internal/fuzz: limit number of consecutive mutations

This makes two changes: (1) mutator.mutate now only applies a single
mutation to the input, and (2) in workerServer.fuzz if, after five
mutations are applied to the input, no new coverage is found the input
is reset to its initial state. This process is repeated until new
coverage is found, or the fuzz call times out.

This results in finding new coverage expanding inputs which have less
divergence from the initial input they were mutated from, which makes
traversing certain types of call graphs significantly more efficient.

Fixes #49601
Fixes #48179
Fixes #47090

Change-Id: I74d18a56ca2669f20192951090b281f58ee0b5dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/364214
Trust: Roland Shoemaker <roland@golang.org>
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>

2 files changed, 35 insertions, 27 deletions

diff --git a/src/internal/fuzz/mutator.go b/src/internal/fuzz/mutator.go
index da7200dcbe..a3161c04ea 100644
--- a/src/internal/fuzz/mutator.go
+++ b/src/internal/fuzz/mutator.go
@@ -125,15 +125,13 @@ func (m *mutator) mutate(vals []interface{}, maxBytes int) {
 }
 
 func (m *mutator) mutateInt(v, maxValue int64) int64 {
-	numIters := 1 + m.r.exp2()
 	var max int64
-	for iter := 0; iter < numIters; iter++ {
+	for {
 		max = 100
 		switch m.rand(2) {
 		case 0:
 			// Add a random number
 			if v >= maxValue {
-				iter--
 				continue
 			}
 			if v > 0 && maxValue-v < max {
@@ -141,10 +139,10 @@ func (m *mutator) mutateInt(v, maxValue int64) int64 {
 				max = maxValue - v
 			}
 			v += int64(1 + m.rand(int(max)))
+			return v
 		case 1:
 			// Subtract a random number
 			if v <= -maxValue {
-				iter--
 				continue
 			}
 			if v < 0 && maxValue+v < max {
@@ -152,21 +150,19 @@ func (m *mutator) mutateInt(v, maxValue int64) int64 {
 				max = maxValue + v
 			}
 			v -= int64(1 + m.rand(int(max)))
+			return v
 		}
 	}
-	return v
 }
 
 func (m *mutator) mutateUInt(v, maxValue uint64) uint64 {
-	numIters := 1 + m.r.exp2()
 	var max uint64
-	for iter := 0; iter < numIters; iter++ {
+	for {
 		max = 100
 		switch m.rand(2) {
 		case 0:
 			// Add a random number
 			if v >= maxValue {
-				iter--
 				continue
 			}
 			if v > 0 && maxValue-v < max {
@@ -175,10 +171,10 @@ func (m *mutator) mutateUInt(v, maxValue uint64) uint64 {
 			}
 
 			v += uint64(1 + m.rand(int(max)))
+			return v
 		case 1:
 			// Subtract a random number
 			if v <= 0 {
-				iter--
 				continue
 			}
 			if v < max {
@@ -186,20 +182,18 @@ func (m *mutator) mutateUInt(v, maxValue uint64) uint64 {
 				max = v
 			}
 			v -= uint64(1 + m.rand(int(max)))
+			return v
 		}
 	}
-	return v
 }
 
 func (m *mutator) mutateFloat(v, maxValue float64) float64 {
-	numIters := 1 + m.r.exp2()
 	var max float64
-	for iter := 0; iter < numIters; iter++ {
+	for {
 		switch m.rand(4) {
 		case 0:
 			// Add a random number
 			if v >= maxValue {
-				iter--
 				continue
 			}
 			max = 100
@@ -208,10 +202,10 @@ func (m *mutator) mutateFloat(v, maxValue float64) float64 {
 				max = maxValue - v
 			}
 			v += float64(1 + m.rand(int(max)))
+			return v
 		case 1:
 			// Subtract a random number
 			if v <= -maxValue {
-				iter--
 				continue
 			}
 			max = 100
@@ -220,11 +214,11 @@ func (m *mutator) mutateFloat(v, maxValue float64) float64 {
 				max = maxValue + v
 			}
 			v -= float64(1 + m.rand(int(max)))
+			return v
 		case 2:
 			// Multiply by a random number
 			absV := math.Abs(v)
 			if v == 0 || absV >= maxValue {
-				iter--
 				continue
 			}
 			max = 10
@@ -233,16 +227,16 @@ func (m *mutator) mutateFloat(v, maxValue float64) float64 {
 				max = maxValue / absV
 			}
 			v *= float64(1 + m.rand(int(max)))
+			return v
 		case 3:
 			// Divide by a random number
 			if v == 0 {
-				iter--
 				continue
 			}
 			v /= float64(1 + m.rand(10))
+			return v
 		}
 	}
-	return v
 }
 
