code: func validateVolumeSourcePath(targetPath string, fldPath *field.Path) field.ErrorList {
  	allErrs := field.ErrorList{}
  	if path.IsAbs(targetPath) {
  		allErrs = append(allErrs, field.Invalid(fldPath, targetPath, "must be a relative path"))
  	}
  	// TODO assume OS of api server & nodes are the same for now
  	items := strings.Split(targetPath, string(os.PathSeparator))
  
  	for _, item := range items {
  		if item == ".." {
  			allErrs = append(allErrs, field.Invalid(fldPath, targetPath, "must not contain '..'"))
  		}
  	}
  	if strings.HasPrefix(items[0], "..") && len(items[0]) > 2 {
  		allErrs = append(allErrs, field.Invalid(fldPath, targetPath, "must not start with '..'"))
  	}
  	return allErrs
  }
docs: // This validate will make sure targetPath:
  // 1. is not abs path
  // 2. does not contain '..'
  // 3. does not start with '..'