What order do Netfilter hooks go in, if they all specify NF_IP_PRI_LAST or NF_IP_PRI_FIRST?

Question

If I write a few kernel modules, and in all of them specify they should be the first (or last) netfilter hook called, in what order will they actually be called?

netfilter_ops_out.hook     = hook_func_out;
netfilter_ops_out.pf       = PF_INET;
netfilter_ops_out.hooknum  = NF_IP_LOCAL_OUT;
netfilter_ops_out.priority = NF_IP_PRI_FIRST;

ret = nf_register_hook(&netfilter_ops_out);
if (0 > ret) {
    printk("Error registering netfilter hook: %d\n", ret);
    return ret;
}    

netfilter_ops_in.hook      = hook_func_in;
netfilter_ops_in.pf        = PF_INET;
netfilter_ops_in.hooknum   = NF_IP_LOCAL_IN;
netfilter_ops_in.priority  = NF_IP_PRI_LAST;

ret = nf_register_hook(&netfilter_ops_in);
if (0 > ret) {
    printk("Error registering netfilter hook: %d\n", ret);
    return ret;
}    

Experimentally, I made two modules, insmoded them in the two different orders - but they gave the same result, implying there's some suborder that's not just 'first come first serve'. (It's also not alphabetical...)

Answer 1

From the nf_register_hook() codes, we can know that if two hooks belong to the same nf_hooks[reg->pf][reg->hooknum], the hook execute sequence is decided by priority. If the priority is also the same, the sequence is 'first come first serve'. See following codes:

int nf_register_hook(struct nf_hook_ops *reg)
{
    struct nf_hook_ops *elem;
    int err;

    err = mutex_lock_interruptible(&nf_hook_mutex);
    if (err < 0)
        return err;
    list_for_each_entry(elem, &nf_hooks[reg->pf][reg->hooknum], list) {
        if (reg->priority < elem->priority)
            break;
    }
    list_add_rcu(&reg->list, elem->list.prev);
    mutex_unlock(&nf_hook_mutex);
#if defined(CONFIG_JUMP_LABEL)
    static_key_slow_inc(&nf_hooks_needed[reg->pf][reg->hooknum]);
#endif
    return 0;
}