IP包碎片重组过程

(1) 当内核接收到本地的IP包, 在传递给上层协议处理之前, 先进行碎片重组.IP包片段之间的标识号(id)是相同的. 
当IP包片偏量(frag_off)第14位(IP_MF)为1时, 表示该IP包有后继片段. 片偏量的低13位则为该片段在完整数据包中
的偏移量, 以8字节为单位. 当IP_MF位为0时, 表示IP包是最后一块碎片.

(2) 碎片重组由重组队列完成, 每一重组队列对应于(daddr,saddr,protocol,id)构成的键值, 它们存在于ipq结构构成
的散列链之中. 重组队列将IP包按照将片段偏量的顺序进行排列, 当所有的片段都到齐后, 就可以将队列中的包碎片
按顺序拼合成一个完整的IP包.

(3) 如果30秒后重组队列内包未到齐, 则重组过程失败, 重组队列被释放, 同时向发送方以ICMP协议通知失败信息.
 重组队列的内存消耗不得大于256k(sysctl_ipfrag_high_thresh), 否则将会调用(ip_evictor)释放每支散列尾端的重
组队列.

; net/ipv4/ip_input.c:

/*

 *  Deliver IP Packets to the higher protocol layers.

 */ 

int ip_local_deliver(struct sk_buff *skb)

{

 struct iphdr *iph = skb->nh.iph;

 /*

  * Reassemble IP fragments.

  */

 if (iph->frag_off & htons(IP_MF|IP_OFFSET)) {

  skb = ip_defrag(skb);

  if (!skb)

   return 0;

 }

 return NF_HOOK(PF_INET, NF_IP_LOCAL_IN, skb, skb->dev, NULL,

         ip_local_deliver_finish);

}

; net/ipv4/ip_fragment.c:

/* NOTE. Logic of IP defragmentation is parallel to corresponding IPv6

 * code now. If you change something here, _PLEASE_ update ipv6/reassembly.c

 * as well. Or notify me, at least. --ANK

 */

/* Fragment cache limits. We will commit 256K at one time. Should we

 * cross that limit we will prune down to 192K. This should cope with

 * even the most extreme cases without allowing an attacker to measurably

 * harm machine performance.

 */

int sysctl_ipfrag_high_thresh = 256*1024; (256k字节)

int sysctl_ipfrag_low_thresh = 192*1024; (192k字节)

/* Important NOTE! Fragment queue must be destroyed before MSL expires.

 * RFC791 is wrong proposing to prolongate timer each fragment arrival by TTL.

 */

int sysctl_ipfrag_time = IP_FRAG_TIME; (30秒)

struct ipfrag_skb_cb

{

 struct inet_skb_parm h;

 int   offset;

};

#define FRAG_CB(skb) ((struct ipfrag_skb_cb*)((skb)->cb))

/* Describe an entry in the "incomplete datagrams" queue. */

struct ipq {

 struct ipq *next;  /* linked list pointers   */

 u32  saddr;

 u32  daddr;

 u16  id;

 u8  protocol;

 u8  last_in;

#define COMPLETE  4

#define FIRST_IN  2

#define LAST_IN   1

 struct sk_buff *fragments; /* linked list of received fragments */

 int  len;  /* total length of original datagram */

 int  meat;

 spinlock_t lock;

 atomic_t refcnt;

 struct timer_list timer; /* when will this queue expire?  */

 struct ipq **pprev;

 int  iif;  /* Device index - for icmp replies */

};

/* Hash table. */

#define IPQ_HASHSZ 64

/* Per-bucket lock is easy to add now. */

static struct ipq *ipq_hash[IPQ_HASHSZ];

static rwlock_t ipfrag_lock = RW_LOCK_UNLOCKED;

int ip_frag_nqueues = 0;

/* Process an incoming IP datagram fragment. */

struct sk_buff *ip_defrag(struct sk_buff *skb)

{

 struct iphdr *iph = skb->nh.iph;

 struct ipq *qp;

 struct net_device *dev;

 

 IP_INC_STATS_BH(IpReasmReqds);

 /* Start by cleaning up the memory. */

 if (atomic_read(&ip_frag_mem) > sysctl_ipfrag_high_thresh)

  ip_evictor();

 dev = skb->dev;

