encoding/asn1: return error for unexported fields in Marshal, Unmarshal

The old code cannot handle unexported fields, it panics.
The new code returns error instead.

Fixes #17462

Change-Id: I927fc46b21d60e86cb52e84c65f2122f9159b21d
Reviewed-on: https://go-review.googlesource.com/31540
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/encoding/asn1/asn1.go

@@ -841,6 +841,13 @@ func parseField(v reflect.Value, bytes []byte, initOffset int, params fieldParam
 	case reflect.Struct:
 		structType := fieldType
 
+		for i := 0; i < structType.NumField(); i++ {
+			if structType.Field(i).PkgPath != "" {
+				err = StructuralError{"struct contains unexported fields"}
+				return
+			}
+		}
+
 		if structType.NumField() > 0 &&
 			structType.Field(0).Type == rawContentsType {
 			bytes := bytes[initOffset:offset]

src/encoding/asn1/asn1_test.go

@@ -967,7 +967,7 @@ func TestUnmarshalInvalidUTF8(t *testing.T) {
 func TestMarshalNilValue(t *testing.T) {
 	nilValueTestData := []interface{}{
 		nil,
-		struct{ v interface{} }{},
+		struct{ V interface{} }{},
 	}
 	for i, test := range nilValueTestData {
 		if _, err := Marshal(test); err == nil {
@@ -975,3 +975,32 @@ func TestMarshalNilValue(t *testing.T) {
 		}
 	}
 }
+
+type unexported struct {
+	X int
+	y int
+}
+
+type exported struct {
+	X int
+	Y int
+}
+
+func TestUnexportedStructField(t *testing.T) {
+	want := StructuralError{"struct contains unexported fields"}
+
+	_, err := Marshal(unexported{X: 5, y: 1})
+	if err != want {
+		t.Errorf("got %v, want %v", err, want)
+	}
+
+	bs, err := Marshal(exported{X: 5, Y: 1})
+	if err != nil {
+		t.Fatal(err)
+	}
+	var u unexported
+	_, err = Unmarshal(bs, &u)
+	if err != want {
+		t.Errorf("got %v, want %v", err, want)
+	}
+}

src/encoding/asn1/marshal.go

@@ -427,6 +427,12 @@ func makeBody(value reflect.Value, params fieldParameters) (e encoder, err error
 	case reflect.Struct:
 		t := v.Type()
 
+		for i := 0; i < t.NumField(); i++ {
+			if t.Field(i).PkgPath != "" {
+				return nil, StructuralError{"struct contains unexported fields"}
+			}
+		}
+
 		startingField := 0
 
 		n := t.NumField()