diff --git a/src/cmd/godoc/httpzip.go b/src/cmd/godoc/httpzip.go
index 9f3da08749f..12e99646df8 100644
--- a/src/cmd/godoc/httpzip.go
+++ b/src/cmd/godoc/httpzip.go
@@ -47,6 +47,7 @@ func (fi *fileInfo) Mode() os.FileMode  { return fi.mode }
 func (fi *fileInfo) Size() int64        { return fi.size }
 func (fi *fileInfo) ModTime() time.Time { return fi.mtime }
 func (fi *fileInfo) IsDir() bool        { return fi.mode.IsDir() }
+func (fi *fileInfo) Sys() interface{}   { return nil }
 
 // httpZipFile is the zip-file based implementation of http.File
 type httpZipFile struct {
diff --git a/src/cmd/godoc/zip.go b/src/cmd/godoc/zip.go
index cd38ed92bcb..8c4b1101b58 100644
--- a/src/cmd/godoc/zip.go
+++ b/src/cmd/godoc/zip.go
@@ -65,6 +65,10 @@ func (fi zipFI) IsDir() bool {
 	return fi.file == nil
 }
 
+func (fi zipFI) Sys() interface{} {
+	return nil
+}
+
 // zipFS is the zip-file based implementation of FileSystem
 type zipFS struct {
 	*zip.ReadCloser
diff --git a/src/pkg/archive/zip/struct.go b/src/pkg/archive/zip/struct.go
index 3da84357e1c..abbe635e281 100644
--- a/src/pkg/archive/zip/struct.go
+++ b/src/pkg/archive/zip/struct.go
@@ -71,6 +71,7 @@ func (fi headerFileInfo) Size() int64        { return int64(fi.fh.UncompressedSi
 func (fi headerFileInfo) IsDir() bool        { return fi.Mode().IsDir() }
 func (fi headerFileInfo) ModTime() time.Time { return fi.fh.ModTime() }
 func (fi headerFileInfo) Mode() os.FileMode  { return fi.fh.Mode() }
+func (fi headerFileInfo) Sys() interface{}   { return fi.fh }
 
 // FileInfoHeader creates a partially-populated FileHeader from an
 // os.FileInfo.
diff --git a/src/pkg/archive/zip/zip_test.go b/src/pkg/archive/zip/zip_test.go
index acd3d938218..1a260cc569f 100644
--- a/src/pkg/archive/zip/zip_test.go
+++ b/src/pkg/archive/zip/zip_test.go
@@ -85,4 +85,7 @@ func TestFileHeaderRoundTrip(t *testing.T) {
 	if !reflect.DeepEqual(fh, fh2) {
 		t.Errorf("mismatch\n input=%#v\noutput=%#v\nerr=%v", fh, fh2, err)
 	}
+	if sysfh, ok := fi.Sys().(*FileHeader); !ok && sysfh != fh {
+		t.Errorf("Sys didn't return original *FileHeader")
+	}
 }
diff --git a/src/pkg/net/http/fs_test.go b/src/pkg/net/http/fs_test.go
index 85cad3ec71b..feea9209e6a 100644
--- a/src/pkg/net/http/fs_test.go
+++ b/src/pkg/net/http/fs_test.go
@@ -190,7 +190,7 @@ func TestDirJoin(t *testing.T) {
 		if err != nil {
 			t.Fatalf("stat of %s: %v", name, err)
 		}
-		if !gfi.(*os.FileStat).SameFile(wfi.(*os.FileStat)) {
+		if !os.SameFile(gfi, wfi) {
 			t.Errorf("%s got different file", name)
 		}
 	}
diff --git a/src/pkg/os/file_unix.go b/src/pkg/os/file_unix.go
index ae5e908339e..e337d2b078f 100644
--- a/src/pkg/os/file_unix.go
+++ b/src/pkg/os/file_unix.go
@@ -152,7 +152,7 @@ func (f *File) readdir(n int) (fi []FileInfo, err error) {
 		if err == nil {
 			fi[i] = fip
 		} else {
-			fi[i] = &FileStat{name: filename}
+			fi[i] = &fileStat{name: filename}
 		}
 	}
 	return fi, err
diff --git a/src/pkg/os/getwd.go b/src/pkg/os/getwd.go
index a0d3c99a503..56836434dbe 100644
--- a/src/pkg/os/getwd.go
+++ b/src/pkg/os/getwd.go
@@ -30,7 +30,7 @@ func Getwd() (pwd string, err error) {
 	pwd = Getenv("PWD")
 	if len(pwd) > 0 && pwd[0] == '/' {
 		d, err := Stat(pwd)
-		if err == nil && dot.(*FileStat).SameFile(d.(*FileStat)) {
+		if err == nil && SameFile(dot, d) {
 			return pwd, nil
 		}
 	}
@@ -42,7 +42,7 @@ func Getwd() (pwd string, err error) {
 		// Can't stat root - no hope.
 		return "", err
 	}
-	if root.(*FileStat).SameFile(dot.(*FileStat)) {
+	if SameFile(root, dot) {
 		return "/", nil
 	}
 
