archive/tar: add support for long binary strings in GNU format

The GNU tar format defines the following type flags:
	TypeGNULongName = 'L' // Next file has a long name
	TypeGNULongLink = 'K' // Next file symlinks to a file w/ a long name

Anytime a string exceeds the field dedicated to store it, the GNU format
permits a fake "file" to be prepended where that file entry has a Typeflag
of 'L' or 'K' and the contents of the file is a NUL-terminated string.

Contrary to previous TODO comments,
the GNU format supports arbitrary strings (without NUL) rather UTF-8 strings.
The manual says the following:
<<<
The name, linkname, magic, uname, and gname are
null-terminated character strings
> <<<
> All characters in header blocks are represented
> by using 8-bit characters in the local variant of ASCII.

From this description, we gather the following:
* We must forbid NULs in any GNU strings
* Any 8-bit value (other than NUL) is permitted

Since the modern world has moved to UTF-8, it is really difficult to
determine what a "local variant of ASCII" means. For this reason,
we treat strings as just an arbitrary binary string (without NUL)
and leave it to the user to determine the encoding of this string.
(Practically, it seems that UTF-8 is the typical encoding used
in GNU archives seen in the wild).

The implementation of GNU tar seems to confirm this interpretation
of the manual where it permits any arbitrary binary string to exist
within these fields so long as they do not contain the NUL character.

 $ touch `echo -e "not\x80\x81\x82\x83utf8"`
 $ gnutar -H gnu --tar -cvf gnu-not-utf8.tar $(echo -e "not\x80\x81\x82\x83utf8")

The fact that we permit arbitrary binary in GNU strings goes
hand-in-hand with the fact that GNU also permits a "base-256" encoding
of numeric fields, which is effectively two-complement binary.

Change-Id: Ic037ec6bed306d07d1312f0058594bd9b64d9880
Reviewed-on: https://go-review.googlesource.com/55573Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/archive/tar/common.go

@@ -85,14 +85,13 @@ func (h *Header) allowedFormats() (format int, paxHdrs map[string]string) {
 	format = formatUSTAR | formatPAX | formatGNU
 	paxHdrs = make(map[string]string)
 
-	verifyString := func(s string, size int, gnuLong bool, paxKey string) {
-
+	verifyString := func(s string, size int, paxKey string) {
 		// NUL-terminator is optional for path and linkpath.
 		// Technically, it is required for uname and gname,
 		// but neither GNU nor BSD tar checks for it.
 		tooLong := len(s) > size
-		if !isASCII(s) || (tooLong && !gnuLong) {
-			// TODO(dsnet): GNU supports UTF-8 (without NUL) for strings.
+		allowLongGNU := paxKey == paxPath || paxKey == paxLinkpath
+		if hasNUL(s) || (tooLong && !allowLongGNU) {
 			format &^= formatGNU // No GNU
 		}
 		if !isASCII(s) || tooLong {
@@ -120,15 +119,16 @@ func (h *Header) allowedFormats() (format int, paxHdrs map[string]string) {
 			}
 		}
 	}
-	verifyTime := func(ts time.Time, size int, ustarField bool, paxKey string) {
+	verifyTime := func(ts time.Time, size int, paxKey string) {
 		if ts.IsZero() {
 			return // Always okay
 		}
 		needsNano := ts.Nanosecond() != 0
+		hasFieldUSTAR := paxKey == paxMtime
 		if !fitsInBase256(size, ts.Unix()) || needsNano {
 			format &^= formatGNU // No GNU
 		}
-		if !fitsInOctal(size, ts.Unix()) || needsNano || !ustarField {
+		if !fitsInOctal(size, ts.Unix()) || needsNano || !hasFieldUSTAR {
 			format &^= formatUSTAR // No USTAR
 			if paxKey == paxNone {
 				format &^= formatPAX // No PAX
@@ -138,26 +138,23 @@ func (h *Header) allowedFormats() (format int, paxHdrs map[string]string) {
 		}
 	}
 
