lib/osutil: Replace IsDir with TraversesSymlink (fixes #3839)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3883
LGTM: calmh

lib/model/model.go

@@ -121,7 +121,6 @@ var (
 	errDevicePaused        = errors.New("device is paused")
 	errDeviceIgnored       = errors.New("device is ignored")
 	errNotRelative         = errors.New("not a relative path")
-	errNotDir              = errors.New("parent is not a directory")
 )
 
 // NewModel creates and starts a new model. The model starts in read-only mode,
@@ -1159,8 +1158,8 @@ func (m *Model) Request(deviceID protocol.DeviceID, folder, name string, offset
 		return protocol.ErrNoSuchFile
 	}
 
-	if !osutil.IsDir(folderPath, filepath.Dir(name)) {
+	if err := osutil.TraversesSymlink(folderPath, filepath.Dir(name)); err != nil {
-		l.Debugf("%v REQ(in) for file not in dir: %s: %q / %q o=%d s=%d", m, deviceID, folder, name, offset, len(buf))
+		l.Debugf("%v REQ(in) traversal check: %s - %s: %q / %q o=%d s=%d", m, err, deviceID, folder, name, offset, len(buf))
 		return protocol.ErrNoSuchFile
 	}
 

lib/model/rwfolder.go

@@ -431,8 +431,8 @@ func (f *sendReceiveFolder) pullerIteration(ignores *ignore.Matcher) int {
 	for _, fi := range processDirectly {
 		// Verify that the thing we are handling lives inside a directory,
 		// and not a symlink or empty space.
-		if !osutil.IsDir(f.dir, filepath.Dir(fi.Name)) {
+		if err := osutil.TraversesSymlink(f.dir, filepath.Dir(fi.Name)); err != nil {
-			f.newError(fi.Name, errNotDir)
+			f.newError(fi.Name, err)
 			continue
 		}
 
@@ -520,8 +520,8 @@ nextFile:
 
 		// Verify that the thing we are handling lives inside a directory,
 		// and not a symlink or empty space.
-		if !osutil.IsDir(f.dir, filepath.Dir(fi.Name)) {
+		if err := osutil.TraversesSymlink(f.dir, filepath.Dir(fi.Name)); err != nil {
-			f.newError(fi.Name, errNotDir)
+			f.newError(fi.Name, err)
 			continue
 		}
 

lib/osutil/isdir.go

@@ -1,49 +0,0 @@
-// Copyright (C) 2016 The Syncthing Authors.
-//
-// This Source Code Form is subject to the terms of the Mozilla Public
-// License, v. 2.0. If a copy of the MPL was not distributed with this file,
-// You can obtain one at http://mozilla.org/MPL/2.0/.
-
-package osutil
-
-import (
-	"os"
-	"path/filepath"
-	"strings"
-)
-
-// IsDir returns true if base and every path component of name up to and
-// including filepath.Join(base, name) is a directory (and not a symlink or
-// similar). Base and name must both be clean and name must be relative to
-// base.
-func IsDir(base, name string) bool {
-	path := base
-	info, err := Lstat(path)
-	if err != nil {
-		return false
-	}
-	if !info.IsDir() {
-		return false
-	}
-
-	if name == "." {
-		// The result of calling IsDir("some/where", filepath.Dir("foo"))
-		return true
-	}
-
-	parts := strings.Split(name, string(os.PathSeparator))
-	for _, part := range parts {
-		path = filepath.Join(path, part)
-		info, err := Lstat(path)
-		if err != nil {
-			return false
-		}
-		if info.Mode()&os.ModeSymlink != 0 {
-			return false
-		}
-		if !info.IsDir() {
-			return false
-		}
-	}
-	return true
-}