@@ -67,7 +67,7 @@ func Getwd() (pwd string, err error) {
 			}
 			for _, name := range names {
 				d, _ := Lstat(parent + "/" + name)
-				if d.(*FileStat).SameFile(dot.(*FileStat)) {
+				if SameFile(d, dot) {
 					pwd = "/" + name + pwd
 					goto Found
 				}
@@ -82,7 +82,7 @@ func Getwd() (pwd string, err error) {
 			return "", err
 		}
 		fd.Close()
-		if pd.(*FileStat).SameFile(root.(*FileStat)) {
+		if SameFile(pd, root) {
 			break
 		}
 		// Set up for next round.
diff --git a/src/pkg/os/os_test.go b/src/pkg/os/os_test.go
index ec8c50986fe..25d9cbc73ac 100644
--- a/src/pkg/os/os_test.go
+++ b/src/pkg/os/os_test.go
@@ -408,7 +408,7 @@ func TestHardLink(t *testing.T) {
 	if err != nil {
 		t.Fatalf("stat %q failed: %v", from, err)
 	}
-	if !tostat.(*FileStat).SameFile(fromstat.(*FileStat)) {
+	if !SameFile(tostat, fromstat) {
 		t.Errorf("link %q, %q did not create hard link", to, from)
 	}
 }
@@ -444,7 +444,7 @@ func TestSymLink(t *testing.T) {
 	if err != nil {
 		t.Fatalf("stat %q failed: %v", from, err)
 	}
-	if !tostat.(*FileStat).SameFile(fromstat.(*FileStat)) {
+	if !SameFile(tostat, fromstat) {
 		t.Errorf("symlink %q, %q did not create symlink", to, from)
 	}
 	fromstat, err = Lstat(from)
@@ -658,7 +658,7 @@ func TestChtimes(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Stat %s: %s", f.Name(), err)
 	}
-	preStat := st.(*FileStat)
+	preStat := st
 
 	// Move access and modification time back a second
 	at := Atime(preStat)
@@ -672,7 +672,7 @@ func TestChtimes(t *testing.T) {
 	if err != nil {
 		t.Fatalf("second Stat %s: %s", f.Name(), err)
 	}
-	postStat := st.(*FileStat)
+	postStat := st
 
 	/* Plan 9:
 		Mtime is the time of the last change of content.  Similarly, atime is set whenever the
diff --git a/src/pkg/os/os_unix_test.go b/src/pkg/os/os_unix_test.go
index 1bdcd748bc0..f8e330beba4 100644
--- a/src/pkg/os/os_unix_test.go
+++ b/src/pkg/os/os_unix_test.go
@@ -18,7 +18,7 @@ func checkUidGid(t *testing.T, path string, uid, gid int) {
 	if err != nil {
 		t.Fatalf("Stat %q (looking for uid/gid %d/%d): %s", path, uid, gid, err)
 	}
-	sys := dir.(*FileStat).Sys.(*syscall.Stat_t)
+	sys := dir.Sys().(*syscall.Stat_t)
 	if int(sys.Uid) != uid {
 		t.Errorf("Stat %q: uid %d want %d", path, sys.Uid, uid)
 	}
@@ -52,7 +52,7 @@ func TestChown(t *testing.T) {
 	if err = Chown(f.Name(), -1, gid); err != nil {
 		t.Fatalf("chown %s -1 %d: %s", f.Name(), gid, err)
 	}
-	sys := dir.(*FileStat).Sys.(*syscall.Stat_t)
+	sys := dir.Sys().(*syscall.Stat_t)
 	checkUidGid(t, f.Name(), int(sys.Uid), gid)
 