 type byteSliceMutator func(*mutator, []byte) []byte
@@ -279,15 +273,12 @@ func (m *mutator) mutateBytes(ptrB *[]byte) {
 		*ptrB = b
 	}()
 
-	numIters := 1 + m.r.exp2()
-	for iter := 0; iter < numIters; iter++ {
+	for {
 		mut := byteSliceMutators[m.rand(len(byteSliceMutators))]
-		mutated := mut(m, b)
-		if mutated == nil {
-			iter--
-			continue
+		if mutated := mut(m, b); mutated != nil {
+			b = mutated
+			return
 		}
-		b = mutated
 	}
 }
 
diff --git a/src/internal/fuzz/worker.go b/src/internal/fuzz/worker.go
index 48a3923112..e7d824bea1 100644
--- a/src/internal/fuzz/worker.go
+++ b/src/internal/fuzz/worker.go
@@ -661,6 +661,17 @@ func (ws *workerServer) serve(ctx context.Context) error {
 	}
 }
 
+// chainedMutations is how many mutations are applied before the worker
+// resets the input to it's original state.
+// NOTE: this number was picked without much thought. It is low enough that
+// it seems to create a significant diversity in mutated inputs. We may want
+// to consider looking into this more closely once we have a proper performance
+// testing framework. Another option is to randomly pick the number of chained
+// mutations on each invocation of the workerServer.fuzz method (this appears to
+// be what libFuzzer does, although there seems to be no documentation which
+// explains why this choice was made.)
+const chainedMutations = 5
+
 // fuzz runs the test function on random variations of the input value in shared
 // memory for a limited duration or number of iterations.
 //
@@ -699,11 +710,13 @@ func (ws *workerServer) fuzz(ctx context.Context, args fuzzArgs) (resp fuzzRespo
 		return resp
 	}
 
-	vals, err := unmarshalCorpusFile(mem.valueCopy())
+	originalVals, err := unmarshalCorpusFile(mem.valueCopy())
 	if err != nil {
 		resp.InternalErr = err.Error()
 		return resp
 	}
+	vals := make([]interface{}, len(originalVals))
+	copy(vals, originalVals)
 
 	shouldStop := func() bool {
 		return args.Limit > 0 && mem.header().count >= args.Limit
@@ -742,9 +755,13 @@ func (ws *workerServer) fuzz(ctx context.Context, args fuzzArgs) (resp fuzzRespo
 		select {
 		case <-ctx.Done():
 			return resp
-
 		default:
+			if mem.header().count%chainedMutations == 0 {
+				copy(vals, originalVals)
+				ws.m.r.save(&mem.header().randState, &mem.header().randInc)
+			}
 			ws.m.mutate(vals, cap(mem.valueRef()))
+
 			entry := CorpusEntry{Values: vals}
 			dur, cov, errMsg := fuzzOnce(entry)
 			if errMsg != "" {
@@ -1094,7 +1111,7 @@ func (wc *workerClient) fuzz(ctx context.Context, entryIn CorpusEntry, args fuzz
 		wc.m.r.restore(mem.header().randState, mem.header().randInc)
 		if !args.Warmup {
 			// Only mutate the valuesOut if fuzzing actually occurred.
-			for i := int64(0); i < resp.Count; i++ {
+			for i := int64(0); i < resp.Count%chainedMutations; i++ {
 				wc.m.mutate(valuesOut, cap(mem.valueRef()))
 			}
 		}