lib/osutil/traversessymlink.go

@@ -0,0 +1,76 @@
+// Copyright (C) 2016 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package osutil
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// TraversesSymlinkError is an error indicating symlink traversal
+type TraversesSymlinkError struct {
+	path string
+}
+
+func (e TraversesSymlinkError) Error() string {
+	return fmt.Sprintf("traverses symlink: %s", e.path)
+}
+
+// NotADirectoryError is an error indicating an expected path is not a directory
+type NotADirectoryError struct {
+	path string
+}
+
+func (e NotADirectoryError) Error() string {
+	return fmt.Sprintf("not a directory: %s", e.path)
+}
+
+// TraversesSymlink returns an error if base and any path component of name up to and
+// including filepath.Join(base, name) traverses a symlink.
+// Base and name must both be clean and name must be relative to base.
+func TraversesSymlink(base, name string) error {
+	path := base
+	info, err := Lstat(path)
+	if err != nil {
+		return err
+	}
+	if !info.IsDir() {
+		return &NotADirectoryError{
+			path: base,
+		}
+	}
+
+	if name == "." {
+		// The result of calling TraversesSymlink("some/where", filepath.Dir("foo"))
+		return nil
+	}
+
+	parts := strings.Split(name, string(os.PathSeparator))
+	for _, part := range parts {
+		path = filepath.Join(path, part)
+		info, err := Lstat(path)
+		if err != nil {
+			if os.IsNotExist(err) {
+				return nil
+			}
+			return err
+		}
+		if info.Mode()&os.ModeSymlink != 0 {
+			return &TraversesSymlinkError{
+				path: strings.TrimPrefix(path, base),
+			}
+		}
+		if !info.IsDir() {
+			return &NotADirectoryError{
+				path: strings.TrimPrefix(path, base),
+			}
+		}
+	}
+	return nil
+}

lib/osutil/traversessymlink_test.go

@@ -14,7 +14,7 @@ import (
 	"github.com/syncthing/syncthing/lib/symlinks"
 )
 
-func TestIsDir(t *testing.T) {
+func TestTraversesSymlink(t *testing.T) {
 	if !symlinks.Supported {
 		t.Skip("pointless test")
 		return
@@ -35,40 +35,42 @@ func TestIsDir(t *testing.T) {
 	}
 
 	cases := []struct {
-		name  string
+		name      string
-		isDir bool
+		traverses bool
 	}{
 		// Exist
-		{".", true},
+		{".", false},
-		{"a", true},
+		{"a", false},
-		{"a/b", true},
+		{"a/b", false},
-		{"a/b/c", true},
+		{"a/b/c", false},
 		// Don't exist
 		{"x", false},
 		{"a/x", false},
 		{"a/b/x", false},
 		{"a/x/c", false},
 		// Symlink or behind symlink
-		{"a/l", false},
+		{"a/l", true},
-		{"a/l/c", false},
+		{"a/l/c", true},
+		// Non-existing behind a symlink
+		{"a/l/x", true},
 	}
 
 	for _, tc := range cases {
-		if res := osutil.IsDir("testdata", tc.name); res != tc.isDir {
+		if res := osutil.TraversesSymlink("testdata", tc.name); tc.traverses == (res == nil) {
-			t.Errorf("IsDir(%q) = %v, should be %v", tc.name, res, tc.isDir)
+			t.Errorf("TraversesSymlink(%q) = %v, should be %v", tc.name, res, tc.traverses)
 		}
 	}
 }
 
-var isDirResult bool
+var traversesSymlinkResult error
 
-func BenchmarkIsDir(b *testing.B) {
+func BenchmarkTraversesSymlink(b *testing.B) {
 	os.RemoveAll("testdata")
 	defer os.RemoveAll("testdata")
 	os.MkdirAll("testdata/a/b/c", 0755)
 
 	for i := 0; i < b.N; i++ {
-		isDirResult = osutil.IsDir("testdata", "a/b/c")
+		traversesSymlinkResult = osutil.TraversesSymlink("testdata", "a/b/c")
 	}
 
 	b.ReportAllocs()