本文分析基于Linux Kernel 3.2.1

原创作品，转载请标明http://blog.csdn.net/yming0221/article/details/7996528

更多请查看专栏http://blog.csdn.net/column/details/linux-kernel-net.html

作者：闫明

1、套接字的绑定

创建完套接字服务器端会在应用层使用bind函数进行套接字的绑定，这时会产生系统调用，sys_bind内核函数进行套接字。

系统调用函数的具体实现

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

    sock = sockfd_lookup_light(fd, &err, &fput_needed);
    if (sock) {
        err = move_addr_to_kernel(umyaddr, addrlen, (struct sockaddr *)&address);
        if (err >= 0) {
            err = security_socket_bind(sock,
                           (struct sockaddr *)&address,
                           addrlen);
            if (!err)
                err = sock->ops->bind(sock,
                              (struct sockaddr *)
                              &address, addrlen);
        }
        fput_light(sock->file, fput_needed);
    }
    return err;
}

首先调用函数sockfd_lookup_light()函数通过文件描述符来查找对应的套接字sock。

static struct socket *sockfd_lookup_light(int fd, int *err, int *fput_needed)
{
    struct file *file;
    struct socket *sock;

    *err = -EBADF;
    file = fget_light(fd, fput_needed);
    if (file) {
        sock = sock_from_file(file, err);
        if (sock)
            return sock;
        fput_light(file, *fput_needed);
    }
    return NULL;
}

上面函数中先调用fget_light函数通过文件描述符返回对应的文件结构，然后调用函数sock_from_file函数返回该文件对应的套接字结构体地址，它存储在file->private_data属性中。

再回到sys_bind函数，在返回了对应的套接字结构之后，调用move_addr_to_kernel将用户地址空间的socket拷贝到内核空间。

然后调用INET协议族的操作集中bind函数inet_bind函数将socket地址（内核空间）和socket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);
    unsigned short snum;
    int chk_addr_ret;
    int err;

    //RAW类型套接字若有自己的bind函数，则使用之
    if (sk->sk_prot->bind) {
        err = sk->sk_prot->bind(sk, uaddr, addr_len);
        goto out;
    }
    err = -EINVAL;
    .....................
        //地址合法性检查
    chk_addr_ret = inet_addr_type(sock_net(sk), addr->sin_addr.s_addr);

    /* 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;
    if (!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;
    if (snum && snum < PROT_SOCK && !capable(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;
    if (sk->sk_state != TCP_CLOSE || inet->inet_num)//如果sk的状态是CLOSE或者本地端口已经被绑定
        goto out_release_sock;

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

    /* Make sure we are allowed to bind here. */
    if (sk->sk_prot->get_port(sk, snum)) {
        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);//设置源端口号，标明该端口已经被占用
    inet->inet_daddr = 0;
    inet->inet_dport = 0;
    sk_dst_reset(sk);
    err = 0;
out_release_sock:
    release_sock(sk);
out:
    return err;
}

这样套接字绑定结束。

2、套接字的监听

SYSCALL_DEFINE2(listen, int, fd, int, backlog)
{
    struct socket *sock;
    int err, fput_needed;
    int somaxconn;

    sock = sockfd_lookup_light(fd, &err, &fput_needed);
    if (sock) {
        ......................

        err = security_socket_listen(sock, backlog);
        if (!err)
            err = sock->ops->listen(sock, backlog);

        fput_light(sock->file, fput_needed);
    }
    return err;
}

该函数先通过文件描述符查找到对应的套接字结构，然后调用inet_listen函数对将套接字sk的状态设置为TCP_LISTEN。

int inet_listen(struct socket *sock, int backlog)
{
    struct sock *sk = sock->sk;
    unsigned char old_state;
    int err;
    lock_sock(sk);

    err = -EINVAL;
    if (sock->state != SS_UNCONNECTED || sock->type != SOCK_STREAM)
        goto out;

    old_state = sk->sk_state;
    if (!((1 << old_state) & (TCPF_CLOSE | TCPF_LISTEN)))
        goto out;

    if (old_state != TCP_LISTEN) {
        err = inet_csk_listen_start(sk, backlog);//该函数将sk的状态设置为TCP_LISTEN
        if (err)
            goto out;
    }
    sk->sk_max_ack_backlog = backlog;
    err = 0;
out:
    release_sock(sk);
    return err;
}

