use fuzzing helper functions to generate transport parameter seed corpus

fuzzing/internal/helper/helper.go

@@ -8,6 +8,14 @@ import (
 	"path/filepath"
 )
 
+// NthBit gets the n-th bit of a byte (counting starts at 0).
+func NthBit(val uint8, n int) bool {
+	if n < 0 || n > 7 {
+		panic("invalid value for n")
+	}
+	return val>>n&0x1 == 1
+}
+
 // WriteCorpusFile writes data to a corpus file in directory path.
 // The filename is calculated from the SHA1 sum of the file contents.
 func WriteCorpusFile(path string, data []byte) error {

fuzzing/internal/helper/helper_test.go

@@ -57,4 +57,16 @@ var _ = Describe("exporting", func() {
 		Expect(WriteCorpusFile(subdir, []byte("lorem ipsum"))).To(Succeed())
 		Expect(subdir).To(BeADirectory())
 	})
+
+	It("gets the nth bit of a byte", func() {
+		const val = 0b10010001
+		Expect(NthBit(val, 0)).To(BeTrue())
+		Expect(NthBit(val, 1)).To(BeFalse())
+		Expect(NthBit(val, 2)).To(BeFalse())
+		Expect(NthBit(val, 3)).To(BeFalse())
+		Expect(NthBit(val, 4)).To(BeTrue())
+		Expect(NthBit(val, 5)).To(BeFalse())
+		Expect(NthBit(val, 6)).To(BeFalse())
+		Expect(NthBit(val, 7)).To(BeTrue())
+	})
 })

fuzzing/transportparameters/cmd/corpus.go

@@ -1,14 +1,15 @@
 package main
 
 import (
-	"fmt"
+	"bytes"
 	"log"
 	"math"
 	"math/rand"
 	"net"
-	"os"
 	"time"
 
+	"github.com/lucas-clemente/quic-go/fuzzing/internal/helper"
+	"github.com/lucas-clemente/quic-go/fuzzing/transportparameters"
 	"github.com/lucas-clemente/quic-go/internal/protocol"
 
 	"github.com/lucas-clemente/quic-go/internal/wire"
@@ -26,8 +27,7 @@ func getRandomValue() uint64 {
 }
 
 func main() {
-	rand.Seed(1337)
-	for i := 0; i < 20; i++ {
+	for i := 0; i < 30; i++ {
 		tp := &wire.TransportParameters{
 			InitialMaxStreamDataBidiLocal:  protocol.ByteCount(getRandomValue()),
 			InitialMaxStreamDataBidiRemote: protocol.ByteCount(getRandomValue()),
@@ -69,24 +69,21 @@ func main() {
 				StatelessResetToken: token,
 			}
 		}
+
+		var data []byte
+		if rand.Int()%2 == 0 {
 			pers := protocol.PerspectiveServer
 			if rand.Int()%2 == 0 {
 				pers = protocol.PerspectiveClient
 			}
-		if err := writeCorpusFile(fmt.Sprintf("tp%d", i), tp.Marshal(pers)); err != nil {
+			data = tp.Marshal(pers)
+		} else {
+			b := &bytes.Buffer{}
+			tp.MarshalForSessionTicket(b)
+			data = b.Bytes()
+		}
+		if err := helper.WriteCorpusFileWithPrefix("corpus", data, transportparameters.PrefixLen); err != nil {
 			log.Fatal(err)
 		}
 	}
 }
-
-func writeCorpusFile(name string, data []byte) error {
-	file, err := os.Create("corpus/" + name)
-	if err != nil {
-		return err
-	}
-	data = append(getRandomData(2), data...)
-	if _, err := file.Write(data); err != nil {
-		return err
-	}
-	return file.Close()
-}

fuzzing/transportparameters/fuzz.go

@@ -4,28 +4,32 @@ import (
 	"bytes"
 	"fmt"
 
+	"github.com/lucas-clemente/quic-go/fuzzing/internal/helper"
 	"github.com/lucas-clemente/quic-go/internal/protocol"
 	"github.com/lucas-clemente/quic-go/internal/wire"
 )
 
+// PrefixLen is the number of bytes used for configuration
+const PrefixLen = 1
+
+// Fuzz fuzzes the QUIC transport parameters.
 //go:generate go run ./cmd/corpus.go
 func Fuzz(data []byte) int {
-	if len(data) <= 1 {
+	if len(data) <= PrefixLen {
 		return 0
 	}
 
-	if data[0]%2 == 0 {
-		return fuzzTransportParametersForSessionTicket(data[1:])
+	if helper.NthBit(data[0], 0) {
+		return fuzzTransportParametersForSessionTicket(data[PrefixLen:])
 	}
-	return fuzzTransportParameters(data[1:])
+	return fuzzTransportParameters(data[PrefixLen:], helper.NthBit(data[0], 1))
 }
 
-func fuzzTransportParameters(data []byte) int {
-	perspective := protocol.PerspectiveServer
-	if data[0]%2 == 1 {
+func fuzzTransportParameters(data []byte, isServer bool) int {
+	perspective := protocol.PerspectiveClient
+	if isServer {
 		perspective = protocol.PerspectiveServer
 	}
-	data = data[1:]
 
 	tp := &wire.TransportParameters{}
 	if err := tp.Unmarshal(data, perspective); err != nil {