-	// TODO(dsnet): Add GNU long name support.
-	const supportGNULong = false
-
 	var blk block
 	v7 := blk.V7()
 	ustar := blk.USTAR()
 	gnu := blk.GNU()
-	verifyString(h.Name, len(v7.Name()), supportGNULong, paxPath)
-	verifyString(h.Linkname, len(v7.LinkName()), supportGNULong, paxLinkpath)
-	verifyString(h.Uname, len(ustar.UserName()), false, paxUname)
-	verifyString(h.Gname, len(ustar.GroupName()), false, paxGname)
+	verifyString(h.Name, len(v7.Name()), paxPath)
+	verifyString(h.Linkname, len(v7.LinkName()), paxLinkpath)
+	verifyString(h.Uname, len(ustar.UserName()), paxUname)
+	verifyString(h.Gname, len(ustar.GroupName()), paxGname)
 	verifyNumeric(h.Mode, len(v7.Mode()), paxNone)
 	verifyNumeric(int64(h.Uid), len(v7.UID()), paxUid)
 	verifyNumeric(int64(h.Gid), len(v7.GID()), paxGid)
 	verifyNumeric(h.Size, len(v7.Size()), paxSize)
 	verifyNumeric(h.Devmajor, len(ustar.DevMajor()), paxNone)
 	verifyNumeric(h.Devminor, len(ustar.DevMinor()), paxNone)
-	verifyTime(h.ModTime, len(v7.ModTime()), true, paxMtime)
-	verifyTime(h.AccessTime, len(gnu.AccessTime()), false, paxAtime)
-	verifyTime(h.ChangeTime, len(gnu.ChangeTime()), false, paxCtime)
+	verifyTime(h.ModTime, len(v7.ModTime()), paxMtime)
+	verifyTime(h.AccessTime, len(gnu.AccessTime()), paxAtime)
+	verifyTime(h.ChangeTime, len(gnu.ChangeTime()), paxCtime)
 
 	if !isHeaderOnlyType(h.Typeflag) && h.Size < 0 {
 		return formatUnknown, nil

src/archive/tar/reader_test.go

@@ -350,6 +350,41 @@ func TestReader(t *testing.T) {
 			Uname:    "rawr",
 			Gname:    "dsnet",
 		}},
+	}, {
+		// This archive was generated by Writer but is readable by both
+		// GNU and BSD tar utilities.
+		// The archive generated by GNU is nearly byte-for-byte identical
+		// to the Go version except the Go version sets a negative Devminor
+		// just to force the GNU format.
+		file: "testdata/gnu-utf8.tar",
+		headers: []*Header{{
+			Name: "☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹",
+			Mode: 0644,
+			Uid:  1000, Gid: 1000,
+			ModTime:  time.Unix(0, 0),
+			Typeflag: '0',
+			Uname:    "☺",
+			Gname:    "⚹",
+			Devminor: -1,
+		}},
+	}, {
+		// This archive was generated by Writer but is readable by both
+		// GNU and BSD tar utilities.
+		// The archive generated by GNU is nearly byte-for-byte identical
+		// to the Go version except the Go version sets a negative Devminor
+		// just to force the GNU format.
+		file: "testdata/gnu-not-utf8.tar",
+		headers: []*Header{{
+			Name:     "hi\x80\x81\x82\x83bye",
+			Mode:     0644,
+			Uid:      1000,
+			Gid:      1000,
+			ModTime:  time.Unix(0, 0),
+			Typeflag: '0',
+			Uname:    "rawr",
+			Gname:    "dsnet",
+			Devminor: -1,
+		}},
 	}, {
 		// BSD tar v3.1.2 and GNU tar v1.27.1 both rejects PAX records
 		// with NULs in the key.

src/archive/tar/strconv.go

@@ -12,6 +12,11 @@ import (
 	"time"
 )
 
+// hasNUL reports whether the NUL character exists within s.
+func hasNUL(s string) bool {
+	return strings.IndexByte(s, 0) >= 0
+}
+
 // isASCII reports whether the input is an ASCII C-style string.
 func isASCII(s string) bool {
 	for _, c := range s {
@@ -59,7 +64,6 @@ func (f *formatter) formatString(b []byte, s string) {
 	if len(s) > len(b) {
 		f.err = ErrFieldTooLong
 	}
-	s = toASCII(s) // TODO(dsnet): Remove this for UTF-8 support in GNU format
 	copy(b, s)
 	if len(s) < len(b) {
 		b[len(s)] = 0
@@ -307,8 +311,8 @@ func validPAXRecord(k, v string) bool {
 	}
 	switch k {
 	case paxPath, paxLinkpath, paxUname, paxGname:
-		return strings.IndexByte(v, 0) < 0
+		return !hasNUL(v)
 	default:
-		return strings.IndexByte(k, 0) < 0
+		return !hasNUL(k)
 	}
 }

