config: prevent use of windows reserved names in config file name

fs/config/config.go

@@ -22,6 +22,7 @@ import (
 	"github.com/rclone/rclone/fs/config/obscure"
 	"github.com/rclone/rclone/fs/fspath"
 	"github.com/rclone/rclone/fs/rc"
+	"github.com/rclone/rclone/lib/file"
 	"github.com/rclone/rclone/lib/random"
 )
 
@@ -226,16 +227,20 @@ func GetConfigPath() string {
 //
 // Checks for empty string, os null device, or special path, all of which indicates in-memory config.
 func SetConfigPath(path string) (err error) {
+	var cfgPath string
 	if path == "" || path == os.DevNull {
-		configPath = ""
+		cfgPath = ""
+	} else if err = file.IsReserved(path); err != nil {
+		return err
 	} else {
-		if configPath, err = filepath.Abs(path); err != nil {
+		if cfgPath, err = filepath.Abs(path); err != nil {
 			return err
 		}
-		if configPath == noConfigPath {
-			configPath = ""
+		if cfgPath == noConfigPath {
+			cfgPath = ""
 		}
 	}
+	configPath = cfgPath
 	return nil
 }
 

lib/file/file_other.go

@@ -13,3 +13,8 @@ import "os"
 // Under both Unix and Windows this will allow open files to be
 // renamed and or deleted.
 var OpenFile = os.OpenFile
+
+// IsReserved checks if path contains a reserved name
+func IsReserved(path string) error {
+	return nil
+}

lib/file/file_test.go

@@ -6,6 +6,7 @@ import (
 	"io/ioutil"
 	"os"
 	"path"
+	"runtime"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -152,3 +153,29 @@ func TestOpenFileOperations(t *testing.T) {
 	})
 
 }
+
+// Smoke test the IsReserved function
+func TestIsReserved(t *testing.T) {
+	if runtime.GOOS != "windows" {
+		t.Skip("Skipping test on !windows")
+	}
+	// Regular name
+	require.NoError(t, IsReserved("readme.txt"))
+	require.NoError(t, IsReserved("some/path/readme.txt"))
+	// Empty
+	require.Error(t, IsReserved(""))
+	// Separators only
+	require.Error(t, IsReserved("/"))
+	require.Error(t, IsReserved("////"))
+	require.Error(t, IsReserved("./././././"))
+	// Legacy device name
+	require.Error(t, IsReserved("NUL"))
+	require.Error(t, IsReserved("nul"))
+	require.Error(t, IsReserved("Nul"))
+	require.Error(t, IsReserved("NUL.txt"))
+	require.Error(t, IsReserved("some/path/to/nul.txt"))
+	require.NoError(t, IsReserved("NULL"))
+	// Name end with a space or a period
+	require.Error(t, IsReserved("test."))
+	require.Error(t, IsReserved("test "))
+}

lib/file/file_windows.go

@@ -3,7 +3,10 @@
 package file
 
 import (
+	"errors"
 	"os"
+	"path/filepath"
+	"regexp"
 	"syscall"
 )
 
@@ -64,3 +67,36 @@ func OpenFile(path string, mode int, perm os.FileMode) (*os.File, error) {
 	}
 	return os.NewFile(uintptr(h), path), nil
 }
+
+// IsReserved checks if path contains a reserved name
+func IsReserved(path string) error {
+	if path == "" {
+		return errors.New("path is empty")
+	}
+	base := filepath.Base(path)
+	// If the path is empty or reduces to ".", Base returns ".".
+	if base == "." {
+		return errors.New("path is '.'")
+	}
+	// If the path consists entirely of separators, Base returns a single separator.
+	if base == string(filepath.Separator) {
+		return errors.New("path consists entirely of separators")
+	}
+	// Do not end a file or directory name with a space or a period. Although the underlying
+	// file system may support such names, the Windows shell and user interface does not.
+	// (https://docs.microsoft.com/en-gb/windows/win32/fileio/naming-a-file)
+	suffix := base[len(base)-1]
+	switch suffix {
+	case ' ':
+		return errors.New("base file name ends with a space")
+	case '.':
+		return errors.New("base file name ends with a period")
+	}
+	// Do not use names of legacy (DOS) devices, not even as basename without extension,
+	// as this will refer to the actual device.
+	// (https://docs.microsoft.com/en-gb/windows/win32/fileio/naming-a-file)
+	if reserved, _ := regexp.MatchString(`^(?i:con|prn|aux|nul|com[1-9]|lpt[1-9])(?:\.|$)`, base); reserved {
+		return errors.New("base file name is reserved windows device name (CON, PRN, AUX, NUL, COM[1-9], LPT[1-9])")
+	}
+	return nil
+}