compress/gzip: only encode MTIME if it is valid

The GZIP format records the ModTime as an uint32 counting seconds since
the Unix epoch. The zero value is explicitly defined in section 2.3.1
as meaning no timestamp is available.

Currently, the Writer always encodes the ModTime even if it is the zero
time.Time value, which causes the Writer to try and encode the value
-62135596800 into the uint32 MTIME field. This causes an overflow and
results in our GZIP files having MTIME fields indicating a date in 2042-07-13.

We alter the Writer to only encode ModTime if the value does not underflow
the MTIME field (i.e., it is newer than the Unix epoch). We do not attempt
to fix what happens when the timestamp overflows in the year 2106.

We alter the Reader to only decode ModTime if the value is non-zero.
There is no risk of overflowing time.Time when decoding.

Fixes #17663

Change-Id: Ie1b65770c6342cd7b14aeebe10e5a49e6c9eb730
Reviewed-on: https://go-review.googlesource.com/32325
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

4 files changed, 22 insertions, 11 deletions

diff --git a/src/compress/gzip/gunzip.go b/src/compress/gzip/gunzip.go
index bc303898b3..8bd750bd8b 100644
--- a/src/compress/gzip/gunzip.go
+++ b/src/compress/gzip/gunzip.go
@@ -186,7 +186,11 @@ func (z *Reader) readHeader() (hdr Header, err error) {
 		return hdr, ErrHeader
 	}
 	flg := z.buf[3]
-	hdr.ModTime = time.Unix(int64(le.Uint32(z.buf[4:8])), 0)
+	if t := int64(le.Uint32(z.buf[4:8])); t > 0 {
+		// Section 2.3.1, the zero value for MTIME means that the
+		// modified time is not set.
+		hdr.ModTime = time.Unix(t, 0)
+	}
 	// z.buf[8] is XFL and is currently ignored.
 	hdr.OS = z.buf[9]
 	z.digest = crc32.ChecksumIEEE(z.buf[:10])
diff --git a/src/compress/gzip/gzip.go b/src/compress/gzip/gzip.go
index c70321970b..aafb442a66 100644
--- a/src/compress/gzip/gzip.go
+++ b/src/compress/gzip/gzip.go
@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"hash/crc32"
 	"io"
+	"time"
 )
 
 // These constants are copied from the flate package, so that code that imports
@@ -143,10 +144,7 @@ func (z *Writer) Write(p []byte) (int, error) {
 	// Write the GZIP header lazily.
 	if !z.wroteHeader {
 		z.wroteHeader = true
-		z.buf[0] = gzipID1
-		z.buf[1] = gzipID2
-		z.buf[2] = gzipDeflate
-		z.buf[3] = 0
+		z.buf = [10]byte{0: gzipID1, 1: gzipID2, 2: gzipDeflate}
 		if z.Extra != nil {
 			z.buf[3] |= 0x04
 		}
@@ -156,13 +154,15 @@ func (z *Writer) Write(p []byte) (int, error) {
 		if z.Comment != "" {
 			z.buf[3] |= 0x10
 		}
-		le.PutUint32(z.buf[4:8], uint32(z.ModTime.Unix()))
+		if z.ModTime.After(time.Unix(0, 0)) {
+			// Section 2.3.1, the zero value for MTIME means that the
+			// modified time is not set.
+			le.PutUint32(z.buf[4:8], uint32(z.ModTime.Unix()))
+		}
 		if z.level == BestCompression {
 			z.buf[8] = 2
 		} else if z.level == BestSpeed {
 			z.buf[8] = 4
-		} else {
-			z.buf[8] = 0
 		}
 		z.buf[9] = z.OS
 		n, z.err = z.w.Write(z.buf[:10])
diff --git a/src/compress/gzip/gzip_test.go b/src/compress/gzip/gzip_test.go
index 09271b24e9..865c529f55 100644
--- a/src/compress/gzip/gzip_test.go
+++ b/src/compress/gzip/gzip_test.go
@@ -8,6 +8,7 @@ import (
 	"bufio"
 	"bytes"
 	"io/ioutil"
+	"reflect"
 	"testing"
 	"time"
 )
@@ -24,6 +25,9 @@ func TestEmpty(t *testing.T) {
 	if err != nil {
 		t.Fatalf("NewReader: %v", err)
 	}
+	if want := (Header{OS: 255}); !reflect.DeepEqual(r.Header, want) {
+		t.Errorf("Header mismatch:\ngot  %#v\nwant %#v", r.Header, want)
+	}
 	b, err := ioutil.ReadAll(r)
 	if err != nil {
 		t.Fatalf("ReadAll: %v", err)
diff --git a/src/compress/gzip/issue14937_test.go b/src/compress/gzip/issue14937_test.go
index 432ad16b1c..e76d47cc4e 100644
--- a/src/compress/gzip/issue14937_test.go
+++ b/src/compress/gzip/issue14937_test.go
@@ -7,7 +7,6 @@ import (
 	"runtime"
 	"strings"
 	"testing"
-	"time"
 )
 
 // Per golang.org/issue/14937, check that every .gz file
@@ -16,8 +15,12 @@ func TestGZIPFilesHaveZeroMTimes(t *testing.T) {
 	if testing.Short() && testenv.Builder() == "" {
 		t.Skip("skipping in short mode")
 	}
+	goroot, err := filepath.EvalSymlinks(runtime.GOROOT())
+	if err != nil {
+		t.Fatal("error evaluating GOROOT: ", err)
+	}
 	var files []string
-	err := filepath.Walk(runtime.GOROOT(), func(path string, info os.FileInfo, err error) error {
+	err = filepath.Walk(goroot, func(path string, info os.FileInfo, err error) error {
 		if err != nil {
 			return err
 		}
@@ -53,7 +56,7 @@ func checkZeroMTime(t *testing.T, path string) {
 		return
 	}
 	defer gz.Close()
-	if !gz.ModTime.Equal(time.Unix(0, 0)) {
+	if !gz.ModTime.IsZero() {
 		t.Errorf("gzip file %s has non-zero mtime (%s)", path, gz.ModTime)
 	}
 }