bind系统调用

在socket创建成功之后，调用bind函数以完成对指定地址和端口的绑定工作；

下面详细分析bind相关代码；

 /*
  *    Bind a name to a socket. Nothing much to do here since it's
  *    the protocol's responsibility to handle the local address.
  *
  *    We move the socket address to kernel space before we call
  *    the protocol layer (having also checked the address is ok).
  */

 SYSCALL_DEFINE3(bind, int, fd, struct sockaddr __user *, umyaddr, int, addrlen)
 {
     struct socket *sock;
     struct sockaddr_storage address;
     int err, fput_needed;

     /* 获取socket ，fput_need标识是否需要减少文件引用计数*/
     sock = sockfd_lookup_light(fd, &err, &fput_needed);
     if (sock) {
         /* 将用户空间地址复制到内核空间 */
         err = move_addr_to_kernel(umyaddr, addrlen, &address);
         if (err >= ) {
             /* 安全模块的bind检查 */
             err = security_socket_bind(sock,
                            (struct sockaddr *)&address,
                            addrlen);
             if (!err)
                 /* 调用socket的bind操作 */
                 err = sock->ops->bind(sock,
                               (struct sockaddr *)
                               &address, addrlen);
         }

         /* 根据fput_needed决定是否减少引用计数 */
         fput_light(sock->file, fput_needed);
     }
     return err;
 }

首先根据传入的socket描述符来获取socket结构；

 static struct socket *sockfd_lookup_light(int fd, int *err, int *fput_needed)
 {
     /* 获取fd结构 */
     struct fd f = fdget(fd);
     struct socket *sock;

     *err = -EBADF;
     if (f.file) {
         /* 从文件的私有数据中获取socket */
         sock = sock_from_file(f.file, err);
         if (likely(sock)) {
             /* 设置是否需要减少引用计数的标志 */
             *fput_needed = f.flags;
             return sock;
         }
         fdput(f);
     }
     return NULL;
 }

之后，将用户空间的地址拷贝到内核空间；

 /*
  * Support routines.
  * Move socket addresses back and forth across the kernel/user
  * divide and look after the messy bits.
  */

 /**
  *    move_addr_to_kernel    -    copy a socket address into kernel space
  *    @uaddr: Address in user space
  *    @kaddr: Address in kernel space
  *    @ulen: Length in user space
  *
  *    The address is copied into kernel space. If the provided address is
  *    too long an error code of -EINVAL is returned. If the copy gives
  *    invalid addresses -EFAULT is returned. On a success 0 is returned.
  */

 /* 复制socket地址到内核空间 */
 int move_addr_to_kernel(void __user *uaddr, int ulen, struct sockaddr_storage *kaddr)
 {
     /* 长度检查 */
     if (ulen <  || ulen > sizeof(struct sockaddr_storage))
         return -EINVAL;
     if (ulen == )
         return ;

     /* 从用户空间拷贝数据 */
     if (copy_from_user(kaddr, uaddr, ulen))
         return -EFAULT;

     /* 审计信息 */
     return audit_sockaddr(ulen, kaddr);
 }

最后，来看最核心的函数，sock->ops->bind调用inet_bind(为什么? 具体可以参考本博套接字调用关系那片文章)，inet_bind将会进行一些列检查之后，调用传输层的sk->sk_prot->get_port函数来执行更详细的绑定工作，比如tcp会调用inet_csk_get_port函数；

/* 地址绑定 */
int inet_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
{
    struct sockaddr_in *addr = (struct sockaddr_in *)uaddr;
    struct sock *sk = sock->sk;
    struct inet_sock *inet = inet_sk(sk);
    struct net *net = sock_net(sk);
    unsigned short snum;
    int chk_addr_ret;
    u32 tb_id = RT_TABLE_LOCAL;
    int err;

    /* If the socket has its own bind function then use it. (RAW) */
    /*
        如果传输控制块有自己的bind操作则调用，
        目前只有raw实现了自己的bind
    */
    if (sk->sk_prot->bind) {
        err = sk->sk_prot->bind(sk, uaddr, addr_len);
        goto out;
    }

    err = -EINVAL;
    /* 地址长度错误 */
    if (addr_len < sizeof(struct sockaddr_in))
        goto out;

