IP包碎片重组过程

playmud

            
(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 !
"));
        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) 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 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 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 ip_summed != CHECKSUM_UNNECESSARY)
                                skb->ip_summed = CHECKSUM_NONE;
                }
        }
        ; next是当前包的后一包
        while (next && FRAG_CB(next)->offset offset; /* overlap is 'i' bytes */
                if (i 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 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
",
                        qp));
        goto out_fail;
out_oversize:
        if (net_ratelimit())
                printk(KERN_INFO
                        "Oversized IP packet from %d.%d.%d.%d.
",
                        NIPQUAD(qp->saddr));
out_fail:
        IP_INC_STATS_BH(IpReasmFails);
        return NULL;
}
Edited by lucian_yao on 05/17/01 10:40
AM.


本文来自ChinaUnix博客，如果查看原文请点：http://blog.chinaunix.net/u/1574/showart_39241.html