 /* Lookup (or create) queue header */

 if ((qp = ip_find(iph)) != NULL) {

  struct sk_buff *ret = NULL;

  spin_lock(&qp->lock);

  ip_frag_queue(qp, skb);

  if (qp->last_in == (FIRST_IN|LAST_IN) && 首包与尾包已收到

      qp->meat == qp->len) 队列中字节数恰好等于队列尾部

   ret = ip_frag_reasm(qp, dev); 重组碎片

  spin_unlock(&qp->lock);

  ipq_put(qp);

  return ret;

 }

 IP_INC_STATS_BH(IpReasmFails);

 kfree_skb(skb);

 return NULL;

}

/*

 * Was: ((((id) >> 1) ^ (saddr) ^ (daddr) ^ (prot)) & (IPQ_HASHSZ - 1))

 *

 * I see, I see evil hand of bigendian mafia. On Intel all the packets hit

 * one hash bucket with this hash function. 8)

 */

static __inline__ unsigned int ipqhashfn(u16 id, u32 saddr, u32 daddr, u8 prot)

{

 unsigned int h = saddr ^ daddr;

 h ^= (h>>16)^id;

 h ^= (h>>8)^prot;

 return h & (IPQ_HASHSZ - 1);

}

/* Find the correct entry in the "incomplete datagrams" queue for

 * this IP datagram, and create new one, if nothing is found.

 */

static inline struct ipq *ip_find(struct iphdr *iph)

{

 __u16 id = iph->id;

 __u32 saddr = iph->saddr;

 __u32 daddr = iph->daddr;

 __u8 protocol = iph->protocol;

 unsigned int hash = ipqhashfn(id, saddr, daddr, protocol);

 struct ipq *qp;

 read_lock(&ipfrag_lock);

 for(qp = ipq_hash[hash]; qp; qp = qp->next) {

  if(qp->id == id  &&

     qp->saddr == saddr &&

     qp->daddr == daddr &&

     qp->protocol == protocol) {

   atomic_inc(&qp->refcnt);

   read_unlock(&ipfrag_lock);

   return qp;

  }

 }

 read_unlock(&ipfrag_lock);

 return ip_frag_create(hash, iph); 创建一条IP片段队列

}

/* Add an entry to the 'ipq' queue for a newly received IP datagram. */

static struct ipq *ip_frag_create(unsigned hash, struct iphdr *iph)

{

 struct ipq *qp;

 if ((qp = frag_alloc_queue()) == NULL)

  goto out_nomem;

 qp->protocol = iph->protocol;

 qp->last_in = 0;

 qp->id = iph->id;

 qp->saddr = iph->saddr;

 qp->daddr = iph->daddr;

 qp->len = 0;

 qp->meat = 0;

 qp->fragments = NULL;

 qp->iif = 0;

 /* Initialize a timer for this entry. */

 init_timer(&qp->timer);

 qp->timer.data = (unsigned long) qp; /* pointer to queue */

 qp->timer.function = ip_expire;  /* expire function */

 qp->lock = SPIN_LOCK_UNLOCKED;

 atomic_set(&qp->refcnt, 1);

 return ip_frag_intern(hash, qp);

out_nomem:

 NETDEBUG(printk(KERN_ERR "ip_frag_create: no memory left !/n"));

 return NULL;

}

extern __inline__ struct ipq *frag_alloc_queue(void)

{

 struct ipq *qp = kmalloc(sizeof(struct ipq), GFP_ATOMIC);

 if(!qp)

  return NULL;

 atomic_add(sizeof(struct ipq), &ip_frag_mem);

 return qp;

}

/* Creation primitives. */

static struct ipq *ip_frag_intern(unsigned int hash, struct ipq *qp_in)

{

 struct ipq *qp;

 write_lock(&ipfrag_lock);

#ifdef CONFIG_SMP

 /* With SMP race we have to recheck hash table, because

  * such entry could be created on other cpu, while we

  * promoted read lock to write lock.

