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// net/netfilter/core.c

static void __net_init
__netfilter_net_init(struct nf_hook_entries __rcu **e, int max)
{
	int h;

	for (h = 0; h < max; h++)
		RCU_INIT_POINTER(e[h], NULL);
}

static int __net_init netfilter_net_init(struct net *net)
{
	// 初始化 ipv4 钩子数组
	__netfilter_net_init(net->nf.hooks_ipv4, ARRAY_SIZE(net->nf.hooks_ipv4));
	// 初始化 ipv6 钩子数组
	__netfilter_net_init(net->nf.hooks_ipv6, ARRAY_SIZE(net->nf.hooks_ipv6));
	// 初始化 arp 钩子数组
	__netfilter_net_init(net->nf.hooks_arp, ARRAY_SIZE(net->nf.hooks_arp));
	// 初始化 bridge 钩子数组
	__netfilter_net_init(net->nf.hooks_bridge, ARRAY_SIZE(net->nf.hooks_bridge));
	// 初始化 decnet 钩子数组
	__netfilter_net_init(net->nf.hooks_decnet, ARRAY_SIZE(net->nf.hooks_decnet));

	...

	return 0;
}

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|-->NF_HOOK()                           // include/linux/netfilter.h
    |-->nf_hook()
        |-->nf_hook_slow()              // net/netfilter/core.c
            |-->nf_hook_entry_hookfn()  // include/linux/netfilter.h

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// net/ipv4/ip_input.c

/*
 * IP 层入口
 */
int ip_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt,
	   struct net_device *orig_dev)
{
	struct net *net = dev_net(dev);
	...
	return NF_HOOK(NFPROTO_IPV4, NF_INET_PRE_ROUTING, // 执行 ipv4 PRE_ROUTING 列表里的钩子
			   net, NULL, skb, dev, NULL,
			   ip_rcv_finish);
}

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// include/linux/netfilter.h

static inline int
NF_HOOK(...)
{
	int ret = nf_hook(pf, hook, net, sk, skb, in, out, okfn);
	if (ret == 1)
		ret = okfn(net, sk, skb);
	return ret;
}

/**
 *	nf_hook - 调用 netfilter 钩子
 */
static inline int nf_hook(...)
{
	struct nf_hook_entries *hook_head = NULL;
	int ret = 1;
	...
	switch (pf) { // 在此处区分 ipv4, ipv6, arp, bridge, decnet，并获取指定钩子序号的钩子列表
	case NFPROTO_IPV4:
		hook_head = rcu_dereference(net->nf.hooks_ipv4[hook]);
		break;
	case NFPROTO_IPV6:
		hook_head = rcu_dereference(net->nf.hooks_ipv6[hook]);
		break;
	case NFPROTO_ARP:
		hook_head = rcu_dereference(net->nf.hooks_arp[hook]);
		break;
	case NFPROTO_BRIDGE:
		hook_head = rcu_dereference(net->nf.hooks_bridge[hook]);
		break;
	case NFPROTO_DECNET:
		hook_head = rcu_dereference(net->nf.hooks_decnet[hook]);
		break;
	default:
		WARN_ON_ONCE(1);
		break;
	}

	if (hook_head) {
		struct nf_hook_state state;
		nf_hook_state_init(&state, hook, pf, indev, outdev,
				   sk, net, okfn);
		ret = nf_hook_slow(skb, &state, hook_head, 0); // 执行列表里的钩子
	}

	return ret;
}

// net/netfilter/core.c

int nf_hook_slow(...)
{
	unsigned int verdict;
	int ret;

	for (; s < e->num_hook_entries; s++) { // 遍历钩子列表
		verdict = nf_hook_entry_hookfn(&e->hooks[s], skb, state); // 执行钩子函数
		switch (verdict & NF_VERDICT_MASK) {
		case NF_ACCEPT:    // -j ACCEPT
			break;
		case NF_DROP:      // -j DROP
			kfree_skb(skb);
			...
			return ret;
		case NF_QUEUE:     // -j NFQUEUE
			ret = nf_queue(skb, state, s, verdict); // NFQUEUE 的处理入口
			if (ret == 1)
				continue;
			return ret;
		default:
			/* Implicit handling for NF_STOLEN, as well as any other
			 * non conventional verdicts.
			 */
			return 0;
		}
	}

	return 1;
}

// include/linux/netfilter.h

static inline int
nf_hook_entry_hookfn(const struct nf_hook_entry *entry, struct sk_buff *skb,
			 struct nf_hook_state *state)
{
	return entry->hook(entry->priv, skb, state);
}