encoding/gob: fix two crashes on corrupted data.

Fixes #6323.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/56870043

src/pkg/encoding/gob/codec_test.go

@@ -1364,11 +1364,7 @@ type DT struct {
 	S     []string
 }
 
-func TestDebugStruct(t *testing.T) {
-	if debugFunc == nil {
-		return
-	}
-	Register(OnTheFly{})
+func newDT() DT {
 	var dt DT
 	dt.A = 17
 	dt.B = "hello"
@@ -1379,6 +1375,15 @@ func TestDebugStruct(t *testing.T) {
 	dt.M = map[string]int{"one": 1, "two": 2}
 	dt.T = [3]int{11, 22, 33}
 	dt.S = []string{"hi", "joe"}
+	return dt
+}
+
+func TestDebugStruct(t *testing.T) {
+	if debugFunc == nil {
+		return
+	}
+	Register(OnTheFly{})
+	dt := newDT()
 	b := new(bytes.Buffer)
 	err := NewEncoder(b).Encode(dt)
 	if err != nil {
@@ -1458,3 +1463,44 @@ func testFuzz(t *testing.T, seed int64, n int, input ...interface{}) {
 		}
 	}
 }
+
+// TestFuzzOneByte tries to decode corrupted input sequences
+// and checks that no panic occurs.
+func TestFuzzOneByte(t *testing.T) {
+	buf := new(bytes.Buffer)
+	Register(OnTheFly{})
+	dt := newDT()
+	if err := NewEncoder(buf).Encode(dt); err != nil {
+		t.Fatal(err)
+	}
+	s := buf.String()
+
+	indices := make([]int, 0, len(s))
+	for i := 0; i < len(s); i++ {
+		switch i {
+		case 14, 167, 231, 265: // a slice length, corruptions are not handled yet.
+			continue
+		}
+		indices = append(indices, i)
+	}
+	if testing.Short() {
+		indices = []int{1, 111, 178} // known fixed panics
+	}
+	for _, i := range indices {
+		for j := 0; j < 256; j += 3 {
+			b := []byte(s)
+			b[i] ^= byte(j)
+			var e DT
+			func() {
+				defer func() {
+					if p := recover(); p != nil {
+						t.Errorf("crash for b[%d] ^= 0x%x", i, j)
+						panic(p)
+					}
+				}()
+				err := NewDecoder(bytes.NewReader(b)).Decode(&e)
+				_ = err
+			}()
+		}
+	}
+}

src/pkg/encoding/gob/decode.go

@@ -701,6 +701,9 @@ func (dec *Decoder) decodeInterface(ityp reflect.Type, state *decoderState, p un
 	if nr < 0 || nr > 1<<31 { // zero is permissible for anonymous types
 		errorf("invalid type name length %d", nr)
 	}
+	if nr > uint64(state.b.Len()) {
+		errorf("invalid type name length %d: exceeds input size", nr)
+	}
 	b := make([]byte, nr)
 	state.b.Read(b)
 	name := string(b)
@@ -1237,7 +1240,8 @@ func (dec *Decoder) decodeValue(wireId typeId, val reflect.Value) {
 	}
 	engine := *enginePtr
 	if st := base; st.Kind() == reflect.Struct && ut.externalDec == 0 {
-		if engine.numInstr == 0 && st.NumField() > 0 && len(dec.wireType[wireId].StructT.Field) > 0 {
+		if engine.numInstr == 0 && st.NumField() > 0 &&
+			dec.wireType[wireId] != nil && len(dec.wireType[wireId].StructT.Field) > 0 {
 			name := base.Name()
 			errorf("type mismatch: no fields matched compiling decoder for %s", name)
 		}

src/pkg/encoding/gob/encoder_test.go

@@ -129,6 +129,8 @@ func TestBadData(t *testing.T) {
 	corruptDataCheck("", io.EOF, t)
 	corruptDataCheck("\x7Fhi", io.ErrUnexpectedEOF, t)
 	corruptDataCheck("\x03now is the time for all good men", errBadType, t)
+	// issue 6323.
+	corruptDataCheck("\x04\x24foo", errRange, t)
 }
 
 // Types not supported at top level by the Encoder.