  */

 for(qp = ipq_hash[hash]; qp; qp = qp->next) {

  if(qp->id == qp_in->id  &&

     qp->saddr == qp_in->saddr &&

     qp->daddr == qp_in->daddr &&

     qp->protocol == qp_in->protocol) {

   atomic_inc(&qp->refcnt);

   write_unlock(&ipfrag_lock);

   qp_in->last_in |= COMPLETE;

   ipq_put(qp_in);

   return qp;

  }

 }

#endif

 qp = qp_in;

 if (!mod_timer(&qp->timer, jiffies + sysctl_ipfrag_time))

  atomic_inc(&qp->refcnt); 安装重组队列超时监视器

 atomic_inc(&qp->refcnt);

 if((qp->next = ipq_hash[hash]) != NULL)

  qp->next->pprev = &qp->next;

 ipq_hash[hash] = qp;

 qp->pprev = &ipq_hash[hash];

 ip_frag_nqueues++;

 write_unlock(&ipfrag_lock);

 return qp;

}

/*

 * Oops, a fragment queue timed out.  Kill it and send an ICMP reply.

 */

static void ip_expire(unsigned long arg)

{

 struct ipq *qp = (struct ipq *) arg;

 spin_lock(&qp->lock);

 if (qp->last_in & COMPLETE)

  goto out;

 ipq_kill(qp); 将重组队列从散列中删除

 IP_INC_STATS_BH(IpReasmTimeout);

 IP_INC_STATS_BH(IpReasmFails);

 if ((qp->last_in&FIRST_IN) && qp->fragments != NULL) {

  struct sk_buff *head = qp->fragments;

  /* Send an ICMP "Fragment Reassembly Timeout" message. */

  if ((head->dev = dev_get_by_index(qp->iif)) != NULL) {

   icmp_send(head, ICMP_TIME_EXCEEDED, ICMP_EXC_FRAGTIME, 0);

   dev_put(head->dev);

  }

 }

out:

 spin_unlock(&qp->lock);

 ipq_put(qp); 释放重组队列成员

}

/* Kill ipq entry. It is not destroyed immediately,

 * because caller (and someone more) holds reference count.

 */

static __inline__ void ipq_kill(struct ipq *ipq)

{

 if (del_timer(&ipq->timer))

  atomic_dec(&ipq->refcnt);

 if (!(ipq->last_in & COMPLETE)) {

  ipq_unlink(ipq);

  atomic_dec(&ipq->refcnt);

  ipq->last_in |= COMPLETE;

 }

}

static __inline__ void __ipq_unlink(struct ipq *qp)

{

 if(qp->next)

  qp->next->pprev = qp->pprev;

 *qp->pprev = qp->next;

 ip_frag_nqueues--;

}

static __inline__ void ipq_put(struct ipq *ipq)

{

 if (atomic_dec_and_test(&ipq->refcnt))

  ip_frag_destroy(ipq);

}

/* Complete destruction of ipq. */

static void ip_frag_destroy(struct ipq *qp)

{

 struct sk_buff *fp;

 BUG_TRAP(qp->last_in&COMPLETE);

 BUG_TRAP(del_timer(&qp->timer) == 0);

 /* Release all fragment data. */

 fp = qp->fragments;

 while (fp) {

  struct sk_buff *xp = fp->next;

  frag_kfree_skb(fp);

  fp = xp;

 }

 /* Finally, release the queue descriptor itself. */

 frag_free_queue(qp);

}

/* Memory Tracking Functions. */

extern __inline__ void frag_kfree_skb(struct sk_buff *skb)

{

 atomic_sub(skb->truesize, &ip_frag_mem);

 kfree_skb(skb);

}

extern __inline__ void frag_free_queue(struct ipq *qp)

{

 atomic_sub(sizeof(struct ipq), &ip_frag_mem);

 kfree(qp);

}

/* Memory limiting on fragments.  Evictor trashes the oldest 

 * fragment queue until we are back under the low threshold.

 */

static void ip_evictor(void) 重组队列驱逐器

{

 int i, progress;

 do {

  if (atomic_read(&ip_frag_mem) <= sysctl_ipfrag_low_thresh)

   return;

  progress = 0;

  /* FIXME: Make LRU queue of frag heads. -DaveM */

  for (i = 0; i < IPQ_HASHSZ; i++) {

   struct ipq *qp;

   if (ipq_hash[ i ] == NULL)

    continue;

   write_lock(&ipfrag_lock);

   if ((qp = ipq_hash[ i ]) != NULL) {

    /* find the oldest queue for this hash bucket */

    while (qp->next)

     qp = qp->next; 取每个散列的最后一项

    __ipq_unlink(qp);

    write_unlock(&ipfrag_lock);

    spin_lock(&qp->lock);

    if (del_timer(&qp->timer))

     atomic_dec(&qp->refcnt);

    qp->last_in |= COMPLETE;

    spin_unlock(&qp->lock);

    ipq_put(qp);

