Linux网络层 ip_rcv()函数代码分析

int ip_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt) 

//几个结构sk_buff套接字缓存,net_device网络设备结构,是所有网络结构的基础数据结构

,packet_type原型

struct packet_type
{
 unsigned short  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 *);
 void   *data; /* Private to the packet type  */
 struct packet_type *next;
};

{
 struct iphdr *iph;

//指向IP协议头的结构指针

 /* When the interface is in promisc. mode, drop all the crap
  * that it receives, do not try to analyse it.
  */
 if (skb->pkt_type == PACKET_OTHERHOST)
  goto drop;
         //pkt_type表示报文类型 。PACKET_OTHERHOST表示非去往本机但是在特定模式下被接受的报文
         //此处的作用是根据包类型,来丢弃并非去往本地的包

 IP_INC_STATS_BH(IpInReceives);

 if ((skb = skb_share_check(skb, GFP_ATOMIC)) == NULL)
  goto out;

/*skb_share_check定义在linux/Skbuff.h里面,如果users是一个就返回假,其他返回真。
[
static inline struct sk_buff *skb_share_check(struct sk_buff *skb, int pri)
{
 if (skb_shared(skb)) {
  struct sk_buff *nskb;
  nskb = skb_clone(skb, pri);  //返回了一个clone过的套接字缓存
  kfree_skb(skb); //Free an skbuff
  return nskb;
 }
 return skb;
}
]

#define GFP_ATOMIC (__GFP_HIGH)
#define __GFP_HIGH 0x20
*/
注: skb_clone的原型是struct sk_buff *skb_clone(struct sk_buff *skb, int gfp_mask)
gfp_mask说明    gfp_mask: get_free_pages mask, passed to alloc_skb


 if (!pskb_may_pull(skb, sizeof(struct iphdr)))
  goto inhdr_error;
//
static inline int pskb_may_pull(struct sk_buff *skb, unsigned int len)
{
 if (len <= skb_headlen(skb))
  return 1;
 if (len > skb->len)
  return 0;
 return (__pskb_pull_tail(skb, len-skb_headlen(skb)) != NULL);
}

pskb_may_pull在调用 skb_pull() 去掉外层协议头之前,通常先调用此函数判断一下是否有足够的数据用于“pull”。

如果线性 buffer足够 pull,则返回1;

如果需要 pull 的数据超过 skb->len,则返回0;

最后,调用__pskb_pull_tail() 来检查 page buffer 有没有足够的数据用于 pull
   */
 iph = skb->nh.iph;

 /*
  * RFC1122: 3.1.2.2 MUST silently discard any IP frame that fails the checksum.
  *
  * Is the datagram acceptable?
  *
  * 1. Length at least the size of an ip header
  * 2. Version of 4
  * 3. Checksums correctly. [Speed optimisation for later, skip loopback checksums]
  * 4. Doesn't have a bogus length
  */

 if (iph->ihl < 5 || iph->version != 4)
  goto inhdr_error; 
       //iph->ihl<5说明iph->ihl指的是IP包的首部长度,首部一行是32bit也就是4byte(字节)注:1byte=8bit,byte是计算机中最小文件

单位,普通IP数据包首部长度(不包含任何选项)字段的值是5.

 if (!pskb_may_pull(skb, iph->ihl*4))
  goto inhdr_error;
       //iph->ihl*4是20,是首部最长的长度,此语句是说如果头部长度不能pull,则error

 iph = skb->nh.iph;

 if (ip_fast_csum((u8 *)iph, iph->ihl) != 0)
  goto inhdr_error; 
        //用汇编来校验和

 {
  __u32 len = ntohs(iph->tot_len);
  if (skb->len < len || len < (iph->ihl<<2))
   goto inhdr_error;

  /* Our transport medium may have padded the buffer out. Now we know it
   * is IP we can trim to the true length of the frame.
   * Note this now means skb->len holds ntohs(iph->tot_len).
   */
  if (skb->len > len) {
   __pskb_trim(skb, len);
   if (skb->ip_summed == CHECKSUM_HW)
    skb->ip_summed = CHECKSUM_NONE;
  }
 }

 return NF_HOOK(PF_INET, NF_IP_PRE_ROUTING, skb, dev, NULL,
         ip_rcv_finish);

inhdr_error:
 IP_INC_STATS_BH(IpInHdrErrors);
drop:
        kfree_skb(skb);
out:
        return NET_RX_DROP;

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