rem: return EINVAL;
add: return FS_EINVAL;
context: do_rename(int argc, char **argv){    int  err;    char oldname[128], newname[128];    if (cur_fd >= 0)        do_close(0, NULL);    if (argc < 3) {        printf("rename needs two arguments!\n");        return EINVAL;    }        strcpy(oldname, "/myfs/");    strcpy(newname, "/myfs/");    strcat(oldname, &argv[1][0]);    strcat(newname, &argv[2][0]);    err = sys_rename(1, -1, oldname, -1, newname);    if (err)        printf("rename failed with err: %s\n", strerror(err));	return err;}