Linux系统内核接收以太帧的处理程序

1. 前言

以太头中除了6字节目的MAC地址、6字节源MAC地址外，还有两字节的以太帧类型值，如IPv4为0x0800，ARP为0x0806等，网卡驱动收到以太帧后通过接口函数netif_receive_skb()(netif_rx实际最后也是调用netif_receive_skb)交到上层，而这个接口函数就完成对以太帧类型的区分，交到不同的协议处理程序。如果想自己编写某一以太类型帧的处理程序，需要自己添加相应的代码。以下为Linux内核2.6代码。

2. 数据结构

每种协议都要定义一个packet_type结构，引导进入相关的协议数据处理函数，所有节点组成一个链表(HASH链表)。

/* include/linux/netdevice.h */struct packet_type {__be16 type; /* This is really htons(ether_type). */struct net_device *dev; /* NULL is wildcarded here */int (*func) (struct sk_buff *,struct net_device *,struct packet_type *,struct net_device *);void *af_packet_priv;struct list_head list;            };

参数说明：

type：以太帧类型，16位。

dev：所附着的网卡设备，如果为NULL则匹配全部网卡。

func：协议入口接收处理函数。

af_packet_priv：协议私有数据。

list：链表扣。

一般各协议的packet_type结构都是静态存在，初始化时只提供type和func两个参数就可以了，每个协议在初始化时都要将此结构加入到系统类型链表中。

3. 处理函数

3.1 添加节点

/* net/core/dev.c *//*** dev_add_pack - add packet handler* @pt: packet type declaration** Add a protocol handler to the networking stack. The passed &packet_type* is linked into kernel lists and may not be freed until it has been* removed from the kernel lists.** This call does not sleep therefore it can not * guarantee all CPU's that are in middle of receiving packets* will see the new packet type (until the next received packet).*/void dev_add_pack(struct packet_type *pt){int hash;spin_lock_bh(&ptype_lock);// 如果类型是全部以太类型，则节点链接到ptype_all链if (pt->type == htons(ETH_P_ALL)) {netdev_nit++;list_add_rcu(&pt->list, &ptype_all);} else {// 根据协议类型取个HASH，共15个HASH链表hash = ntohs(pt->type) & 15;// 将节点链接到HASH链表中，list_add_rcu是加了smp_wmb()保护的list_add链表操作list_add_rcu(&pt->list, &ptype_base[hash]);}spin_unlock_bh(&ptype_lock);            }

2 删除节点

/*** __dev_remove_pack - remove packet handler* @pt: packet type declaration** Remove a protocol handler that was previously added to the kernel* protocol handlers by dev_add_pack(). The passed &packet_type is removed* from the kernel lists and can be freed or reused once this function* returns. ** The packet type might still be in use by receivers* and must not be freed until after all the CPU's have gone* through a quiescent state.*/void __dev_remove_pack(struct packet_type *pt){struct list_head *head;struct packet_type *pt1;spin_lock_bh(&ptype_lock);// 根据协议类型找是在ptype_all表还是某一HASH链表中if (pt->type == htons(ETH_P_ALL)) {netdev_nit--;head = &ptype_all;} elsehead = &ptype_base[ntohs(pt->type) & 15];// 直接用地址比对进行查找，而不是类型，因为同一个类型也可能有多个节点list_for_each_entry(pt1, head, list) {if (pt == pt1) {list_del_rcu(&pt->list);goto out;}}printk(KERN_WARNING "dev_remove_pack: %p not found.\n", pt);out:spin_unlock_bh(&ptype_lock);}/*** dev_remove_pack - remove packet handler* @pt: packet type declaration** Remove a protocol handler that was previously added to the kernel* protocol handlers by dev_add_pack(). The passed &packet_type is removed* from the kernel lists and can be freed or reused once this function* returns.** This call sleeps to guarantee that no CPU is looking at the packet* type after return.*/// 只是__dev_remove_pack()的包裹函数void dev_remove_pack(struct packet_type *pt){__dev_remove_pack(pt);synchronize_net();            }

4. 实例

4.1 IP

/* net/ipv4/af_inet.c */static struct packet_type ip_packet_type = {.type = __constant_htons(ETH_P_IP),.func = ip_rcv, // IP接收数据的入口点};static int __init inet_init(void){......dev_add_pack(&ip_packet_type);            ......

由于IP协议部分不能作为内核模块，所以是没有卸载函数的，没必要调用dev_remove_pack()函数。 

4.2 8021q vlan

/* net/8021q/vlan.c */static struct packet_type vlan_packet_type = {.type = __constant_htons(ETH_P_8021Q),.func = vlan_skb_recv, /* VLAN receive method */};......static int __init vlan_proto_init(void){......dev_add_pack(&vlan_packet_type);......static void __exit vlan_cleanup_module(void){......dev_remove_pack(&vlan_packet_type);      ......

由于VLAN可为模块方式存在，所以在模块清除函数中要调用dev_remove_pack()。

5. 网络接收

网卡驱动收到数据包构造出skb后，通过接口函数netif_receive_skb()传递到上层进行协议处理分配。

/* net/core/dev.c */int netif_receive_skb(struct sk_buff *skb){......// 先查处理所有以太类型的链表各节点list_for_each_entry_rcu(ptype, &ptype_all, list) {if (!ptype->dev || ptype->dev == skb->dev) {if (pt_prev) ret = deliver_skb(skb, pt_prev, orig_dev);pt_prev = ptype;}}......// 再查指定协议的HASH链表list_for_each_entry_rcu(ptype, &ptype_base[ntohs(type)&15], list) {if (ptype->type == type &&(!ptype->dev || ptype->dev == skb->dev)) {if (pt_prev) ret = deliver_skb(skb, pt_prev, orig_dev);pt_prev = ptype;}}......// 该函数就是调用个协议的接收函数处理该skb包，进入第三层网络层处理static __inline__ int deliver_skb(struct sk_buff *skb,struct packet_type *pt_prev,struct net_device *orig_dev){atomic_inc(&skb->users);return pt_prev->func(skb, skb->dev, pt_prev, orig_dev);      }

6. 结论

通过链表挂接方式，Linux内核可以很容易的添加各种协议的接收处理函数。

数据流程:

网卡驱动--->netif_rx()--->netif_receive_skb()->deliver_skb()->packet_type.func