Subject: Re: [PATCH bpf-next 4/7] netfilter: bpf: Support BPF_F_NETFILTER_IP_DEFRAG in netfilter link

From: Florian Westphal <[email-masked]>
Date: Tue, 27 Jun 2023 13:12:48 +0200

Daniel Xu <[email-masked]> wrote:
> +static int bpf_nf_enable_defrag(struct bpf_nf_link *link)
> +{
> +	int err;
> +
> +	switch (link->hook_ops.pf) {
> +#if IS_ENABLED(CONFIG_NF_DEFRAG_IPV4)
> +	case NFPROTO_IPV4:
> +		const struct nf_defrag_v4_hook *v4_hook;
> +
> +		err = request_module("nf_defrag_ipv4");
> +		if (err)
> +			return err;
> +
> +		rcu_read_lock();
> +		v4_hook = rcu_dereference(nf_defrag_v4_hook);
> +		err = v4_hook->enable(link->net);
> +		rcu_read_unlock();

I'd reverse this, first try rcu_dereference(), then modprobe
if thats returned NULL.

> +static void bpf_nf_disable_defrag(struct bpf_nf_link *link)
> +{
> +	switch (link->hook_ops.pf) {
> +#if IS_ENABLED(CONFIG_NF_DEFRAG_IPV4)
> +	case NFPROTO_IPV4:
> +		const struct nf_defrag_v4_hook *v4_hook;
> +
> +		rcu_read_lock();
> +		v4_hook = rcu_dereference(nf_defrag_v4_hook);
> +		v4_hook->disable(link->net);
> +		rcu_read_unlock();

if (v4_hook)
	v4_hook->disable()

Else we get trouble on manual 'rmmod'.

> +	/* make sure conntrack confirm is always last */
> +	prio = attr->link_create.netfilter.priority;
> +	if (prio == NF_IP_PRI_FIRST)
> +		return -ERANGE;  /* sabotage_in and other warts */
> +	else if (prio == NF_IP_PRI_LAST)
> +		return -ERANGE;  /* e.g. conntrack confirm */
> +	else if ((attr->link_create.netfilter.flags & BPF_F_NETFILTER_IP_DEFRAG) &&
> +		 (prio > NF_IP_PRI_FIRST && prio <= NF_IP_PRI_CONNTRACK_DEFRAG))
> +		return -ERANGE;  /* cannot use defrag if prog runs before nf_defrag */

You could elide the (prio > NF_IP_PRI_FIRST, its already handled by
first conditional.  Otherwise this looks good to me.