Linux-4.20.8内核桥收包源码解析（六）----------决策函数br_handle_frame_finish

本文链接：https://blog.youkuaiyun.com/Sophisticated_/article/details/87922712

作者：lwyang?
内核版本：Linux-4.20.8

br_handle_frame_finish函数主要是决策将不同类别的数据包做不同的分发路径，它会决定数据包是转发还是交给上层协议栈去处理，所以我给这个函数取名为决策函数，也是即将离开NF_BR_PRE_ROUTING链时做的处理

int br_handle_frame_finish(struct net *net, struct sock *sk, struct sk_buff *skb)
{
	//获取网桥端口 dev->rx_handler_data
	struct net_bridge_port *p = br_port_get_rcu(skb->dev);
	enum br_pkt_type pkt_type = BR_PKT_UNICAST;
	struct net_bridge_fdb_entry *dst = NULL;
	struct net_bridge_mdb_entry *mdst;
	bool local_rcv, mcast_hit = false;
	const unsigned char *dest;
	struct net_bridge *br;
	u16 vid = 0;

	//如果网桥端口不存在或者网桥端口状态为BR_STATE_DISABLED，则丢弃
	if (!p || p->state == BR_STATE_DISABLED)
		goto drop;

	//判断是否允许进入桥内，如果没有开启vlan则所有的数据包都可以进入， 
	//如果开启了vlan则根据vlan相应的规则,从桥上进行数据包转发
	if (!br_allowed_ingress(p->br, nbp_vlan_group_rcu(p), skb, &vid))
		goto out;

	//BR_INPUT_SKB_CB(skb)->offload_fwd_mark = p->offload_fwd_mark
	nbp_switchdev_frame_mark(p, skb);

	/* insert into forwarding database after filtering to avoid spoofing */
	//获取网桥，下面会将网桥的device放入skb的私有数据中
	br = p->br;

	//如果网桥端口标志有BR_LEARNING，则更新fdb表
	//一般新建网桥端口p->flags = BR_LEARNING | BR_FLOOD | BR_MCAST_FLOOD | BR_BCAST_FLOOD
	if (p->flags & BR_LEARNING)
		br_fdb_update(br, p, eth_hdr(skb)->h_source, vid, false);

	//发往本地数据包标记，!!的作用是转换为bool值
	local_rcv = !!(br->dev->flags & IFF_PROMISC);
	dest = eth_hdr(skb)->h_dest;
	if (is_multicast_ether_addr(dest)) {
		/* by definition the broadcast is also a multicast address */
		//若目的mac地址为广播包 （FF:FF:FF:FF:FF:FF），会发往本地一份
		if (is_broadcast_ether_addr(dest)) {
			pkt_type = BR_PKT_BROADCAST;
			local_rcv = true;
		} else {
			//若为组播包
			pkt_type = BR_PKT_MULTICAST;

			//igmp snooping留给网桥子系统的外部接口函数，当网桥接收了igmp数据包后就会调用该函数进行后续处理
			if (br_multicast_rcv(br, p, skb, vid))
				goto drop;
		}
	}

	//如果网桥端口状态此时还是BR_STATE_LEARNING，则丢弃
	if (p->state == BR_STATE_LEARNING)
		goto drop;

	//将网桥所属的net_device放入skb的私有数据中（struct br_input_skb_cb）
	BR_INPUT_SKB_CB(skb)->brdev = br->dev;
	BR_INPUT_SKB_CB(skb)->src_port_isolated = !!(p->flags & BR_ISOLATED);

	//进行与arp协议相关的处理 ？TODO
	if (IS_ENABLED(CONFIG_INET) &&
	    (skb->protocol == htons(ETH_P_ARP) ||
	     skb->protocol == htons(ETH_P_RARP))) {
		br_do_proxy_suppress_arp(skb, br, vid, p);
	} else if (IS_ENABLED(CONFIG_IPV6) &&
		   skb->protocol == htons(ETH_P_IPV6) &&
		   br_opt_get(br, BROPT_NEIGH_SUPPRESS_ENABLED) &&
		   pskb_may_pull(skb, sizeof(struct ipv6hdr) +
				 sizeof(struct nd_msg)) &&
		   ipv6_hdr(skb)->nexthdr == IPPROTO_ICMPV6) {
			struct nd_msg *msg, _msg;

			msg = br_is_nd_neigh_msg(skb, &_msg);
			if (msg)
				br_do_suppress_nd(skb, br, vid, p, msg);
	}

	switch (pkt_type) {

	//若为组播包
	case BR_PKT_MULTICAST:

		//获取组播转发项，设置local_rcv为true，组播包也要发往本地一份
		mdst = br_mdb_get(br, skb, vid);
		if ((mdst || BR_INPUT_SKB_CB_MROUTERS_ONLY(skb)) &&
		    br_multicast_querier_exists(br, eth_hdr(skb))) {
			if ((mdst && mdst->host_joined) ||
			    br_multicast_is_router(br)) {
				local_rcv = true;
				br->dev->stats.multicast++;
			}
			
			//可以获取到数据包对应的组播转发信息
			mcast_hit = true;
		} else {
			local_rcv = true;
			br->dev->stats.multicast++;
		}
		break;
	
	//既不是广播包，也不是组播包，则是单播包
	case BR_PKT_UNICAST:
		
		//根据目的mac地址查找fdb表，看是否有对应的表项
		dst = br_fdb_find_rcu(br, dest, vid);
	default:
		break;
	}

	//如果找到目的mac对应转发表项
	if (dst) {
		unsigned long now = jiffies;

		//dst->is_local为真，送入上层处理
		if (dst->is_local)
			return br_pass_frame_up(skb);

		if (now != dst->used)
			dst->used = now;

		//根据fdb转发表项进行转发，若这里local_rcv 为1,（即端口处于混杂模式IFF_PROMISC），则会克隆一份再转发
		//传入的第一个参数dst->dst 即为要转发的目的端口
		br_forward(dst->dst, skb, local_rcv, false);
	} else {  //如果没有找到目的mac对应转发表项
		
		//进行广播或者组播洪泛
		if (!mcast_hit)
			br_flood(br, skb, pkt_type, local_rcv, false);
		else
			br_multicast_flood(mdst, skb, local_rcv, false);
	}

	//local_rcv标记为1，送入上层处理
	if (local_rcv)
		return br_pass_frame_up(skb);

out:
	return 0;
drop:
	kfree_skb(skb);
	goto out;
}