    IP_INC_STATS_BH(IpReasmFails);

    progress = 1;

    continue;

   }

   write_unlock(&ipfrag_lock);

  }

 } while (progress);

}

/* Add new segment to existing queue. */

static void ip_frag_queue(struct ipq *qp, struct sk_buff *skb)

{

 struct iphdr *iph = skb->nh.iph;

 struct sk_buff *prev, *next;

 int flags, offset;

 int ihl, end;

 if (qp->last_in & COMPLETE)

  goto err;

 offset = ntohs(iph->frag_off); 取片偏移量描述字

 flags = offset & ~IP_OFFSET; 取片标志

 offset &= IP_OFFSET; 求片偏移量

 offset <<= 3;  /* offset is in 8-byte chunks */

 ihl = iph->ihl * 4;

 /* Determine the position of this fragment. */

 end = offset + (ntohs(iph->tot_len) - ihl); 求该片段尾部的数据偏移量

 /* Is this the final fragment? */

 if ((flags & IP_MF) == 0) { 最后一片段

  /* If we already have some bits beyond end

   * or have different end, the segment is corrrupted.

   */

  if (end < qp->len || 最后一个片段的未尾小于队列内最大的未尾

      ((qp->last_in & LAST_IN) && end != qp->len))

   goto err;

  qp->last_in |= LAST_IN;

  qp->len = end; 设取队列最大未尾

 } else { 是中间某个片段

  if (end&7) { 如果片段尾部不在8字节上对齐

   end &= ~7; 

   if (skb->ip_summed != CHECKSUM_UNNECESSARY)

    skb->ip_summed = CHECKSUM_NONE; 不计算校验和

  }

  if (end > qp->len) { 该片段比队列内其它成员位置要大

   /* Some bits beyond end -> corruption. */

   if (qp->last_in & LAST_IN)

    goto err;

   qp->len = end;

  }

 }

 if (end == offset) 片段的数据区长度为零

  goto err;

 /* Point into the IP datagram 'data' part. */

 skb_pull(skb, (skb->nh.raw+ihl) - skb->data); 删除IP包的头部

 skb_trim(skb, end - offset); 去队尾部可能的衬垫

 /* Find out which fragments are in front and at the back of us

  * in the chain of fragments so far.  We must know where to put

  * this fragment, right?

  */

 prev = NULL; 扫描重组队列中的包片段, 取偏移大于或等于当前包偏移的前一成员作为插入位置

 for(next = qp->fragments; next != NULL; next = next->next) {

  if (FRAG_CB(next)->offset >= offset)

   break; /* bingo! */

  prev = next; 

 }

当前偏移的包

 /* We found where to put this one.  Check for overlap with

  * preceding fragment, and, if needed, align things so that

  * any overlaps are eliminated.

  */

 if (prev) {

  int i = (FRAG_CB(prev)->offset + prev->len) - offset; 求prev尾部与当前偏移之差

  if (i > 0) { 插入点成员尾部大于当前包开始, 说明当前包与前一包重叠

   offset += i; 当前包起点后移

   if (end <= offset)

    goto err;

   skb_pull(skb, i); 删除当前包前部i字节.

   if (skb->ip_summed != CHECKSUM_UNNECESSARY)

    skb->ip_summed = CHECKSUM_NONE;

  }

 }

 ; next是当前包的后一包

 while (next && FRAG_CB(next)->offset < end) { 后一包与当前包有重叠

  int i = end - FRAG_CB(next)->offset; /* overlap is 'i' bytes */

  if (i < next->len) { 当前包尾部小于后一包尾部

   /* Eat head of the next overlapped fragment

    * and leave the loop. The next ones cannot overlap.

    */

   FRAG_CB(next)->offset += i; 后一包起点后移

   skb_pull(next, i); 删除后一包i字节

   qp->meat -= i; meat为队列已容纳的总字节数

   if (next->ip_summed != CHECKSUM_UNNECESSARY)

    next->ip_summed = CHECKSUM_NONE;

   break;

  } else { 当前包尾部大于或等于后一包尾部, 则删除后一包

   struct sk_buff *free_it = next;

   /* Old fragmnet is completely overridden with

    * new one drop it.

