connect()函数调用过程
connect(fd, servaddr, addrlen);
-> SYSCALL_DEFINE3()
-> sock->ops->connect() == inet_stream_connect (sock->ops即inet_stream_ops)
-> tcp_v4_connect()
-> inet_hash_connect()
-> __inet_hash_connect()
-> check_established()
-> __inet_check_established()
SYSCALL_DEFINE3(connect, int, fd, struct sockaddr __user *, uservaddr,
int, addrlen)
{
struct socket *sock;
struct sockaddr_storage address;
int err, fput_needed;
/* 找到文件描述符对应的BSD socket结构，在前面的socket调用中建立*/
sock = sockfd_lookup_light(fd, &err, &fput_needed);
if (!sock)
goto out;
/* copy对端的地址到内核空间 */
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;
/* 调用该BSD socket对应的connect调用 */
err = sock->ops->connect(sock, (struct sockaddr *)&address, addrlen,
sock->file->f_flags);
out_put:
/* 释放文件的引用 */
fput_light(sock->file, fput_needed);
out:
return err;
}
/*
* Connect to a remote host. There is regrettably still a little
* TCP 'magic' in here.
*/
int inet_stream_connect(struct socket *sock, struct sockaddr *uaddr,
int addr_len, int flags)
{
struct sock *sk = sock->sk;
int err;
long timeo;
lock_sock(sk);
if (uaddr->sa_family == AF_UNSPEC) {
err = sk->sk_prot->disconnect(sk, flags);
sock->state = err ? SS_DISCONNECTING : SS_UNCONNECTED;
goto out;
}
switch (sock->state) {
default:
err = -EINVAL;
goto out;
case SS_CONNECTED: /* 该BSD socket已连接*/
err = -EISCONN;
goto out;
case SS_CONNECTING: /* 该BSD socket正在连接*/
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;
/* INET SOCKET 调用协议特有connect操作符 */
err = sk->sk_prot->connect(sk, uaddr, addr_len);
if (err < 0)
goto out;
/* 上面的调用完成后，连接并没有完成，*/
sock->state = SS_CONNECTING;
/* Just entered SS_CONNECTING state; the only
* difference is that return value in non-blocking
* case is EINPROGRESS, rather than EALREADY.
*/
err = -EINPROGRESS;
break;
}
/* 获取连接超时时间*/
timeo = sock_sndtimeo(sk, flags & O_NONBLOCK);
if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
/* Error code is set above 进入定时等待 */
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;
/* sk->sk_err may be not zero now, if RECVERR was ordered by user
* and error was received after socket entered established state.
* Hence, it is handled normally after connect() return successfully.
*/
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;
}
/* This will initiate an outgoing connection. */
int tcp_v4_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
{
struct inet_sock *inet = inet_sk(sk);
struct tcp_sock *tp = tcp_sk(sk);
struct sockaddr_in *usin = (struct sockaddr_in *)uaddr;
struct rtable *rt;
__be32 daddr, nexthop;
int tmp;
int err;
if (addr_len < sizeof(struct sockaddr_in))
return -EINVAL;
if (usin->sin_family != AF_INET)
return -EAFNOSUPPORT;
/* 开始准备路由 */
nexthop = daddr = usin->sin_addr.s_addr;
if (inet->opt && inet->opt->srr) {
if (!daddr)
return -EINVAL;
nexthop = inet->opt->faddr;
}
/* 调用路由模块获取出口信息，这里不深入 */
tmp = ip_route_connect(&rt, nexthop, inet->saddr,
RT_CONN_FLAGS(sk), sk->sk_bound_dev_if,
IPPROTO_TCP,
inet->sport, usin->sin_port, sk, 1);
if (tmp < 0) {
if (tmp == -ENETUNREACH)
IP_INC_STATS_BH(sock_net(sk), IPSTATS_MIB_OUTNOROUTES);
return tmp;
}
/* 如果获取的路由是广播或多播域， 返回网络不可达，tcp不支持多播与广播 */
if (rt->rt_flags & (RTCF_MULTICAST | RTCF_BROADCAST)) {
ip_rt_put(rt);
return -ENETUNREACH;
}
if (!inet->opt || !inet->opt->srr)
daddr = rt->rt_dst;
if (!inet->saddr)
inet->saddr = rt->rt_src;
inet->rcv_saddr = inet->saddr;
if (tp->rx_opt.ts_recent_stamp && 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 && rt->rt_dst == daddr) {
struct inet_peer *peer = rt_get_peer(rt);
/*
* 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 != NULL &&
peer->tcp_ts_stamp + TCP_PAWS_MSL >= get_seconds()) {
tp->rx_opt.ts_recent_stamp = peer->tcp_ts_stamp;
tp->rx_opt.ts_recent = peer->tcp_ts;
}
}
inet->dport = usin->sin_port;
inet->daddr = daddr;
inet_csk(sk)->icsk_ext_hdr_len = 0;
if (inet->opt)
inet_csk(sk)->icsk_ext_hdr_len = inet->opt->optlen;
/* mss_clamp */
tp->rx_opt.mss_clamp = 536;
/* Socket identity is still unknown (sport may be zero).