 	// Then try all the auxiliary groups.
diff --git a/src/pkg/os/stat_darwin.go b/src/pkg/os/stat_darwin.go
index e1f93fac388..2e5967d5c81 100644
--- a/src/pkg/os/stat_darwin.go
+++ b/src/pkg/os/stat_darwin.go
@@ -9,18 +9,18 @@ import (
 	"time"
 )
 
-func sameFile(fs1, fs2 *FileStat) bool {
-	sys1 := fs1.Sys.(*syscall.Stat_t)
-	sys2 := fs2.Sys.(*syscall.Stat_t)
-	return sys1.Dev == sys2.Dev && sys1.Ino == sys2.Ino
+func sameFile(sys1, sys2 interface{}) bool {
+	stat1 := sys1.(*syscall.Stat_t)
+	stat2 := sys2.(*syscall.Stat_t)
+	return stat1.Dev == stat2.Dev && stat1.Ino == stat2.Ino
 }
 
 func fileInfoFromStat(st *syscall.Stat_t, name string) FileInfo {
-	fs := &FileStat{
+	fs := &fileStat{
 		name:    basename(name),
 		size:    int64(st.Size),
 		modTime: timespecToTime(st.Mtimespec),
-		Sys:     st,
+		sys:     st,
 	}
 	fs.mode = FileMode(st.Mode & 0777)
 	switch st.Mode & syscall.S_IFMT {
@@ -57,5 +57,5 @@ func timespecToTime(ts syscall.Timespec) time.Time {
 
 // For testing.
 func atime(fi FileInfo) time.Time {
-	return timespecToTime(fi.(*FileStat).Sys.(*syscall.Stat_t).Atimespec)
+	return timespecToTime(fi.Sys().(*syscall.Stat_t).Atimespec)
 }
diff --git a/src/pkg/os/stat_freebsd.go b/src/pkg/os/stat_freebsd.go
index 4c1c19729db..6ba84f438ad 100644
--- a/src/pkg/os/stat_freebsd.go
+++ b/src/pkg/os/stat_freebsd.go
@@ -9,18 +9,18 @@ import (
 	"time"
 )
 
-func sameFile(fs1, fs2 *FileStat) bool {
-	sys1 := fs1.Sys.(*syscall.Stat_t)
-	sys2 := fs2.Sys.(*syscall.Stat_t)
-	return sys1.Dev == sys2.Dev && sys1.Ino == sys2.Ino
+func sameFile(sys1, sys2 interface{}) bool {
+	stat1 := sys1.(*syscall.Stat_t)
+	stat2 := sys2.(*syscall.Stat_t)
+	return stat1.Dev == stat2.Dev && stat1.Ino == stat2.Ino
 }
 
 func fileInfoFromStat(st *syscall.Stat_t, name string) FileInfo {
-	fs := &FileStat{
+	fs := &fileStat{
 		name:    basename(name),
 		size:    int64(st.Size),
 		modTime: timespecToTime(st.Mtimespec),
-		Sys:     st,
+		sys:     st,
 	}
 	fs.mode = FileMode(st.Mode & 0777)
 	switch st.Mode & syscall.S_IFMT {
@@ -57,5 +57,5 @@ func timespecToTime(ts syscall.Timespec) time.Time {
 