    */

   next = next->next;

   if (prev)

    prev->next = next; 

   else 说明next包是队列首包

    qp->fragments = next; 

   qp->meat -= free_it->len;

   frag_kfree_skb(free_it);

  }

 }

 FRAG_CB(skb)->offset = offset; 在skb的cb[]块上记录当前包代表的数据位移

 /* Insert this fragment in the chain of fragments. */

 skb->next = next;

 if (prev)

  prev->next = skb;

 else

  qp->fragments = skb; 作为首包

 if (skb->dev)

  qp->iif = skb->dev->ifindex; 取包的输入设号号

 skb->dev = NULL;

 qp->meat += skb->len;

 atomic_add(skb->truesize, &ip_frag_mem); truesize为包描述结构与数据区总长

 if (offset == 0) 首包

  qp->last_in |= FIRST_IN; 

 return;

err:

 kfree_skb(skb);

}

/* Build a new IP datagram from all its fragments.

 *

 * FIXME: We copy here because we lack an effective way of handling lists

 * of bits on input. Until the new skb data handling is in I'm not going

 * to touch this with a bargepole. 

 */

static inline unsigned int csum_add(unsigned int csum, unsigned int addend)

{

 csum += addend;

 return csum + (csum < addend);

}

static struct sk_buff *ip_frag_reasm(struct ipq *qp, struct net_device *dev)

{

 struct sk_buff *skb;

 struct iphdr *iph;

 struct sk_buff *fp, *head = qp->fragments;

 int len;

 int ihlen;

 ipq_kill(qp); 将队列从散列中删除

 BUG_TRAP(head != NULL);

 BUG_TRAP(FRAG_CB(head)->offset == 0);

 /* Allocate a new buffer for the datagram. */

 ihlen = head->nh.iph->ihl*4; 取队列头IP包头长度

 len = ihlen + qp->len; 总长度

 if(len > 65535)

  goto out_oversize;

 skb = dev_alloc_skb(len);

 if (!skb)

  goto out_nomem;

 /* Fill in the basic details. */

 skb->mac.raw = skb->data;

 skb->nh.raw = skb->data;

 FRAG_CB(skb)->h = FRAG_CB(head)->h; 复制IP选项信息

 skb->ip_summed = head->ip_summed;

 skb->csum = 0;

 /* Copy the original IP headers into the new buffer. */

 memcpy(skb_put(skb, ihlen), head->nh.iph, ihlen);

 /* Copy the data portions of all fragments into the new buffer. */

 for (fp=head; fp; fp = fp->next) {

  memcpy(skb_put(skb, fp->len), fp->data, fp->len);

  if (skb->ip_summed != fp->ip_summed)

   skb->ip_summed = CHECKSUM_NONE;

  else if (skb->ip_summed == CHECKSUM_HW)

   skb->csum = csum_add(skb->csum, fp->csum);

 }

 skb->dst = dst_clone(head->dst);

 skb->pkt_type = head->pkt_type;

 skb->protocol = head->protocol;

 skb->dev = dev;

 /*

 *  Clearly bogus, because security markings of the individual

 *  fragments should have been checked for consistency before

 *  gluing, and intermediate coalescing of fragments may have

 *  taken place in ip_defrag() before ip_glue() ever got called.

 *  If we're not going to do the consistency checking, we might

 *  as well take the value associated with the first fragment.

 * --rct

 */

 skb->security = head->security;

#ifdef CONFIG_NETFILTER

 /* Connection association is same as fragment (if any). */

 skb->nfct = head->nfct;

 nf_conntrack_get(skb->nfct);

#ifdef CONFIG_NETFILTER_DEBUG

 skb->nf_debug = head->nf_debug;

#endif

#endif

 /* Done with all fragments. Fixup the new IP header. */

 iph = skb->nh.iph;

 iph->frag_off = 0;

 iph->tot_len = htons(len);

 IP_INC_STATS_BH(IpReasmOKs);

 return skb;

out_nomem:

  NETDEBUG(printk(KERN_ERR 

   "IP: queue_glue: no memory for gluing queue %p/n",

   qp));

 goto out_fail;

out_oversize:

 if (net_ratelimit())

  printk(KERN_INFO

   "Oversized IP packet from %d.%d.%d.%d./n",

   NIPQUAD(qp->saddr));

out_fail:

 IP_INC_STATS_BH(IpReasmFails);

 return NULL;

}