os: make Readlink work with symlinks with target like \??\Volume{ABCD}\

windows-arm TMP directory live inside such link (see
https://github.com/golang/go/issues/29746#issuecomment-456526811 for
details), so symlinks like that will be common at least on windows-arm.

This CL builds on current syscall.Readlink implementation. Main
difference between the two is how new code handles symlink targets,
like \??\Volume{ABCD}\.

New implementation uses Windows CreateFile API with
FILE_FLAG_OPEN_REPARSE_POINT flag to get \??\Volume{ABCD}\ file handle.
And then it uses Windows GetFinalPathNameByHandle with VOLUME_NAME_DOS
flag to convert that handle into standard Windows path.
FILE_FLAG_OPEN_REPARSE_POINT flag ensures that symlink is not followed
when CreateFile opens the file.

Fixes #30463

Change-Id: I33b18227ce36144caed694169ef2e429fd995fb4
Reviewed-on: https://go-review.googlesource.com/c/164201
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/internal/syscall/windows/reparse_windows.go

@@ -4,6 +4,11 @@
 
 package windows
 
+import (
+	"syscall"
+	"unsafe"
+)
+
 const (
 	FSCTL_SET_REPARSE_POINT    = 0x000900A4
 	IO_REPARSE_TAG_MOUNT_POINT = 0xA0000003
@@ -15,6 +20,13 @@ const (
 // in https://msdn.microsoft.com/en-us/library/cc232007.aspx
 // and https://msdn.microsoft.com/en-us/library/cc232006.aspx.
 
+type REPARSE_DATA_BUFFER struct {
+	ReparseTag        uint32
+	ReparseDataLength uint16
+	Reserved          uint16
+	DUMMYUNIONNAME    byte
+}
+
 // REPARSE_DATA_BUFFER_HEADER is a common part of REPARSE_DATA_BUFFER structure.
 type REPARSE_DATA_BUFFER_HEADER struct {
 	ReparseTag uint32
@@ -46,6 +58,12 @@ type SymbolicLinkReparseBuffer struct {
 	PathBuffer [1]uint16
 }
 
+// Path returns path stored in rb.
+func (rb *SymbolicLinkReparseBuffer) Path() string {
+	p := (*[0xffff]uint16)(unsafe.Pointer(&rb.PathBuffer[0]))
+	return syscall.UTF16ToString(p[rb.SubstituteNameOffset/2 : (rb.SubstituteNameOffset+rb.SubstituteNameLength)/2])
+}
+
 type MountPointReparseBuffer struct {
 	// The integer that contains the offset, in bytes,
 	// of the substitute name string in the PathBuffer array,
@@ -62,3 +80,9 @@ type MountPointReparseBuffer struct {
 	PrintNameLength uint16
 	PathBuffer      [1]uint16
 }
+
+// Path returns path stored in rb.
+func (rb *MountPointReparseBuffer) Path() string {
+	p := (*[0xffff]uint16)(unsafe.Pointer(&rb.PathBuffer[0]))
+	return syscall.UTF16ToString(p[rb.SubstituteNameOffset/2 : (rb.SubstituteNameOffset+rb.SubstituteNameLength)/2])
+}

src/os/file_posix.go

@@ -7,32 +7,12 @@
 package os
 
 import (
-	"runtime"
 	"syscall"
 	"time"
 )
 
 func sigpipe() // implemented in package runtime
 
-// Readlink returns the destination of the named symbolic link.
-// If there is an error, it will be of type *PathError.
-func Readlink(name string) (string, error) {
-	for len := 128; ; len *= 2 {
-		b := make([]byte, len)
-		n, e := fixCount(syscall.Readlink(fixLongPath(name), b))
-		// buffer too small
-		if runtime.GOOS == "aix" && e == syscall.ERANGE {
-			continue
-		}
-		if e != nil {
-			return "", &PathError{"readlink", name, e}
-		}
-		if n < len {
-			return string(b[0:n]), nil
-		}
-	}
-}
-
 // syscallMode returns the syscall-specific mode bits from Go's portable mode bits.
 func syscallMode(i FileMode) (o uint32) {
 	o |= uint32(i.Perm())

src/os/file_unix.go

@@ -399,3 +399,22 @@ func (f *File) readdir(n int) (fi []FileInfo, err error) {
 	}
 	return fi, err
 }
+
+// Readlink returns the destination of the named symbolic link.
+// If there is an error, it will be of type *PathError.
+func Readlink(name string) (string, error) {
+	for len := 128; ; len *= 2 {
+		b := make([]byte, len)
+		n, e := fixCount(syscall.Readlink(name, b))
+		// buffer too small
+		if runtime.GOOS == "aix" && e == syscall.ERANGE {
+			continue
+		}
+		if e != nil {
+			return "", &PathError{"readlink", name, e}
+		}
+		if n < len {
+			return string(b[0:n]), nil
+		}
+	}
+}