3、套接字的连接和接受连接

3.1、申请连接

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

    sock = sockfd_lookup_light(fd, &err, &fput_needed);
    if (!sock)
        goto out;
    err = move_addr_to_kernel(uservaddr, addrlen, (struct sockaddr *)&address);
    if (err < 0)
        goto out_put;

    err =
        security_socket_connect(sock, (struct sockaddr *)&address, addrlen);
    if (err)
        goto out_put;

    err = sock->ops->connect(sock, (struct sockaddr *)&address, addrlen,
                 sock->file->f_flags);
out_put:
    fput_light(sock->file, fput_needed);
out:
    return err;
}

还是先调用sockfd_lookup_light函数获得socket指针，然后将用户空间地址移到内核空间，然后调用函数 inet_stream_connect函数。

int inet_stream_connect(struct socket *sock, struct sockaddr *uaddr,
            int addr_len, int flags)
{
    struct sock *sk = sock->sk;
    int err;
    long timeo;

    if (addr_len < sizeof(uaddr->sa_family))
        return -EINVAL;

    lock_sock(sk);

    ......................

    switch (sock->state) {
    default:
        err = -EINVAL;
        goto out;
    case SS_CONNECTED:
        err = -EISCONN;
        goto out;
    case SS_CONNECTING:
        err = -EALREADY;
        /* Fall out of switch with err, set for this state */
        break;
    case SS_UNCONNECTED:
        err = -EISCONN;
        if (sk->sk_state != TCP_CLOSE)
            goto out;

        err = sk->sk_prot->connect(sk, uaddr, addr_len);
        if (err < 0)
            goto out;

        sock->state = SS_CONNECTING;

        err = -EINPROGRESS;
        break;
    }

    timeo = sock_sndtimeo(sk, flags & O_NONBLOCK);

    if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
        /* Error code is set above */
        if (!timeo || !inet_wait_for_connect(sk, timeo))
            goto out;

        err = sock_intr_errno(timeo);
        if (signal_pending(current))
            goto out;
    }

    /* Connection was closed by RST, timeout, ICMP error
     * or another process disconnected us.
     */
    if (sk->sk_state == TCP_CLOSE)
        goto sock_error;

    sock->state = SS_CONNECTED;
    err = 0;
out:
    release_sock(sk);
    return err;

sock_error:
    err = sock_error(sk) ? : -ECONNABORTED;
    sock->state = SS_UNCONNECTED;
    if (sk->sk_prot->disconnect(sk, flags))
        sock->state = SS_DISCONNECTING;
    goto out;
}

调用函数tcp_v4_connect函数后然后将sock的状态置SS_CONNECTING。

int tcp_v4_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
{
    struct sockaddr_in *usin = (struct sockaddr_in *)uaddr;
    struct inet_sock *inet = inet_sk(sk);
    struct tcp_sock *tp = tcp_sk(sk);
    __be16 orig_sport, orig_dport;
    __be32 daddr, nexthop;
    struct flowi4 *fl4;
    struct rtable *rt;
    int err;
    struct ip_options_rcu *inet_opt;
        //合法性检查
    if (addr_len < sizeof(struct sockaddr_in))
        return -EINVAL;

    if (usin->sin_family != AF_INET)
        return -EAFNOSUPPORT;
        //记录吓一跳地址和目的地址
    nexthop = daddr = usin->sin_addr.s_addr;
    inet_opt = rcu_dereference_protected(inet->inet_opt,
                         sock_owned_by_user(sk));
    if (inet_opt && inet_opt->opt.srr) {
        if (!daddr)
            return -EINVAL;
        nexthop = inet_opt->opt.faddr;
    }
        //本地端口和目的端口
    orig_sport = inet->inet_sport;
    orig_dport = usin->sin_port;
    fl4 = &inet->cork.fl.u.ip4;
    rt = ip_route_connect(fl4, nexthop, inet->inet_saddr,
                  RT_CONN_FLAGS(sk), sk->sk_bound_dev_if,
                  IPPROTO_TCP,
                  orig_sport, orig_dport, sk, true);//维护路由表
    if (IS_ERR(rt)) {
        err = PTR_ERR(rt);
        if (err == -ENETUNREACH)
            IP_INC_STATS_BH(sock_net(sk), IPSTATS_MIB_OUTNOROUTES);
        return err;
    }
        //处理多播或广播
    if (rt->rt_flags & (RTCF_MULTICAST | RTCF_BROADCAST)) {
        ip_rt_put(rt);
        return -ENETUNREACH;
    }

