Prevent overwriting irregular files (cp, save, export commands)

Signed-off-by: Philipp Schmied <pschmied@schutzwerk.com>

cli/command/utils.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/docker/docker/api/types/filters"
 	"github.com/docker/docker/pkg/system"
+	"github.com/pkg/errors"
 	"github.com/spf13/pflag"
 )
 
@@ -125,3 +126,37 @@ func AddPlatformFlag(flags *pflag.FlagSet, target *string) {
 	flags.SetAnnotation("platform", "version", []string{"1.32"})
 	flags.SetAnnotation("platform", "experimental", nil)
 }
+
+// ValidateOutputPath validates the output paths of the `export` and `save` commands.
+func ValidateOutputPath(path string) error {
+	dir := filepath.Dir(path)
+	if dir != "" && dir != "." {
+		if _, err := os.Stat(dir); os.IsNotExist(err) {
+			return errors.Errorf("invalid output path: directory %q does not exist", dir)
+		}
+	}
+	// check whether `path` points to a regular file
+	// (if the path exists and doesn't point to a directory)
+	if fileInfo, err := os.Stat(path); !os.IsNotExist(err) {
+		if fileInfo.Mode().IsDir() || fileInfo.Mode().IsRegular() {
+			return nil
+		}
+
+		if err := ValidateOutputPathFileMode(fileInfo.Mode()); err != nil {
+			return errors.Wrapf(err, fmt.Sprintf("invalid output path: %q must be a directory or a regular file", path))
+		}
+	}
+	return nil
+}
+
+// ValidateOutputPathFileMode validates the output paths of the `cp` command and serves as a
+// helper to `ValidateOutputPath`
+func ValidateOutputPathFileMode(fileMode os.FileMode) error {
+	switch {
+	case fileMode&os.ModeDevice != 0:
+		return errors.New("got a device")
+	case fileMode&os.ModeIrregular != 0:
+		return errors.New("got an irregular file")
+	}
+	return nil
+}