 // For testing.
 func atime(fi FileInfo) time.Time {
-	return timespecToTime(fi.(*FileStat).Sys.(*syscall.Stat_t).Atimespec)
+	return timespecToTime(fi.Sys().(*syscall.Stat_t).Atimespec)
 }
diff --git a/src/pkg/os/stat_linux.go b/src/pkg/os/stat_linux.go
index 8d1323af9c6..00506b2b609 100644
--- a/src/pkg/os/stat_linux.go
+++ b/src/pkg/os/stat_linux.go
@@ -9,18 +9,18 @@ import (
 	"time"
 )
 
-func sameFile(fs1, fs2 *FileStat) bool {
-	sys1 := fs1.Sys.(*syscall.Stat_t)
-	sys2 := fs2.Sys.(*syscall.Stat_t)
-	return sys1.Dev == sys2.Dev && sys1.Ino == sys2.Ino
+func sameFile(sys1, sys2 interface{}) bool {
+	stat1 := sys1.(*syscall.Stat_t)
+	stat2 := sys2.(*syscall.Stat_t)
+	return stat1.Dev == stat2.Dev && stat1.Ino == stat2.Ino
 }
 
 func fileInfoFromStat(st *syscall.Stat_t, name string) FileInfo {
-	fs := &FileStat{
+	fs := &fileStat{
 		name:    basename(name),
 		size:    int64(st.Size),
 		modTime: timespecToTime(st.Mtim),
-		Sys:     st,
+		sys:     st,
 	}
 	fs.mode = FileMode(st.Mode & 0777)
 	switch st.Mode & syscall.S_IFMT {
@@ -57,5 +57,5 @@ func timespecToTime(ts syscall.Timespec) time.Time {
 
 // For testing.
 func atime(fi FileInfo) time.Time {
-	return timespecToTime(fi.(*FileStat).Sys.(*syscall.Stat_t).Atim)
+	return timespecToTime(fi.Sys().(*syscall.Stat_t).Atim)
 }
diff --git a/src/pkg/os/stat_netbsd.go b/src/pkg/os/stat_netbsd.go
index 8d1323af9c6..c58a2874c72 100644
--- a/src/pkg/os/stat_netbsd.go
+++ b/src/pkg/os/stat_netbsd.go
@@ -9,14 +9,14 @@ import (
 	"time"
 )
 
-func sameFile(fs1, fs2 *FileStat) bool {
-	sys1 := fs1.Sys.(*syscall.Stat_t)
-	sys2 := fs2.Sys.(*syscall.Stat_t)
-	return sys1.Dev == sys2.Dev && sys1.Ino == sys2.Ino
+func sameFile(sys1, sys2 interface{}) bool {
+	stat1 := sys1.(*syscall.Stat_t)
+	stat2 := sys2.(*syscall.Stat_t)
+	return stat1.Dev == stat2.Dev && stat1.Ino == stat2.Ino
 }
 
 func fileInfoFromStat(st *syscall.Stat_t, name string) FileInfo {
-	fs := &FileStat{
+	fs := &fileStat{
 		name:    basename(name),
 		size:    int64(st.Size),
 		modTime: timespecToTime(st.Mtim),
@@ -57,5 +57,5 @@ func timespecToTime(ts syscall.Timespec) time.Time {
 
 // For testing.
 func atime(fi FileInfo) time.Time {
-	return timespecToTime(fi.(*FileStat).Sys.(*syscall.Stat_t).Atim)
+	return timespecToTime(fi.Sys().(*syscall.Stat_t).Atim)
 }
diff --git a/src/pkg/os/stat_openbsd.go b/src/pkg/os/stat_openbsd.go
index 8d1323af9c6..00506b2b609 100644
--- a/src/pkg/os/stat_openbsd.go
+++ b/src/pkg/os/stat_openbsd.go
@@ -9,18 +9,18 @@ import (
 	"time"
 )
 
