Linux内核--网络协议栈深入分析(五)--套接字的绑定、监听、连接和断开

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。