* However we set state to SYN-SENT and not releasing socket
* lock select source port, enter ourselves into the hash tables and
* complete initialization after this.
*/
tcp_set_state(sk, TCP_SYN_SENT);
err = inet_hash_connect(&tcp_death_row, sk);
if (err)
goto failure;
err = ip_route_newports(&rt, IPPROTO_TCP,
inet->sport, inet->dport, sk);
if (err)
goto failure;
/* OK, now commit destination to socket. */
sk->sk_gso_type = SKB_GSO_TCPV4;
sk_setup_caps(sk, &rt->u.dst);
if (!tp->write_seq)
tp->write_seq = secure_tcp_sequence_number(inet->saddr,
inet->daddr,
inet->sport,
usin->sin_port);
/* id是IP包头的id域 */
inet->id = tp->write_seq ^ jiffies;
err = tcp_connect(sk);
rt = NULL;
if (err)
goto failure;
return 0;
failure:
/*
* This unhashes the socket and releases the local port,
* if necessary.
*/
tcp_set_state(sk, TCP_CLOSE);
ip_rt_put(rt);
sk->sk_route_caps = 0;
inet->dport = 0;
return err;
}
当snum==0时，表明此时源端口没有指定，此时会随机选择一个空闲端口作为此次连接 的源端口。low和high分别表示可用端口的下限和上限，remaining表示可用端口的数，注意这里的可用只是指端口 可以用作源端口，其中部分端口可能已经作为其它socket的端口号在使用了，所以要循环1~remaining， 直到查找到空闲的源端口。
下面来看下对每个端口的检查，即//choose a valid port部分的代码。 这里要先了解下tcp的内核表组成，udp的表内核表udptable只是一张hash表，tcp的表则稍复杂， 它的名字是tcp_hashinfo，在tcp_init()中被初始化，这个数据结构定义如下(省略了不相关的数据)：
struct inet_hashinfo {
struct inet_ehash_bucket *ehash;
……
struct inet_bind_hashbucket *bhash;
……
struct inet_listen_hashbucket listening_hash[INET_LHTABLE_SIZE]
____cacheline_aligned_in_smp;
};
从定义可以看出，tcp表又分成了三张表ehash, bhash, listening_hash，其中ehash, listening_hash对应于socket处在TCP的ESTABLISHED, LISTEN状态，bhash对应于socket已绑定了本地地址。 三者间并不互斥，如一个socket可同时在bhash和ehash中，由于TIME_WAIT是一个比较特殊的状态，所以ehash又分成了 chain和twchain，为TIME_WAIT的socket单独形成一张表。
回到刚才的代码，现在还只是建立socket连接，使用的就应该是tcp表中的bhash。 首先取得内核tcp表的bind表 – bhash，查看是否已有socket占用：
如果没有，则调用inet_bind_bucket_create()创建一个bind表项tb， 并插入到bind表中，跳转至goto ok代码段； 如果有，则跳转至goto ok代码段。
进入ok代码段表明已找到合适的bind表项(无论是创建的还是查找到的)，调用inet_bind_hash()赋值源端口inet_num。
inet_hash_connect()函数只是对__inet_hash_connect()函数进行了简单的封装 。在__inet_hash_connect()中如果已绑定了端口号，并且是和其他传输控制块共享绑定的端口号， 则会调用check_established参数指向的函数来检查这个绑定的端口号是否可用，代码如下所示：
int __inet_hash_connect(struct inet_timewait_death_row *death_row,
struct sock *sk, u32 port_offset,
int (*check_established)(struct inet_timewait_death_row *,
struct sock *, __u16, struct inet_timewait_sock **),
void (*hash)(struct sock *sk))
{
struct inet_hashinfo *hinfo = death_row->hashinfo;
const unsigned short snum = inet_sk(sk)->num;
struct inet_bind_hashbucket *head;
struct inet_bind_bucket *tb;
int ret;
struct net *net = sock_net(sk);
if (!snum) {
int i, remaining, low, high, port;