-func sameFile(fs1, fs2 *FileStat) bool {
-	sys1 := fs1.Sys.(*syscall.Stat_t)
-	sys2 := fs2.Sys.(*syscall.Stat_t)
-	return sys1.Dev == sys2.Dev && sys1.Ino == sys2.Ino
+func sameFile(sys1, sys2 interface{}) bool {
+	stat1 := sys1.(*syscall.Stat_t)
+	stat2 := sys2.(*syscall.Stat_t)
+	return stat1.Dev == stat2.Dev && stat1.Ino == stat2.Ino
 }
 
 func fileInfoFromStat(st *syscall.Stat_t, name string) FileInfo {
-	fs := &FileStat{
+	fs := &fileStat{
 		name:    basename(name),
 		size:    int64(st.Size),
 		modTime: timespecToTime(st.Mtim),
-		Sys:     st,
+		sys:     st,
 	}
 	fs.mode = FileMode(st.Mode & 0777)
 	switch st.Mode & syscall.S_IFMT {
@@ -57,5 +57,5 @@ func timespecToTime(ts syscall.Timespec) time.Time {
 
 // For testing.
 func atime(fi FileInfo) time.Time {
-	return timespecToTime(fi.(*FileStat).Sys.(*syscall.Stat_t).Atim)
+	return timespecToTime(fi.Sys().(*syscall.Stat_t).Atim)
 }
diff --git a/src/pkg/os/stat_plan9.go b/src/pkg/os/stat_plan9.go
index f731e43740a..7c2d1bd4efc 100644
--- a/src/pkg/os/stat_plan9.go
+++ b/src/pkg/os/stat_plan9.go
@@ -9,18 +9,18 @@ import (
 	"time"
 )
 
-func sameFile(fs1, fs2 *FileStat) bool {
-	a := fs1.Sys.(*Dir)
-	b := fs2.Sys.(*Dir)
+func sameFile(sys1, sys2 interface{}) bool {
+	a := sys1.(*Dir)
+	b := sys2.(*Dir)
 	return a.Qid.Path == b.Qid.Path && a.Type == b.Type && a.Dev == b.Dev
 }
 
 func fileInfoFromStat(d *Dir) FileInfo {
-	fs := &FileStat{
+	fs := &fileStat{
 		name:    d.Name,
 		size:    int64(d.Length),
 		modTime: time.Unix(int64(d.Mtime), 0),
-		Sys:     d,
+		sys:     d,
 	}
 	fs.mode = FileMode(d.Mode & 0777)
 	if d.Mode&syscall.DMDIR != 0 {
@@ -100,5 +100,5 @@ func Lstat(name string) (FileInfo, error) {
 
 // For testing.
 func atime(fi FileInfo) time.Time {
-	return time.Unix(int64(fi.(*FileStat).Sys.(*Dir).Atime), 0)
+	return time.Unix(int64(fi.Sys().(*Dir).Atime), 0)
 }
diff --git a/src/pkg/os/stat_windows.go b/src/pkg/os/stat_windows.go
index e599583b2e5..bbd95a17c94 100644
--- a/src/pkg/os/stat_windows.go
+++ b/src/pkg/os/stat_windows.go
@@ -82,8 +82,12 @@ type winTimes struct {
 }
 
 func toFileInfo(name string, fa, sizehi, sizelo uint32, ctime, atime, mtime syscall.Filetime) FileInfo {
-	fs := new(FileStat)
-	fs.mode = 0
+	fs := &fileStat{
+		name:    name,
+		size:    int64(sizehi)<<32 + int64(sizelo),
+		modTime: time.Unix(0, mtime.Nanoseconds()),
+		sys:     &winTimes{atime, ctime},
+	}
 	if fa&syscall.FILE_ATTRIBUTE_DIRECTORY != 0 {
 		fs.mode |= ModeDir
 	}
@@ -92,14 +96,10 @@ func toFileInfo(name string, fa, sizehi, sizelo uint32, ctime, atime, mtime sysc
 	} else {
 		fs.mode |= 0666
 	}
-	fs.size = int64(sizehi)<<32 + int64(sizelo)
-	fs.name = name
-	fs.modTime = time.Unix(0, mtime.Nanoseconds())
-	fs.Sys = &winTimes{atime, ctime}
 	return fs
 }
 