    /* 如果不是AF_INET协议族 */
    if (addr->sin_family != AF_INET) {
        /* Compatibility games : accept AF_UNSPEC (mapped to AF_INET)
         * only if s_addr is INADDR_ANY.
         */
        err = -EAFNOSUPPORT;

        /* 接受AF_UNSPEC && s_addr=htonl(INADDR_ANY)的情况 */
        if (addr->sin_family != AF_UNSPEC ||
            addr->sin_addr.s_addr != htonl(INADDR_ANY))
            goto out;
    }

    tb_id = l3mdev_fib_table_by_index(net, sk->sk_bound_dev_if) ? : tb_id;
    chk_addr_ret = inet_addr_type_table(net, addr->sin_addr.s_addr, tb_id);

    /* Not specified by any standard per-se, however it breaks too
     * many applications when removed.  It is unfortunate since
     * allowing applications to make a non-local bind solves
     * several problems with systems using dynamic addressing.
     * (ie. your servers still start up even if your ISDN link
     *  is temporarily down)
     */
    err = -EADDRNOTAVAIL;

    /* 合法性检查 */
    if (!net->ipv4.sysctl_ip_nonlocal_bind &&
        !(inet->freebind || inet->transparent) &&
        addr->sin_addr.s_addr != htonl(INADDR_ANY) &&
        chk_addr_ret != RTN_LOCAL &&
        chk_addr_ret != RTN_MULTICAST &&
        chk_addr_ret != RTN_BROADCAST)
        goto out;

    /* 源端口 */
    snum = ntohs(addr->sin_port);
    err = -EACCES;

    /* 绑定特权端口的权限检查 */
    if (snum && snum < inet_prot_sock(net) &&
        !ns_capable(net->user_ns, CAP_NET_BIND_SERVICE))
        goto out;

    /*      We keep a pair of addresses. rcv_saddr is the one
     *      used by hash lookups, and saddr is used for transmit.
     *
     *      In the BSD API these are the same except where it
     *      would be illegal to use them (multicast/broadcast) in
     *      which case the sending device address is used.
     */
    lock_sock(sk);

    /* Check these errors (active socket, double bind). */
    err = -EINVAL;

    /* 传输控制块的状态不是CLOSE || 存在本地端口 */
    if (sk->sk_state != TCP_CLOSE || inet->inet_num)
        goto out_release_sock;

    /* 设置源地址rcv_addr用作hash查找，saddr用作传输 */
    inet->inet_rcv_saddr = inet->inet_saddr = addr->sin_addr.s_addr;

    /* 组播或者广播，使用设备地址 */
    if (chk_addr_ret == RTN_MULTICAST || chk_addr_ret == RTN_BROADCAST)
        inet->inet_saddr = ;  /* Use device */

    /* Make sure we are allowed to bind here. */

    /*
        端口不为0，或者端口为0允许绑定
        则使用协议的具体获取端口函数绑定端口
    */
    if ((snum || !inet->bind_address_no_port) &&
        sk->sk_prot->get_port(sk, snum)) {

        /* 绑定失败 */
        inet->inet_saddr = inet->inet_rcv_saddr = ;

        /* 端口在使用中 */
        err = -EADDRINUSE;
        goto out_release_sock;
    }

    /* 传输控制块已经绑定本地地址或端口标志 */
    if (inet->inet_rcv_saddr)
        sk->sk_userlocks |= SOCK_BINDADDR_LOCK;
    if (snum)
        sk->sk_userlocks |= SOCK_BINDPORT_LOCK;

    /* 设置源端口 */
    inet->inet_sport = htons(inet->inet_num);

    /* 设置目的地址和端口默认值 */
    inet->inet_daddr = ;
    inet->inet_dport = ;

    /* 设置路由默认值 */
    sk_dst_reset(sk);
    err = ;
out_release_sock:
    release_sock(sk);
out:
    return err;
}
EXPORT_SYMBOL(inet_bind);

TCP bind()系统调用实现部分，请阅读<TCP层bind系统调用的实现分析>