rem: return EINVAL;
add: return FS_EINVAL;
context: rootfs_wstat(void *_ns, void *_node, struct my_stat *st, long mask){    nspace      *ns;    vnode       *node;    ns = (nspace *) _ns;    node = (vnode *) _node;    if (mask & WSTAT_SIZE)        return EINVAL;    LOCK(ns->lock);    if (mask & WSTAT_MODE)        node->mode = (node->mode & MY_S_IFMT) | (st->st_mode & ~MY_S_IFMT);    if (mask & WSTAT_UID)        node->uid = st->st_uid;    if (mask & WSTAT_GID)        node->gid = st->st_gid;    if (mask & WSTAT_MTIME)        node->mtime = st->st_mtime;    if (mask & WSTAT_ATIME)        node->mtime = st->st_atime;    UNLOCK(ns->lock);    return 0;}