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_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_SENTtcp_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;//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;elsesock->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。