static u32 hint;
u32 offset = hint + port_offset;
struct hlist_node *node;
struct inet_timewait_sock *tw = NULL;
inet_get_local_port_range(&low, &high);
remaining = (high - low) + 1;
local_bh_enable();
for (i = 1; i <= remaining; i++) {
port = low + (i + offset) % remaining;
if (inet_is_reserved_local_port(port)
continue;
head = &hinfo->bhash[inet_bhashfn(net, port, hinfo->bhash_size)];
spin_lock(&head->lock);
inet_bind_bucket_for_each(tb, node, &head->chain) {
if (net_eq(ib_net(tb), net) && tb->port == port) {
if (tb->fastreuse >= 0)
goto next_port;
WARN_ON(hlist_empty(&tb->owners));
if (!check_established(death_row, sk, port, &tw))
goto ok;
goto next_port;
}
}
tb = inet_bind_bucket_create(hinfo->bind_bucket_cachep, net, head, port);
if (!tb) {
spin_unlock(&head->lock);
break;
}
tb->fastreuse = -1;
tb->fastreuseport = -1;
goto ok;
next_port:
spin_unlock(&head->lock);
}
local_bh_enable();
return -EADDRNOTAVAIL;
ok:
hint += i;
inet_bind_hash(sk, tb, port);
if (sk_unhashed(sk)) {
inet_sk(sk)->sport = htons(port);
hash(sk);
}
spin_unlock(&head->lock);
if (tw) {
inet_twsk_deschedule(tw, death_row);
inet_twsk_put(tw);
}
ret = 0;
goto out;
}
head = &hinfo->bhash[inet_bhashfn(net, snum, hinfo->bhash_size)];
tb = inet_csk(sk)->icsk_bind_hash;
spin_lock_bh(&head->lock);
if (sk_head(&tb->owners) == sk && !sk->sk_bind_node.next) {
hash(sk);
spin_unlock_bh(&head->lock);
return 0;
} else {
spin_unlock(&head->lock);
/* No definite answer... Walk to established hash table */
ret = check_established(death_row, sk, snum, NULL);
out:
local_bh_enable();
return ret;
}
}
(sk_head(&tb->owners) == sk && !sk->sk_bind_node.next)这个判断条件就是用来 判断是不是只有当前传输控制块在使用已绑定的端口，条件为false时，会执行else分支，检查是否可用。 这么看来，调用bind()成功并不意味着这个端口就真的可以用。
check_established参数对应的函数是__inet_check_established()， 在inet_hash_connect()中可以看到。在上面的代码中我们还注意到调用check_established()时第三个参数为NULL， 这在后面的分析中会用到。
__inet_check_established()函数中，会分别在TIME_WAIT传输控制块和除TIME_WIAT、 LISTEN状态外的传输控制块中查找是已绑定的端口是否已经使用，代码片段如下所示：
/* called with local bh disabled */
static int __inet_check_established(struct inet_timewait_death_row *death_row,
struct sock *sk, __u16 lport,
struct inet_timewait_sock **twp)
{
struct inet_hashinfo *hinfo = death_row->hashinfo;
struct inet_sock *inet = inet_sk(sk);
__be32 daddr = inet->rcv_saddr;
__be32 saddr = inet->daddr;
int dif = sk->sk_bound_dev_if;
INET_ADDR_COOKIE(acookie, saddr, daddr)
const __portpair ports = INET_COMBINED_PORTS(inet->dport, lport);
struct net *net = sock_net(sk);
unsigned int hash = inet_ehashfn(net, daddr, lport, saddr, inet->dport);
struct inet_ehash_bucket *head = inet_ehash_bucket(hinfo, hash);
