34, 15, 13,
+		99, 12, 23, 42, 25, 34, 11, 14,
+		99, 12, 54, 12, 25, 34, 99, 11,
+	}
+
+	r := rand.New(New(data))
+
+	got := []int{r.Int(), r.Int(), r.Int(), r.Int(), r.Int()}
+
+	want := []int{
+		3568552425102051206,
+		3568552489526560135,
+		3568569467532292485,
+		7616166771204380295,
+		5210010188159375967,
+	}
+
+	for i := range got {
+		if want[i] != got[i] {
+			t.Errorf("want[%d] = %d, got: %d", i, want[i], got[i])
+		}
+	}
+}

fuzz.go

@@ -22,6 +22,8 @@ import (
 	"reflect"
 	"regexp"
 	"time"
+
+	"github.com/google/gofuzz/bytesource"
 )
 
 // fuzzFuncMap is a map from a type to a fuzzFunc that handles that type.
@@ -61,6 +63,34 @@ func NewWithSeed(seed int64) *Fuzzer {
 	return f
 }
 
+// NewFromGoFuzz is a helper function that enables using gofuzz (this
+// project) with go-fuzz (https://github.com/dvyukov/go-fuzz) for continuous
+// fuzzing. Essentially, it enables translating the fuzzing bytes from
+// go-fuzz to any Go object using this library.
+//
+// This implementation promises a constant translation from a given slice of
+// bytes to the fuzzed objects. This promise will remain over future
+// versions of Go and of this library.
+//
+// Note: the returned Fuzzer should not be shared between multiple goroutines,
+// as its deterministic output will no longer be available.
+//
+// Example: use go-fuzz to test the function `MyFunc(int)` in the package
+// `mypackage`. Add the file: "mypacakge_fuzz.go" with the content:
+//
+// // +build gofuzz
+// package mypacakge
+// import "github.com/google/go-fuzz"
+// func Fuzz(data []byte) int {
+// 	var i int
+// 	fuzz.NewFromGoFuzz(data).Fuzz(&i)
+// 	MyFunc(i)
+// 	return 0
+// }
+func NewFromGoFuzz(data []byte) *Fuzzer {
+	return New().RandSource(bytesource.New(data))
+}
+
 // Funcs adds each entry in fuzzFuncs as a custom fuzzing function.
 //
 // Each entry in fuzzFuncs must be a function taking two parameters.

fuzz_test.go

@@ -553,3 +553,16 @@ func Test_charRange_choose(t *testing.T) {
 		}
 	})
 }
+
+func TestNewFromGoFuzz(t *testing.T) {
+	t.Parallel()
+
+	input := []byte{1, 2, 3}
+
+	var got int
+	NewFromGoFuzz(input).Fuzz(&got)
+
+	if want := 5563767293437588600; want != got {
+		t.Errorf("Fuzz(%q) = %d, want: %d", input, got, want)
+	}
+}