src/os/file_windows.go

@@ -5,6 +5,7 @@
 package os
 
 import (
+	"errors"
 	"internal/poll"
 	"internal/syscall/windows"
 	"runtime"
@@ -396,3 +397,122 @@ func Symlink(oldname, newname string) error {
 	}
 	return nil
 }
+
+// openSymlink calls CreateFile Windows API with FILE_FLAG_OPEN_REPARSE_POINT
+// parameter, so that Windows does not follow symlink, if path is a symlink.
+// openSymlink returns opened file handle.
+func openSymlink(path string) (syscall.Handle, error) {
+	p, err := syscall.UTF16PtrFromString(path)
+	if err != nil {
+		return 0, err
+	}
+	attrs := uint32(syscall.FILE_FLAG_BACKUP_SEMANTICS)
+	// Use FILE_FLAG_OPEN_REPARSE_POINT, otherwise CreateFile will follow symlink.
+	// See https://docs.microsoft.com/en-us/windows/desktop/FileIO/symbolic-link-effects-on-file-systems-functions#createfile-and-createfiletransacted
+	attrs |= syscall.FILE_FLAG_OPEN_REPARSE_POINT
+	h, err := syscall.CreateFile(p, 0, 0, nil, syscall.OPEN_EXISTING, attrs, 0)
+	if err != nil {
+		return 0, err
+	}
+	return h, nil
+}
+
+// normaliseLinkPath converts absolute paths returned by
+// DeviceIoControl(h, FSCTL_GET_REPARSE_POINT, ...)
+// into paths acceptable by all Windows APIs.
+// For example, it coverts
+//  \??\C:\foo\bar into C:\foo\bar
+//  \??\UNC\foo\bar into \\foo\bar
+//  \??\Volume{abc}\ into C:\
+func normaliseLinkPath(path string) (string, error) {
+	if len(path) < 4 || path[:4] != `\??\` {
+		// unexpected path, return it as is
+		return path, nil
+	}
+	// we have path that start with \??\
+	s := path[4:]
+	switch {
+	case len(s) >= 2 && s[1] == ':': // \??\C:\foo\bar
+		return s, nil
+	case len(s) >= 4 && s[:4] == `UNC\`: // \??\UNC\foo\bar
+		return `\\` + s[4:], nil
+	}
+
+	// handle paths, like \??\Volume{abc}\...
+
+	err := windows.LoadGetFinalPathNameByHandle()
+	if err != nil {
+		// we must be using old version of Windows
+		return "", err
+	}
+
+	h, err := openSymlink(path)
+	if err != nil {
+		return "", err
+	}
+	defer syscall.CloseHandle(h)
+
+	buf := make([]uint16, 100)
+	for {
+		n, err := windows.GetFinalPathNameByHandle(h, &buf[0], uint32(len(buf)), windows.VOLUME_NAME_DOS)
+		if err != nil {
+			return "", err
+		}
+		if n < uint32(len(buf)) {
+			break
+		}
+		buf = make([]uint16, n)
+	}
+	s = syscall.UTF16ToString(buf)
+	if len(s) > 4 && s[:4] == `\\?\` {
+		s = s[4:]
+		if len(s) > 3 && s[:3] == `UNC` {
+			// return path like \\server\share\...
+			return `\` + s[3:], nil
+		}
+		return s, nil
+	}
+	return "", errors.New("GetFinalPathNameByHandle returned unexpected path: " + s)
+}
+
+func readlink(path string) (string, error) {
+	h, err := openSymlink(path)
+	if err != nil {
+		return "", err
+	}
+	defer syscall.CloseHandle(h)
+
+	rdbbuf := make([]byte, syscall.MAXIMUM_REPARSE_DATA_BUFFER_SIZE)
+	var bytesReturned uint32
+	err = syscall.DeviceIoControl(h, syscall.FSCTL_GET_REPARSE_POINT, nil, 0, &rdbbuf[0], uint32(len(rdbbuf)), &bytesReturned, nil)
+	if err != nil {
+		return "", err
+	}
+
+	rdb := (*windows.REPARSE_DATA_BUFFER)(unsafe.Pointer(&rdbbuf[0]))
+	switch rdb.ReparseTag {
+	case syscall.IO_REPARSE_TAG_SYMLINK:
+		rb := (*windows.SymbolicLinkReparseBuffer)(unsafe.Pointer(&rdb.DUMMYUNIONNAME))
+		s := rb.Path()
+		if rb.Flags&windows.SYMLINK_FLAG_RELATIVE != 0 {
+			return s, nil
+		}
+		return normaliseLinkPath(s)
+	case windows.IO_REPARSE_TAG_MOUNT_POINT:
+		return normaliseLinkPath((*windows.MountPointReparseBuffer)(unsafe.Pointer(&rdb.DUMMYUNIONNAME)).Path())
+	default:
+		// the path is not a symlink or junction but another type of reparse
+		// point
+		return "", syscall.ENOENT
+	}
+}
+
+// Readlink returns the destination of the named symbolic link.
+// If there is an error, it will be of type *PathError.
+func Readlink(name string) (string, error) {
+	s, err := readlink(fixLongPath(name))
+	if err != nil {
+		return "", &PathError{"readlink", name, err}
+	}
+	return s, nil
+}