    if (!inet_opt || !inet_opt->opt.srr)
        daddr = fl4->daddr;

    if (!inet->inet_saddr)
        inet->inet_saddr = fl4->saddr;
    inet->inet_rcv_saddr = inet->inet_saddr;

    if (tp->rx_opt.ts_recent_stamp && inet->inet_daddr != daddr) {
        /* Reset inherited state */
        tp->rx_opt.ts_recent       = 0;
        tp->rx_opt.ts_recent_stamp = 0;
        tp->write_seq          = 0;
    }

    if (tcp_death_row.sysctl_tw_recycle &&
        !tp->rx_opt.ts_recent_stamp && fl4->daddr == daddr) {
        struct inet_peer *peer = rt_get_peer(rt, fl4->daddr);
        /*
         * VJ's idea. We save last timestamp seen from
         * the destination in peer table, when entering state
         * TIME-WAIT * and initialize rx_opt.ts_recent from it,
         * when trying new connection.
         */
        if (peer) {
            inet_peer_refcheck(peer);
            if ((u32)get_seconds() - peer->tcp_ts_stamp <= TCP_PAWS_MSL) {
                tp->rx_opt.ts_recent_stamp = peer->tcp_ts_stamp;
                tp->rx_opt.ts_recent = peer->tcp_ts;
            }
        }
    }
        //设置套接字中的目的端口和目的地址
    inet->inet_dport = usin->sin_port;
    inet->inet_daddr = daddr;

    inet_csk(sk)->icsk_ext_hdr_len = 0;
    if (inet_opt)
        inet_csk(sk)->icsk_ext_hdr_len = inet_opt->opt.optlen;

    tp->rx_opt.mss_clamp = TCP_MSS_DEFAULT;

    //设置sk的状态为TCP_SYN_SENT
    tcp_set_state(sk, TCP_SYN_SENT);
    err = inet_hash_connect(&tcp_death_row, sk);
    if (err)
        goto failure;

    rt = ip_route_newports(fl4, rt, orig_sport, orig_dport,
                   inet->inet_sport, inet->inet_dport, sk);
    if (IS_ERR(rt)) {
        err = PTR_ERR(rt);
        rt = NULL;
        goto failure;
    }
    /* OK, now commit destination to socket.  */
    sk->sk_gso_type = SKB_GSO_TCPV4;
    sk_setup_caps(sk, &rt->dst);

    if (!tp->write_seq)
        tp->write_seq = secure_tcp_sequence_number(inet->inet_saddr,
                               inet->inet_daddr,
                               inet->inet_sport,
                               usin->sin_port);

    inet->inet_id = tp->write_seq ^ jiffies;

    err = tcp_connect(sk);//创建SYN报文并发送，该函数实现过程挺复杂，需进行TCP连接初始化以及发送
    rt = NULL;
    if (err)
        goto failure;

    return 0;

failure:
    //失败处理
    tcp_set_state(sk, TCP_CLOSE);
    ip_rt_put(rt);
    sk->sk_route_caps = 0;
    inet->inet_dport = 0;
    return err;
}

3.2、接受连接

系统调用函数sys_accept实现如下：

SYSCALL_DEFINE3(accept, int, fd, struct sockaddr __user *, upeer_sockaddr,
        int __user *, upeer_addrlen)
{
    return sys_accept4(fd, upeer_sockaddr, upeer_addrlen, 0);
}

调用系统调用sys_accept4

SYSCALL_DEFINE4(accept4, int, fd, struct sockaddr __user *, upeer_sockaddr,
        int __user *, upeer_addrlen, int, flags)
{
    struct socket *sock, *newsock;
    struct file *newfile;
    int err, len, newfd, fput_needed;
    struct sockaddr_storage address;
    .......................
    sock = sockfd_lookup_light(fd, &err, &fput_needed);//根据fd获得一个socket
    if (!sock)
        goto out;