src/archive/tar/tar_test.go

@@ -386,7 +386,7 @@ func TestHeaderAllowedFormats(t *testing.T) {
 		formats: formatUnknown,
 	}, {
 		header:  &Header{Name: "用戶名", Devmajor: -1 << 56},
-		formats: formatUnknown,
+		formats: formatGNU,
 	}, {
 		header:  &Header{Size: math.MaxInt64},
 		paxHdrs: map[string]string{paxSize: "9223372036854775807"},
@@ -408,10 +408,14 @@ func TestHeaderAllowedFormats(t *testing.T) {
 	}, {
 		header:  &Header{Name: strings.Repeat("a", nameSize)},
 		formats: formatUSTAR | formatPAX | formatGNU,
+	}, {
+		header:  &Header{Name: strings.Repeat("a", nameSize+1)},
+		paxHdrs: map[string]string{paxPath: strings.Repeat("a", nameSize+1)},
+		formats: formatPAX | formatGNU,
 	}, {
 		header:  &Header{Linkname: "用戶名"},
 		paxHdrs: map[string]string{paxLinkpath: "用戶名"},
-		formats: formatPAX,
+		formats: formatPAX | formatGNU,
 	}, {
 		header:  &Header{Linkname: strings.Repeat("用戶名\x00", nameSize)},
 		paxHdrs: map[string]string{paxLinkpath: strings.Repeat("用戶名\x00", nameSize)},

src/archive/tar/writer.go

@@ -121,12 +121,10 @@ func (tw *Writer) writePAXHeader(hdr *Header, paxHdrs map[string]string) error {
 	}
 
 	// Pack the main header.
-	var f formatter
-	blk := tw.templateV7Plus(hdr, f.formatString, f.formatOctal)
+	var f formatter // Ignore errors since they are expected
+	fmtStr := func(b []byte, s string) { f.formatString(b, toASCII(s)) }
+	blk := tw.templateV7Plus(hdr, fmtStr, f.formatOctal)
 	blk.SetFormat(formatPAX)
-	if f.err != nil && len(paxHdrs) == 0 {
-		return f.err // Should never happen, otherwise PAX headers would be used
-	}
 	return tw.writeRawHeader(blk, hdr.Size, hdr.Typeflag)
 }
 
@@ -134,10 +132,23 @@ func (tw *Writer) writeGNUHeader(hdr *Header) error {
 	// TODO(dsnet): Support writing sparse files.
 	// See https://golang.org/issue/13548
 
-	// TODO(dsnet): Support long filenames (with UTF-8) support.
+	// Use long-link files if Name or Linkname exceeds the field size.
+	const longName = "././@LongLink"
+	if len(hdr.Name) > nameSize {
+		data := hdr.Name + "\x00"
+		if err := tw.writeRawFile(longName, data, TypeGNULongName, formatGNU); err != nil {
+			return err
+		}
+	}
+	if len(hdr.Linkname) > nameSize {
+		data := hdr.Linkname + "\x00"
+		if err := tw.writeRawFile(longName, data, TypeGNULongLink, formatGNU); err != nil {
+			return err
+		}
+	}
 
 	// Pack the main header.
-	var f formatter
+	var f formatter // Ignore errors since they are expected
 	blk := tw.templateV7Plus(hdr, f.formatString, f.formatNumeric)
 	if !hdr.AccessTime.IsZero() {
 		f.formatNumeric(blk.GNU().AccessTime(), hdr.AccessTime.Unix())
@@ -146,9 +157,6 @@ func (tw *Writer) writeGNUHeader(hdr *Header) error {
 		f.formatNumeric(blk.GNU().ChangeTime(), hdr.ChangeTime.Unix())
 	}
 	blk.SetFormat(formatGNU)
-	if f.err != nil {
-		return f.err // Should never happen since header is validated
-	}
 	return tw.writeRawHeader(blk, hdr.Size, hdr.Typeflag)
 }
 

src/archive/tar/writer_test.go

@@ -219,6 +219,35 @@ func TestWriter(t *testing.T) {
 			},
 		}},
 		err: ErrHeader,
+	}, {
+		file: "testdata/gnu-utf8.tar",
+		entries: []*entry{{
+			header: &Header{
+				Name: "☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹☺☻☹",
+				Mode: 0644,
+				Uid:  1000, Gid: 1000,
+				ModTime:  time.Unix(0, 0),
+				Typeflag: '0',
+				Uname:    "☺",
+				Gname:    "⚹",
+				Devminor: -1, // Force use of GNU format
+			},
+		}},
+	}, {
+		file: "testdata/gnu-not-utf8.tar",
+		entries: []*entry{{
+			header: &Header{
+				Name:     "hi\x80\x81\x82\x83bye",
+				Mode:     0644,
+				Uid:      1000,
+				Gid:      1000,
+				ModTime:  time.Unix(0, 0),
+				Typeflag: '0',
+				Uname:    "rawr",
+				Gname:    "dsnet",
+				Devminor: -1, // Force use of GNU format
+			},
+		}},
 	}}
 
 	for _, v := range vectors {