关于ip_finish_output2的一点分析

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关于ip_finish_output2的一点分析(转) 2009-10-22 07:33:55

分类： LINUX

首先声明，因为刚开始看linux内核协议栈，所以肯定有许多不正确的地方，如果大家发现不对的地方请帮我指正。在此先谢过了。

ip_finish_output2函数在文件“net/ipv4/ip_output.c”中定义，如下：

staticinline int ip_finish_output2(struct sk_buff*skb) { struct dst_entry *dst = skb->dst; struct hh_cache *hh = dst->hh; struct net_device *dev = dst->dev; int hh_len = LL_RESERVED_SPACE(dev); /* Be paranoid, rather than too clever. */ if (unlikely(skb_headroom(skb)< hh_len && dev->hard_header)){ struct sk_buff *skb2; skb2 = skb_realloc_headroom(skb, LL_RESERVED_SPACE(dev)); if (skb2 ==NULL){ kfree_skb(skb); return -ENOMEM; } if (skb->sk) skb_set_owner_w(skb2, skb->sk); kfree_skb(skb); skb = skb2; } if (hh){ int hh_alen; read_lock_bh(&hh->hh_lock); hh_alen = HH_DATA_ALIGN(hh->hh_len); memcpy(skb->data- hh_alen, hh->hh_data, hh_alen); read_unlock_bh(&hh->hh_lock); skb_push(skb, hh->hh_len); return hh->hh_output(skb); } elseif (dst->neighbour) return dst->neighbour->output(skb); if (net_ratelimit()) printk(KERN_DEBUG "ip_finish_output2: No header cache and no neighbour!\n"); kfree_skb(skb); return -EINVAL; }

对于“hh->hh_output(skb);”，这里hh_output的赋值，在google上没找到，粗略看了一下代码，估计是在函数neigh_hh_init中赋值的（我也不能确定，还需要以后更熟悉协议栈代码后，如有错再更正），此函数在文件“net/core/neighbour.c”中定义，如下

staticvoid neigh_hh_init(struct neighbour*n,struct dst_entry *dst, u16 protocol) { struct hh_cache *hh; struct net_device *dev = dst->dev; for (hh = n->hh; hh; hh= hh->hh_next) if (hh->hh_type== protocol) break; if (!hh&&(hh = kmalloc(sizeof(*hh), GFP_ATOMIC))!=NULL){ memset(hh, 0,sizeof(struct hh_cache)); rwlock_init(&hh->hh_lock); hh->hh_type= protocol; atomic_set(&hh->hh_refcnt, 0); hh->hh_next= NULL; if (dev->hard_header_cache(n, hh)){ kfree(hh); hh = NULL; } else { atomic_inc(&hh->hh_refcnt); hh->hh_next= n->hh; n->hh = hh; if (n->nud_state& NUD_CONNECTED) hh->hh_output= n->ops->hh_output; else hh->hh_output= n->ops->output; } } if (hh) { atomic_inc(&hh->hh_refcnt); dst->hh= hh; } }

我们可以知道赋值为n->ops->hh_output或n->ops->output, n是结构体struct neighbour指针. 这里的

n->ops和ip_finish_output2函数中的“return dst->neighbour->output(skb); ”里的output函数指针应该是在文件"net/ipv4/arp.c"中的arp_constructor函数中赋值的，下面是代码：

staticint arp_constructor(struct neighbour*neigh) { u32 addr = *(u32*)neigh->primary_key; struct net_device *dev = neigh->dev; struct in_device *in_dev; struct neigh_parms *parms; neigh->type= inet_addr_type(addr); rcu_read_lock(); in_dev = __in_dev_get_rcu(dev); if (in_dev== NULL) { rcu_read_unlock(); return -EINVAL; } parms = in_dev->arp_parms; __neigh_parms_put(neigh->parms); neigh->parms= neigh_parms_clone(parms); rcu_read_unlock(); if (dev->hard_header== NULL) { neigh->nud_state= NUD_NOARP; neigh->ops= &arp_direct_ops; neigh->output= neigh->ops->queue_xmit; } else { /* Good devices (checked by reading texts, but only Ethernet is tested) ARPHRD_ETHER: (ethernet, apfddi) ARPHRD_FDDI: (fddi) ARPHRD_IEEE802: (tr) ARPHRD_METRICOM: (strip) ARPHRD_ARCNET: etc. etc. etc. ARPHRD_IPDDP will also work, if author repairs it. I did not it, becausethis driver does not work even in old paradigm. */ #if 1 /* So... these"amateur" devices are hopeless. The only thing, that I can say now: It is very sad that we need to keep ugly obsolete code to make them happy. They should be moved to more reasonable state, now they use rebuild_header INSTEAD OF Besides that, they are sort of out of date (a lot of redundant clones/copies, uselessin 2.1), I wonder why people believe that they work. */ switch (dev->type) { default: break; case ARPHRD_ROSE: #if defined(CONFIG_AX25)|| defined(CONFIG_AX25_MODULE) case ARPHRD_AX25: #if defined(CONFIG_NETROM)|| defined(CONFIG_NETROM_MODULE) case ARPHRD_NETROM: #endif neigh->ops= &arp_broken_ops; neigh->output= neigh->ops->output; return 0; #endif ;} #endif if (neigh->type== RTN_MULTICAST) { neigh->nud_state= NUD_NOARP; arp_mc_map(addr, neigh->ha, dev, 1); } else if (dev->flags&(IFF_NOARP|IFF_LOOPBACK)) { neigh->nud_state= NUD_NOARP; memcpy(neigh->ha, dev->dev_addr, dev->addr_len); } else if (neigh->type== RTN_BROADCAST|| dev->flags&IFF_POINTOPOINT) { neigh->nud_state= NUD_NOARP; memcpy(neigh->ha, dev->broadcast, dev->addr_len); } if (dev->hard_header_cache) neigh->ops= &arp_hh_ops; else neigh->ops= &arp_generic_ops; if (neigh->nud_state&NUD_VALID) neigh->output= neigh->ops->connected_output; else neigh->output= neigh->ops->output; } return 0; }

这个netgh->ops赋值为函数arp_generic_ops或者arp_hh_ops或者arp_broken_ops或者arp_direct_ops的地址, 这四个结构体变量的具体定义在“net/ipv4/arp.c”中找到，代码如下：

staticstruct neigh_ops arp_generic_ops = { .family = AF_INET, .solicit = arp_solicit, .error_report = arp_error_report, .output = neigh_resolve_output, .connected_output = neigh_connected_output, .hh_output = dev_queue_xmit, .queue_xmit = dev_queue_xmit, }; static struct neigh_ops arp_hh_ops= { .family = AF_INET, .solicit = arp_solicit, .error_report = arp_error_report, .output = neigh_resolve_output, .connected_output = neigh_resolve_output, .hh_output = dev_queue_xmit, .queue_xmit = dev_queue_xmit, }; static struct neigh_ops arp_direct_ops= { .family = AF_INET, .output = dev_queue_xmit, .connected_output = dev_queue_xmit, .hh_output = dev_queue_xmit, .queue_xmit = dev_queue_xmit, }; struct neigh_ops arp_broken_ops = { .family = AF_INET, .solicit = arp_solicit, .error_report = arp_error_report, .output = neigh_compat_output, .connected_output = neigh_compat_output, .hh_output = dev_queue_xmit, .queue_xmit = dev_queue_xmit, };

一般情况下指向arp_generic_ops，所以hh->hh_output对应的函数应该是dev_queue_xmit，

而neigh->ops->output对应的就是函数neigh_resolve_output。

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