encoding/xml: reject XML declaration after start of document

The XML specification requires an XML declaration, if present, to only
appear at the very beginning of the document, not even preceded by
whitespace. The parser currently accepts it at any part of the input.

Rejecting whitespace at the beginning of the file might break too many
users. This change instead only rejects an XML declaration preceded by
a non-whitespace token *and* allows the Encoder to emit whitespace
before an XML declaration. This means that a token stream produced by
the Decoder can be passed to the Encoder without error, while we still
don't emit clearly invalid XML.

This might break programs depending on Decoder allowing arbitrary XML
before the XML declaration.

Fixes #65691.

Change-Id: Ib1d4b3116aee63f40fd377f90595780b4befd1ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/564035
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

Author: Merovius

src/encoding/xml/marshal.go

@@ -208,7 +208,6 @@ var (
 // EncodeToken allows writing a [ProcInst] with Target set to "xml" only as the first token
 // in the stream.
 func (enc *Encoder) EncodeToken(t Token) error {
-
 	p := &enc.p
 	switch t := t.(type) {
 	case StartElement:
@@ -228,11 +227,10 @@ func (enc *Encoder) EncodeToken(t Token) error {
 		p.WriteString("<!--")
 		p.Write(t)
 		p.WriteString("-->")
-		return p.cachedWriteError()
 	case ProcInst:
 		// First token to be encoded which is also a ProcInst with target of xml
 		// is the xml declaration. The only ProcInst where target of xml is allowed.
-		if t.Target == "xml" && p.w.Buffered() != 0 {
+		if t.Target == "xml" && p.wroteNonWS {
 			return fmt.Errorf("xml: EncodeToken of ProcInst xml target only valid for xml declaration, first token encoded")
 		}
 		if !isNameString(t.Target) {
@@ -257,9 +255,30 @@ func (enc *Encoder) EncodeToken(t Token) error {
 		p.WriteString(">")
 	default:
 		return fmt.Errorf("xml: EncodeToken of invalid token type")
+	}
+	if err := p.cachedWriteError(); err != nil || enc.p.wroteNonWS {
+		return err
+	}
+	enc.p.wroteNonWS = !isWhitespace(t)
+	return nil
+}
 
+// isWhitespace reports whether t is a CharData token consisting entirely of
+// XML whitespace.
+func isWhitespace(t Token) bool {
+	switch t := t.(type) {
+	case CharData:
+		for _, b := range t {
+			switch b {
+			case ' ', '\r', '\n', '\t':
+			default:
+				return false
+			}
+		}
+		return true
+	default:
+		return false
 	}
-	return p.cachedWriteError()
 }
 
 // isValidDirective reports whether dir is a valid directive text,
@@ -329,6 +348,7 @@ type printer struct {
 	prefixes   []string
 	tags       []Name
 	closed     bool
+	wroteNonWS bool
 	err        error
 }
 

src/encoding/xml/marshal_test.go

@@ -2356,6 +2356,9 @@ func TestProcInstEncodeToken(t *testing.T) {
 	var buf bytes.Buffer
 	enc := NewEncoder(&buf)
 
+	if err := enc.EncodeToken(CharData(" \n\r\t")); err != nil {
+		t.Fatal("enc.EncodeToken: expected to be able to encode whitespace as first token")
+	}
 	if err := enc.EncodeToken(ProcInst{"xml", []byte("Instruction")}); err != nil {
 		t.Fatalf("enc.EncodeToken: expected to be able to encode xml target ProcInst as first token, %s", err)
 	}

src/encoding/xml/xml.go

@@ -212,6 +212,7 @@ type Decoder struct {
 	line           int
 	linestart      int64
 	offset         int64
+	readNonWS      bool
 	unmarshalDepth int
 }
 
@@ -559,6 +560,8 @@ func (d *Decoder) rawToken() (Token, error) {
 		return EndElement{d.toClose}, nil
 	}
 
+	readNonWS := d.readNonWS
+
 	b, ok := d.getc()
 	if !ok {
 		return nil, d.err
@@ -571,8 +574,12 @@ func (d *Decoder) rawToken() (Token, error) {
 		if data == nil {
 			return nil, d.err
 		}
+		if !d.readNonWS && !isWhitespace(CharData(data)) {
+			d.readNonWS = true
+		}
 		return CharData(data), nil
 	}
+	d.readNonWS = true
 
 	if b, ok = d.mustgetc(); !ok {
 		return nil, d.err
@@ -623,6 +630,11 @@ func (d *Decoder) rawToken() (Token, error) {
 		data = data[0 : len(data)-2] // chop ?>
 
 		if target == "xml" {
+			if readNonWS {
+				d.err = errors.New("xml: XML declaration after start of document")
+				return nil, d.err
+			}
+
 			content := string(data)
 			ver := procInst("version", content)
 			if ver != "" && ver != "1.0" {

src/encoding/xml/xml_test.go

@@ -1350,10 +1350,12 @@ func TestParseErrors(t *testing.T) {
 
 		// Header-related errors.
 		{`<?xml version="1.1" encoding="UTF-8"?>`, `unsupported version "1.1"; only version 1.0 is supported`},
+		{`<foo><?xml version="1.0"?>`, `XML declaration after start of document`},
 
 		// Cases below are for "no errors".
 		{withDefaultHeader(`<?ok?>`), ``},
 		{withDefaultHeader(`<?ok version="ok"?>`), ``},
+		{`  <?xml version="1.0"?>`, ``},
 	}
 
 	for _, test := range tests {