📄 2_4网络代码阅读笔记(2) - china linux forum.htm
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<P class=post>发信人: ysqcn (岁月无声), 信区: llkm <BR>标 题: 2.4网络代码阅读笔记(2)
<BR>发信站: UNIX编程 (2001年06月12日21:38:25 星期二), 站内信件
<BR><BR>接下来就分析sys_bind了,这个函数相对简单点,好,让我们开始吧: <BR><BR>asmlinkage long
sys_bind(int fd, struct sockaddr *umyaddr, int addrlen) <BR>{
<BR>struct socket *sock; <BR>char address[MAX_SOCK_ADDR]; <BR>int
err; <BR>if((sock = sockfd_lookup(fd,&err))!=NULL) <BR>{
<BR>if((err=move_addr_to_kernel(umyaddr,addrlen,address))>=0)
<BR>>>>>tcp,udp,raw这里均调用inet_bind,而packet调用了packet_bind
<BR>>>>>这里只分析前者,后者在讨论af_packet.c的时候在说 <BR>err =
sock->ops->bind(sock, (struct sockaddr *)address, addrlen);
<BR>>>>>将sock相联系的file的应用计数减一,如果为0,则释放,此时就引起一连串的操作
<BR>sockfd_put(sock); <BR>} <BR>return err; <BR>}
<BR><BR>对sockfd_lookup没有什么好说的了,current.file--->file_struct.fd[套接字描述符]
<BR>---->file.inde----->inode.socket_i,这样一层层的指下来,就有套接字描述符得到了
<BR>struct socket相应的结构了.
<BR>move_addr_to_kernel就是用copy_from_user拷贝用户给出的地址到字符数组address中
<BR>去,sock->ops->bind就调用inet_stream_ops中的函数了,到了BSD层了,为inet_bind:
<BR><BR>static int inet_bind(struct socket *sock, struct sockaddr
*uaddr, int addr_len) <BR>{ <BR>struct sockaddr_in *addr=(struct
sockaddr_in *)uaddr; <BR>>>>>有struct socket得到相应的struct
sock <BR>struct sock *sk=sock->sk; <BR>unsigned short snum;
<BR>int chk_addr_ret; <BR>int err; <BR>/* If the socket has its own
bind function then use it. (RAW) */ <BR>if(sk->prot->bind)
<BR>>>>>如果INET层有自己的bind就调用它,只有raw有,tcp和udp都没有,为NULL
<BR>>>>>由于udp和tcp的BSD层的bind都是这个inet_bind,而raw又使用udp的BSD层的
<BR>>>>>例程,故对raw的bind也走到这儿了,不过在这儿raw就与tcp和udp分岔了
<BR>return sk->prot->bind(sk, uaddr, addr_len); <BR>if
(addr_len < sizeof(struct sockaddr_in)) <BR>return -EINVAL;
<BR>>>>>得到bind的地址类型,可以有下面的几种:RTN_LOCAL,RTN_MULTICAST,
<BR>>>>>RTN_BROADCAST,RTN_UNICAST <BR>chk_addr_ret =
inet_addr_type(addr->sin_addr.s_addr); <BR>......................
<BR>snum = ntohs(addr->sin_port);
<BR>>>>>只有超级用户才能绑定1024以下的端口 <BR>if (snum && snum
< PROT_SOCK && !capable(CAP_NET_BIND_SERVICE)) <BR>return
-EACCES; <BR>lock_sock(sk); <BR>/* Check these errors (active
socket, double bind). */ <BR>err = -EINVAL; <BR>if ((sk->state !=
TCP_CLOSE) || <BR>>>>>TCP和UDP在创建套接字的时候sk->num并没有赋值，为零
<BR>(sk->num != 0)) <BR>goto out;
<BR>>>>>rcv_saddr用来从hash表中查找，saddr在发包时使用
<BR>sk->rcv_saddr = sk->saddr = addr->sin_addr.s_addr;
<BR>if (chk_addr_ret == RTN_MULTICAST || chk_addr_ret ==
RTN_BROADCAST) <BR>>>>>对广播地址和组播为0 <BR>sk->saddr = 0;
/* Use device */ <BR>/* Make sure we are allowed to bind here. */
<BR>>>>>这里调用了tcp_v4_get_port,如果snum为0,则查找一个合适的端口
<BR>>>>>否则检查端口是不是没有被使用,将最后得到的合法端口赋值给sk->num
<BR>>>>>下面详细分析 <BR>if (sk->prot->get_port(sk,
snum) != 0) { <BR>sk->saddr = sk->rcv_saddr = 0; <BR>err =
-EADDRINUSE; <BR>goto out; <BR>} <BR>if (sk->rcv_saddr)
<BR>sk->userlocks |= SOCK_BINDADDR_LOCK; <BR>if (snum)
<BR>sk->userlocks |= SOCK_BINDPORT_LOCK;
<BR>>>>>这几个赋值,意识很清楚 <BR>sk->sport =
htons(sk->num); <BR>sk->daddr = 0; <BR>sk->dport = 0;
<BR>>>>>sk->dst_cache = NULL; <BR>sk_dst_reset(sk);
<BR>err = 0; <BR>out: <BR>release_sock(sk); <BR>return err; <BR>}
<BR><BR><BR>下面是tcp_v4_get_port了,看看linux是怎样管理端口的: <BR>/* Obtain a
reference to a local port for the given sock, <BR>* if snum is zero
it means select any available local port. <BR>*/ <BR>static int
tcp_v4_get_port(struct sock *sk, unsigned short snum) <BR>{
<BR>>>>>定义了一个桶 <BR>struct tcp_bind_hashbucket *head;
<BR>struct tcp_bind_bucket *tb; <BR>int ret; <BR>local_bh_disable();
<BR>>>>>给定的端口为0,系统分配一个 <BR>if (snum == 0) {
<BR>>>>>本地端口范围,从1024到4999,可以通过sysctl改变. <BR>int low =
sysctl_local_port_range[0]; <BR>int high =
sysctl_local_port_range[1]; <BR>>>>>有多少端口可以用来分配 <BR>int
remaining = (high - low) + 1; <BR>int rover;
<BR>spin_lock(&tcp_portalloc_lock);
<BR>>>>>tcp_port_rover=1023 <BR>rover = tcp_port_rover;
<BR>do { rover++; <BR>if ((rover < low) || (rover > high))
<BR>rover = low;
<BR>>>>>tcp_bhash是已经绑定的端口的hash表,遍历这个表,如果发现有
<BR>>>>>重复的端口,则遍历下一个. <BR>head =
&tcp_bhash[tcp_bhashfn(rover)];
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