permalink is breaking

when I turn on the “Link to source” the permalink was broke. even the other post which are not related to rss..

function csyn_permalink($permalink) {
    global $post;
    list($link) = get_post_custom_values('cyberseo_post_link');
    if (filter_var($link, FILTER_VALIDATE_URL)) {
        $permalink = $link;
    } elseif (filter_var($post->guid, FILTER_VALIDATE_URL)) {
        $permalink = $post->guid;
    }
    return $permalink;
}

https://ww.wp.xz.cn/plugins/cybersyn/