6.3.11.12. bind

bind函数原型为 int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen); 其中sockfd为套接字的文件描述符，addr指向存放地址信息的结构体的首地址，addrlen存放地址信息的结构体的大小。

6.3.11.12.1. 内核实现

bind的内核实现如下：

SYSCALL_DEFINE3(bind, int, fd, struct sockaddr __user *, umyaddr, int, addrlen)
{
    struct socket *sock;
    struct sockaddr_storage address;
    int err, fput_needed;
    // 根据fd获取相应的socket结构体
    sock = sockfd_lookup_light(fd, &err, &fput_needed);
    if (sock) {
        err = move_addr_to_kernel(umyaddr, addrlen, &address);
        if (err >= 0) {
            err = security_socket_bind(sock, (struct sockaddr *)&address,
                           addrlen);//安全相关，暂不关注
            if (!err)
                err = sock->ops->bind(sock, (struct sockaddr *)
                              &address, addrlen);
        }
        fput_light(sock->file, fput_needed);
    }
    return err;
}

其中sock->ops->bind指向的是inet_bind：

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;
    int err;

    if (sk->sk_prot->bind) {
        err = sk->sk_prot->bind(sk, uaddr, addr_len);
        goto out;
    }
    err = -EINVAL;
    if (addr_len < sizeof(struct sockaddr_in))
        goto out;

    if (addr->sin_family != AF_INET) {
        /* Compatibility games : accept AF_UNSPEC (mapped to AF_INET)
         * only if s_addr is INADDR_ANY.
         */
        err = -EAFNOSUPPORT;
        if (addr->sin_family != AF_UNSPEC ||
            addr->sin_addr.s_addr != htonl(INADDR_ANY))
            goto out;
    }

    chk_addr_ret = inet_addr_type(net, addr->sin_addr.s_addr);

    err = -EADDRNOTAVAIL;
    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;

    snum = ntohs(addr->sin_port);
    err = -EACCES;

    // PROT_SOCK的值为1024，非root用户不能使用小于1024的端口号
    if (snum && snum < PROT_SOCK &&
        !ns_capable(net->user_ns, CAP_NET_BIND_SERVICE))
        goto out;

    lock_sock(sk);

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

    if (sk->sk_state != TCP_CLOSE || inet->inet_num)
        goto out_release_sock;

    inet->inet_rcv_saddr = inet->inet_saddr = addr->sin_addr.s_addr;
    if (chk_addr_ret == RTN_MULTICAST || chk_addr_ret == RTN_BROADCAST)
        inet->inet_saddr = 0;  /* Use device */

    if ((snum || !inet->bind_address_no_port) &&
        sk->sk_prot->get_port(sk, snum)) {
        // 绑定失败，绑定的地址已经有人在使用
        inet->inet_saddr = inet->inet_rcv_saddr = 0;
        err = -EADDRINUSE;
        goto out_release_sock;
    }

    if (inet->inet_rcv_saddr)
        sk->sk_userlocks |= SOCK_BINDADDR_LOCK;
    if (snum)
        sk->sk_userlocks |= SOCK_BINDPORT_LOCK;
    inet->inet_sport = htons(inet->inet_num);
    inet->inet_daddr = 0;
    inet->inet_dport = 0;
    sk_dst_reset(sk);
    err = 0;
out_release_sock:
    release_sock(sk);
out:
    return err;
}