src/os/os_windows_test.go

@@ -1050,3 +1050,118 @@ func TestRootDirAsTemp(t *testing.T) {
 		t.Fatalf("unexpected child process output %q, want %q", have, want)
 	}
 }
+
+func testReadlink(t *testing.T, path, want string) {
+	got, err := os.Readlink(path)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	if got != want {
+		t.Errorf(`Readlink(%q): got %q, want %q`, path, got, want)
+	}
+}
+
+func mklink(t *testing.T, link, target string) {
+	output, err := osexec.Command("cmd", "/c", "mklink", link, target).CombinedOutput()
+	if err != nil {
+		t.Fatalf("failed to run mklink %v %v: %v %q", link, target, err, output)
+	}
+}
+
+func mklinkj(t *testing.T, link, target string) {
+	output, err := osexec.Command("cmd", "/c", "mklink", "/J", link, target).CombinedOutput()
+	if err != nil {
+		t.Fatalf("failed to run mklink %v %v: %v %q", link, target, err, output)
+	}
+}
+
+func mklinkd(t *testing.T, link, target string) {
+	output, err := osexec.Command("cmd", "/c", "mklink", "/D", link, target).CombinedOutput()
+	if err != nil {
+		t.Fatalf("failed to run mklink %v %v: %v %q", link, target, err, output)
+	}
+}
+
+func TestWindowsReadlink(t *testing.T) {
+	tmpdir, err := ioutil.TempDir("", "TestWindowsReadlink")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpdir)
+
+	// Make sure tmpdir is not a symlink, otherwise tests will fail.
+	tmpdir, err = filepath.EvalSymlinks(tmpdir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	wd, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = os.Chdir(tmpdir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.Chdir(wd)
+
+	vol := filepath.VolumeName(tmpdir)
+	output, err := osexec.Command("cmd", "/c", "mountvol", vol, "/L").CombinedOutput()
+	if err != nil {
+		t.Fatalf("failed to run mountvol %v /L: %v %q", vol, err, output)
+	}
+	ntvol := strings.Trim(string(output), " \n\r")
+
+	dir := filepath.Join(tmpdir, "dir")
+	err = os.MkdirAll(dir, 0777)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	absdirjlink := filepath.Join(tmpdir, "absdirjlink")
+	mklinkj(t, absdirjlink, dir)
+	testReadlink(t, absdirjlink, dir)
+
+	ntdirjlink := filepath.Join(tmpdir, "ntdirjlink")
+	mklinkj(t, ntdirjlink, ntvol+absdirjlink[len(filepath.VolumeName(absdirjlink)):])
+	testReadlink(t, ntdirjlink, absdirjlink)
+
+	ntdirjlinktolink := filepath.Join(tmpdir, "ntdirjlinktolink")
+	mklinkj(t, ntdirjlinktolink, ntvol+absdirjlink[len(filepath.VolumeName(absdirjlink)):])
+	testReadlink(t, ntdirjlinktolink, absdirjlink)
+
+	mklinkj(t, "reldirjlink", "dir")
+	testReadlink(t, "reldirjlink", dir) // relative directory junction resolves to absolute path
+
+	// Make sure we have sufficient privilege to run mklink command.
+	testenv.MustHaveSymlink(t)
+
+	absdirlink := filepath.Join(tmpdir, "absdirlink")
+	mklinkd(t, absdirlink, dir)
+	testReadlink(t, absdirlink, dir)
+
+	ntdirlink := filepath.Join(tmpdir, "ntdirlink")
+	mklinkd(t, ntdirlink, ntvol+absdirlink[len(filepath.VolumeName(absdirlink)):])
+	testReadlink(t, ntdirlink, absdirlink)
+
+	mklinkd(t, "reldirlink", "dir")
+	testReadlink(t, "reldirlink", "dir")
+
+	file := filepath.Join(tmpdir, "file")
+	err = ioutil.WriteFile(file, []byte(""), 0666)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	filelink := filepath.Join(tmpdir, "filelink")
+	mklink(t, filelink, file)
+	testReadlink(t, filelink, file)
+
+	linktofilelink := filepath.Join(tmpdir, "linktofilelink")
+	mklink(t, linktofilelink, ntvol+filelink[len(filepath.VolumeName(filelink)):])
+	testReadlink(t, linktofilelink, filelink)
+
+	mklink(t, "relfilelink", "file")
+	testReadlink(t, "relfilelink", "file")
+}