AF_PACKET套接字解密

 原文出处：

http://blog.chinaunix.net/uid-22362479-id-3220107.html

http://blog.chinaunix.net/uid-22362479-id-3220136.html

 

使用socket(AF_PACKET, SOCK_RAW, ETH_P_ALL)创建的套接字到底为何于众不同，今日追踪了一下。使用Linux 3.2.5版内核

net/socket.c

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SYSCALL_DEFINE3(socket, int, family, int, type, int, protocol)
{
......
    retval = sock_create(family,  type, protocol, &sock);
......
}

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int sock_create(int family, int type, int protocol, struct socket **res)
{
    return __sock_create(current->nsproxy->net_ns, family, type, protocol, res, 0);
}

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int __sock_create(struct net *net, int family, int type, int protocol,
 struct socket **res, int kern)
{
......
    pf = rcu_dereference(net_families[family]);
......
   err = pf->create(net, sock, protocol, kern);
......
}

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static const struct net_proto_family __rcu *net_families[NPROTO] __read_mostly;

int sock_register(const struct net_proto_family *ops)
{
......
    if (rcu_dereference_protected(net_families[ops->family],
                      lockdep_is_held(&net_family_lock)))
        err = -EEXIST;
    else {
rcu_assign_pointer(net_families[ops->family], ops);
        err = 0;
    }
......
}

net/packet/af_packet.c

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static const struct net_proto_family packet_family_ops = {
    .family =    PF_PACKET,
    .create =    packet_create,
    .owner    =    THIS_MODULE,
};

static int __init packet_init(void)
{
......
    sock_register(&packet_family_ops);
......
}

module_init(packet_init);

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static int packet_create(struct net *net, struct socket *sock, int protocol,
             int kern)
{
    struct sock *sk;
    struct packet_sock *po;
    __be16 proto = (__force __be16)protocol; /* weird, but documented */
......
    sk = sk_alloc(net, PF_PACKET, GFP_KERNEL, &packet_proto);
......
    sock->ops = &packet_ops;
......
    po = pkt_sk(sk);    sk->sk_family = PF_PACKET;
    po->num = proto;
......
    po->prot_hook.func = packet_rcv;
......
    po->prot_hook.af_packet_priv = sk;
    if (proto) {
po->prot_hook.type = proto;
register_prot_hook(sk);
    }
......
}

AF_PACKET套接字的功能来源于prot_hook，其本身是struct packet_type类型:
1：type成员设定为了socket()传递的参数（这里是ETH_P_ALL)
2：过滤得到的包的处理函数保存于func成员，这里被设定为 packet_rcv()
3：dev成员用于对net_device的过滤，可以在bind()中指定

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static int packet_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
{
......
if(sll->sll_ifindex){
err =-ENODEV;
        dev = dev_get_by_index(sock_net(sk), sll->sll_ifindex);
if(dev ==NULL)
            goto out;
}
err =packet_do_bind(sk, dev, sll->sll_protocol ?: pkt_sk(sk)->num);
......
}

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static intpacket_do_bind(struct sock *sk, struct net_device *dev, __be16 protocol)
{
    struct packet_sock *po = pkt_sk(sk);
......
    po->prot_hook.dev = dev;

    po->ifindex = dev ? dev->ifindex : 0;
......
if(!dev ||(dev->flags & IFF_UP)){
register_prot_hook(sk);
......
}

可见在bind()中若指定了绑定的net_device同样会触发prot_hook的注册动作。

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static void register_prot_hook(struct sock *sk)
{
    struct packet_sock *po = pkt_sk(sk);
if(!po->running){
if(po->fanout)
            __fanout_link(sk, po);
else
            dev_add_pack(&po->prot_hook);
        sock_hold(sk);
        po->running = 1;
}
}

net/core/dev.c

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void dev_add_pack(struct packet_type *pt)
{
    struct list_head *head =ptype_head(pt);
    spin_lock(&ptype_lock);
    list_add_rcu(&pt->list, head);
    spin_unlock(&ptype_lock);
}

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static struct list_head ptype_all __read_mostly;    /* Taps */
static inline struct list_head *ptype_head(const struct packet_type *pt)
{
if(pt->type == htons(ETH_P_ALL))
        return &ptype_all;
else
        return &ptype_base[ntohs(pt->type)& PTYPE_HASH_MASK];
}

历尽千辛万苦，终于知道AF_PACKET套接字把自己的prot_hook挂到了ptype_all链表上或ptype_base链表上。当AF_PACKET套接字注册了prot_hook后，怎样进行监听呢，先来看发送：

当协议栈准备将数据交给net_device发送时，它将调用dev_queue_xmit():

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int dev_queue_xmit(struct sk_buff *skb)
{
    struct net_device *dev = skb->dev;
......
                    rc = dev_hard_start_xmit(skb, dev, txq);
......
}

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int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
            struct netdev_queue *txq)
{
......
if(!list_empty(&ptype_all))
            dev_queue_xmit_nit(skb, dev);
......
}

由于AF_PACKET套接字注册了prot_hook，将导致dev_queue_xmit_nit()被调用：

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static void dev_queue_xmit_nit(struct sk_buff *skb, struct net_device *dev)
{
    struct packet_type *ptype;
    struct sk_buff *skb2 =NULL;
    struct packet_type *pt_prev =NULL;

    rcu_read_lock();
    list_for_each_entry_rcu(ptype,&ptype_all, list){
/* Never send packets back to the socket
* they originated from - MvS (miquels@drinkel.ow.org)
*/
if((ptype->dev == dev ||!ptype->dev)&&
(ptype->af_packet_priv ==NULL||
(struct sock *)ptype->af_packet_priv != skb->sk)){
if(pt_prev){
                deliver_skb(skb2, pt_prev, skb->dev);
                pt_prev = ptype;
                continue;
}

            skb2 = skb_clone(skb, GFP_ATOMIC);
if(!skb2)
                break;
......
}
}
if(pt_prev)
        pt_prev->func(skb2, skb->dev, pt_prev, skb->dev);
    rcu_read_unlock();
}

在遍历ptype_all链表时，这里有几点需要着重说明：

1：对于发送的包过滤条件有：
    1). net_device是否是prot_hook指定的dev（NULL代表全部匹配)。
    2). ptype->af_packet_priv在packet_creat()中被设定为自己，故自己发送的包不会被监听。

2：遍历ptype_all时，第一次会复制skb；只有ptype_all中不止1个entry时，将调用deliver_skb()。

3：当退出遍历时，将调用prot_hook的func成员，即packet_rcv()。

4: deliver_skb()仅仅是在调用prot_hook的func成员前增加skb的引用计算数。

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static int packet_rcv(struct sk_buff *skb, struct net_device *dev,
              struct packet_type *pt, struct net_device *orig_dev)
{
......
    __skb_queue_tail(&sk->sk_receive_queue, skb);
......
}

经过packet_rcv()，发送的数据包被加入到了AF_PACKET套接字的接收队列，等待我们的读取。。。一切就是这么简单！