-func sameFile(fs1, fs2 *FileStat) bool {
+func sameFile(sys1, sys2 interface{}) bool {
 	// TODO(rsc): Do better than this, but this matches what
 	// used to happen when code compared .Dev and .Ino,
 	// which were both always zero.  Obviously not all files
@@ -109,5 +109,5 @@ func sameFile(fs1, fs2 *FileStat) bool {
 
 // For testing.
 func atime(fi FileInfo) time.Time {
-	return time.Unix(0, fi.(*FileStat).Sys.(*winTimes).atime.Nanoseconds())
+	return time.Unix(0, fi.Sys().(*winTimes).atime.Nanoseconds())
 }
diff --git a/src/pkg/os/types.go b/src/pkg/os/types.go
index a3f187c25cb..c7c5199be1c 100644
--- a/src/pkg/os/types.go
+++ b/src/pkg/os/types.go
@@ -19,6 +19,7 @@ type FileInfo interface {
 	Mode() FileMode     // file mode bits
 	ModTime() time.Time // modification time
 	IsDir() bool        // abbreviation for Mode().IsDir()
+	Sys() interface{}   // underlying data source (can return nil)
 }
 
 // A FileMode represents a file's mode and permission bits.
@@ -92,28 +93,33 @@ func (m FileMode) Perm() FileMode {
 	return m & ModePerm
 }
 
-// A FileStat is the implementation of FileInfo returned by Stat and Lstat.
-// Clients that need access to the underlying system-specific stat information
-// can test for *os.FileStat and then consult the Sys field.
-type FileStat struct {
+// A fileStat is the implementation of FileInfo returned by Stat and Lstat.
+type fileStat struct {
 	name    string
 	size    int64
 	mode    FileMode
 	modTime time.Time
-
-	Sys interface{}
+	sys     interface{}
 }
 
-func (fs *FileStat) Name() string       { return fs.name }
-func (fs *FileStat) Size() int64        { return fs.size }
-func (fs *FileStat) Mode() FileMode     { return fs.mode }
-func (fs *FileStat) ModTime() time.Time { return fs.modTime }
-func (fs *FileStat) IsDir() bool        { return fs.mode.IsDir() }
+func (fs *fileStat) Name() string       { return fs.name }
+func (fs *fileStat) Size() int64        { return fs.size }
+func (fs *fileStat) Mode() FileMode     { return fs.mode }
+func (fs *fileStat) ModTime() time.Time { return fs.modTime }
+func (fs *fileStat) IsDir() bool        { return fs.mode.IsDir() }
+func (fs *fileStat) Sys() interface{}   { return fs.sys }
 
-// SameFile reports whether fs and other describe the same file.
+// SameFile reports whether fi1 and fi2 describe the same file.
 // For example, on Unix this means that the device and inode fields
 // of the two underlying structures are identical; on other systems
 // the decision may be based on the path names.
-func (fs *FileStat) SameFile(other *FileStat) bool {
-	return sameFile(fs, other)
+// SameFile only applies to results returned by this package's Stat.
+// It returns false in other cases.
+func SameFile(fi1, fi2 FileInfo) bool {
+	fs1, ok1 := fi1.(*fileStat)
+	fs2, ok2 := fi2.(*fileStat)
+	if !ok1 || !ok2 {
+		return false
+	}
+	return sameFile(fs1.sys, fs2.sys)
 }
diff --git a/src/pkg/path/filepath/path_test.go b/src/pkg/path/filepath/path_test.go
index 5b0630fdc9b..a1b0c9d584d 100644
--- a/src/pkg/path/filepath/path_test.go
+++ b/src/pkg/path/filepath/path_test.go
@@ -667,7 +667,7 @@ func TestAbs(t *testing.T) {
 			continue
 		}
 		absinfo, err := os.Stat(abspath)
-		if err != nil || !absinfo.(*os.FileStat).SameFile(info.(*os.FileStat)) {
+		if err != nil || !os.SameFile(absinfo, info) {
 			t.Errorf("Abs(%q)=%q, not the same file", path, abspath)
 		}
 		if !filepath.IsAbs(abspath) {