bind函数（套接字的绑定）

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。

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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，表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_LISTENif (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; }//本地端口和目的地端口 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; }//处理多媒体或广播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socketif (!sock)goto out; err = -ENFILE; newsock = sock_alloc();//重新创建一个新的socketif (!newsock)goto out_put;<span style="white-space:pre"> </span>//复制套接字部分属性 newsock->type = sock->type; newsock->ops = sock->ops; __module_get(newsock->ops->owner);<span style="white-space:pre"> </span>//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;<span style="white-space:pre"> </span>//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. */<span style="white-space:pre"> </span>//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<span style="font-family: Consolas, 'Courier New', Courier, mono, serif; line-height: 18px;">inet_csk_accept函数从队列icsk_accept_queue取出已经连接的套接字</span>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<span style="font-family: Consolas, 'Courier New', Courier, mono, serif; line-height: 18px;">tcp_shutdown强制关闭连接</span>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<span style="font-family: Consolas, 'Courier New', Courier, mono, serif; line-height: 18px; background-color: rgb(248, 248, 248);">tcp_disconnect断开连接</span> 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;}