Wordpress login by user Role

add_action('login_form', 'wdm_login_form_role');
function wdm_login_form_role()
{
 ?>
<p><label for="user_role">Role<br>
<select name="role" style="flaot:left;width:100%;margin-bottom:10px;padding: 10px;">
<option value="publishers">Publishers</option>
<option value="advertisers">Advertisers</option>
</select></label>
<p>
 <?php
  }
add_filter('wp_authenticate_user','wdm_validate_login_role',10,2);
function wdm_validate_login_role($user, $password) {

   $ruser= get_user_by('login',$_POST['log']);

  if($ruser){
       if($ruser->roles[0]==$_POST['role']){
          return $user;
                   }
      else{
     return null;
      }
 }
}