    err = -ENFILE;
    newsock = sock_alloc();//重新创建一个新的socket
    if (!newsock)
        goto out_put;
    //复制套接字部分属性
    newsock->type = sock->type;
    newsock->ops = sock->ops;
    __module_get(newsock->ops->owner);
    //给新建的socket分配文件结构，并返回新的文件描述符
    newfd = sock_alloc_file(newsock, &newfile, flags);
    if (unlikely(newfd < 0)) {
        err = newfd;
        sock_release(newsock);
        goto out_put;
    }

    err = security_socket_accept(sock, newsock);
    if (err)
        goto out_fd;
    //调用inet_accept接受连接
    err = sock->ops->accept(sock, newsock, sock->file->f_flags);
    if (err < 0)
        goto out_fd;

    if (upeer_sockaddr) {//将地址信息从内核移到用户空间
        if (newsock->ops->getname(newsock, (struct sockaddr *)&address,
                      &len, 2) < 0) {
            err = -ECONNABORTED;
            goto out_fd;
        }
        err = move_addr_to_user((struct sockaddr *)&address,
                    len, upeer_sockaddr, upeer_addrlen);
        if (err < 0)
            goto out_fd;
    }

    /* File flags are not inherited via accept() unlike another OSes. */
    //安装文件描述符
    fd_install(newfd, newfile);
    err = newfd;

out_put:
    fput_light(sock->file, fput_needed);
out:
    return err;
out_fd:
    fput(newfile);
    put_unused_fd(newfd);
    goto out_put;
}

该函数创建一个新的套接字，设置客户端连接并唤醒客户端并返回一个新的文件描述符fd。

下面是inet_accept函数的实现

int inet_accept(struct socket *sock, struct socket *newsock, int flags)
{
    struct sock *sk1 = sock->sk;
    int err = -EINVAL;
    struct sock *sk2 = sk1->sk_prot->accept(sk1, flags, &err);//调用inet_csk_accept函数从队列icsk_accept_queue取出已经连接的套接字

    if (!sk2)
        goto do_err;

    lock_sock(sk2);

    sock_rps_record_flow(sk2);
    WARN_ON(!((1 << sk2->sk_state) &
          (TCPF_ESTABLISHED | TCPF_CLOSE_WAIT | TCPF_CLOSE)));

    sock_graft(sk2, newsock);

    newsock->state = SS_CONNECTED;//设置套接字状态
    err = 0;
    release_sock(sk2);
do_err:
    return err;
}

4、关闭连接

关闭一个socket连接，系统调用sys_shutdown

SYSCALL_DEFINE2(shutdown, int, fd, int, how)
{
    int err, fput_needed;
    struct socket *sock;

    sock = sockfd_lookup_light(fd, &err, &fput_needed);
    if (sock != NULL) {
        err = security_socket_shutdown(sock, how);
        if (!err)
            err = sock->ops->shutdown(sock, how);
        fput_light(sock->file, fput_needed);
    }
    return err;
}

函数最后调用inet_shutdown关闭套接字

int inet_shutdown(struct socket *sock, int how)
{
    struct sock *sk = sock->sk;
    int err = 0;
    .................
    lock_sock(sk);
    if (sock->state == SS_CONNECTING) {
        if ((1 << sk->sk_state) &
            (TCPF_SYN_SENT | TCPF_SYN_RECV | TCPF_CLOSE))
            sock->state = SS_DISCONNECTING;
        else
            sock->state = SS_CONNECTED;
    }

    switch (sk->sk_state) {
    case TCP_CLOSE:
        err = -ENOTCONN;
    default:
        sk->sk_shutdown |= how;
        if (sk->sk_prot->shutdown)
            sk->sk_prot->shutdown(sk, how);//调用tcp_shutdown强制关闭连接
        break;

    /* Remaining two branches are temporary solution for missing
     * close() in multithreaded environment. It is _not_ a good idea,
     * but we have no choice until close() is repaired at VFS level.
     */
    case TCP_LISTEN:
        if (!(how & RCV_SHUTDOWN))
            break;
        /* Fall through */
    case TCP_SYN_SENT:
        err = sk->sk_prot->disconnect(sk, O_NONBLOCK);//调用tcp_disconnect断开连接
        sock->state = err ? SS_DISCONNECTING : SS_UNCONNECTED;//设置套接字状态
        break;
    }

    sk->sk_state_change(sk);
    release_sock(sk);
    return err;
}

后面会详细分析TCP协议的发送和接收过程。