[release-branch.go1.21] encoding/gob: prevent panic from index out of range in Decoder.typeString

I believe this bug is introduced by CL 460543 which optimizes the allocations
by changing the type of `idToType` from map to slice, but didn't update the
access code in `Decoder.typeString` that is safe for map but not for slice.

For #62117
Fixes #62154

Change-Id: I0f2e4cc2f34c54dada1f83458ba512a6fde6dcbe
Reviewed-on: https://go-review.googlesource.com/c/go/+/520757
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Andy Pan <panjf2000@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
(cherry picked from commit ba626ac327)
Reviewed-on: https://go-review.googlesource.com/c/go/+/521156
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/encoding/gob/codec_test.go

@@ -1607,3 +1607,15 @@ func TestLargeSlice(t *testing.T) {
 		testEncodeDecode(t, st, rt)
 	})
 }
+
+func TestLocalRemoteTypesMismatch(t *testing.T) {
+	// Test data is from https://go.dev/issue/62117.
+	testData := []byte{9, 127, 3, 1, 2, 255, 128, 0, 0, 0, 3, 255, 128, 0}
+
+	var v []*struct{}
+	buf := bytes.NewBuffer(testData)
+	err := NewDecoder(buf).Decode(&v)
+	if err == nil {
+		t.Error("Encode/Decode: expected error but got err == nil")
+	}
+}

src/encoding/gob/decode.go

@@ -1082,7 +1082,7 @@ func (dec *Decoder) compatibleType(fr reflect.Type, fw typeId, inProgress map[re
 func (dec *Decoder) typeString(remoteId typeId) string {
 	typeLock.Lock()
 	defer typeLock.Unlock()
-	if t := idToType[remoteId]; t != nil {
+	if t := idToType(remoteId); t != nil {
 		// globally known type.
 		return t.string()
 	}

src/encoding/gob/type.go

@@ -173,9 +173,18 @@ type gobType interface {
 	safeString(seen map[typeId]bool) string
 }
 
-var types = make(map[reflect.Type]gobType, 32)
-var idToType = make([]gobType, 1, firstUserId)
-var builtinIdToTypeSlice [firstUserId]gobType // set in init() after builtins are established
+var (
+	types                = make(map[reflect.Type]gobType, 32)
+	idToTypeSlice        = make([]gobType, 1, firstUserId)
+	builtinIdToTypeSlice [firstUserId]gobType // set in init() after builtins are established
+)
+
+func idToType(id typeId) gobType {
+	if id < 0 || int(id) >= len(idToTypeSlice) {
+		return nil
+	}
+	return idToTypeSlice[id]
+}
 
 func builtinIdToType(id typeId) gobType {
 	if id < 0 || int(id) >= len(builtinIdToTypeSlice) {
@@ -189,16 +198,16 @@ func setTypeId(typ gobType) {
 	if typ.id() != 0 {
 		return
 	}
-	nextId := typeId(len(idToType))
+	nextId := typeId(len(idToTypeSlice))
 	typ.setId(nextId)
-	idToType = append(idToType, typ)
+	idToTypeSlice = append(idToTypeSlice, typ)
 }
 
 func (t typeId) gobType() gobType {
 	if t == 0 {
 		return nil
 	}
-	return idToType[t]
+	return idToType(t)
 }
 
 // string returns the string representation of the type associated with the typeId.
@@ -277,14 +286,14 @@ func init() {
 	checkId(21, mustGetTypeInfo(reflect.TypeOf((*fieldType)(nil)).Elem()).id)
 	checkId(23, mustGetTypeInfo(reflect.TypeOf((*mapType)(nil)).Elem()).id)
 
-	copy(builtinIdToTypeSlice[:], idToType)
+	copy(builtinIdToTypeSlice[:], idToTypeSlice)
 
 	// Move the id space upwards to allow for growth in the predefined world
 	// without breaking existing files.
-	if nextId := len(idToType); nextId > firstUserId {
+	if nextId := len(idToTypeSlice); nextId > firstUserId {
 		panic(fmt.Sprintln("nextId too large:", nextId))
 	}
-	idToType = idToType[:firstUserId]
+	idToTypeSlice = idToTypeSlice[:firstUserId]
 	registerBasics()
 	wireTypeUserInfo = userType(wireTypeType)
 }
@@ -526,7 +535,7 @@ func newTypeObject(name string, ut *userTypeInfo, rt reflect.Type) (gobType, err
 	case reflect.Struct:
 		st := newStructType(name)
 		types[rt] = st
-		idToType[st.id()] = st
+		idToTypeSlice[st.id()] = st
 		for i := 0; i < t.NumField(); i++ {
 			f := t.Field(i)
 			if !isSent(&f) {