spinlock_t *lock = inet_ehash_lockp(hinfo, hash);
struct sock *sk2;
const struct hlist_nulls_node *node;
struct inet_timewait_sock *tw;
spin_lock(lock);
/* Check TIME-WAIT sockets first. */
sk_nulls_for_each(sk2, node, &head->twchain) {
tw = inet_twsk(sk2);
if (INET_TW_MATCH(sk2, net, hash, acookie,
saddr, daddr, ports, dif)) {
if (twsk_unique(sk, sk2, twp))
goto unique;
else
goto not_unique;
}
}
tw = NULL;
/* And established part... */
sk_nulls_for_each(sk2, node, &head->chain) {
if (INET_MATCH(sk2, net, hash, acookie,
saddr, daddr, ports, dif))
goto not_unique;
}
unique:
......
return 0;
not_unique:
spin_unlock(lock);
return -EADDRNOTAVAIL;
}
如果是TCP套接字，twsk_uniqueue()中会调用tcp_twsk_uniqueue()来判断，返回true的条件如下所示：
int tcp_twsk_unique(struct sock *sk, struct sock *sktw, void *twp)
{
const struct tcp_timewait_sock *tcptw = tcp_twsk(sktw);
struct tcp_sock *tp = tcp_sk(sk);
if (tcptw->tw_ts_recent_stamp &&
(twp == NULL || (sysctl_tcp_tw_reuse &&
get_seconds() - tcptw->tw_ts_recent_stamp > 1))) {
......
return 1;
}
return 0;
}
/*
* Build a SYN and send it off.
*/
int tcp_connect(struct sock *sk)
{
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *buff;
/* 初始化连接对应的INET socket结构的参数，为连接做准备 */
tcp_connect_init(sk);
/* 获取一个skb，由于是syn包，没有数据，所以大小是MAX_TCP_HEADER的16位对齐 */
buff = alloc_skb_fclone(MAX_TCP_HEADER + 15, sk->sk_allocation);
if (unlikely(buff == NULL))
return -ENOBUFS;
/* Reserve space for headers. */
skb_reserve(buff, MAX_TCP_HEADER);
tp->snd_nxt = tp->write_seq;
/* 设置skb相关参数 */
tcp_init_nondata_skb(buff, tp->write_seq++, TCPCB_FLAG_SYN);
/* 设置ECN */
TCP_ECN_send_syn(sk, buff);
/* Send it off. */
/* 保存该数据包的发送时间*/
TCP_SKB_CB(buff)->when = tcp_time_stamp;
tp->retrans_stamp = TCP_SKB_CB(buff)->when;
skb_header_release(buff);
/* 加入发送队列，待确认后在丢弃*/
__tcp_add_write_queue_tail(sk, buff);
sk->sk_wmem_queued += buff->truesize;
sk_mem_charge(sk, buff->truesize);
tp->packets_out += tcp_skb_pcount(buff);
tcp_transmit_skb(sk, buff, 1, GFP_KERNEL);
/* We change tp->snd_nxt after the tcp_transmit_skb() call
* in order to make this packet get counted in tcpOutSegs.
*/
tp->snd_nxt = tp->write_seq;
tp->pushed_seq = tp->write_seq;
TCP_INC_STATS(sock_net(sk), TCP_MIB_ACTIVEOPENS);
/* Timer for repeating the SYN until an answer. */
inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
inet_csk(sk)->icsk_rto, TCP_RTO_MAX);
return 0;
}
/*
* Do all connect socket setups that can be done AF independent.
*/
static void tcp_connect_init(struct sock *sk)
{
struct dst_entry *dst = __sk_dst_get(sk);
struct tcp_sock *tp = tcp_sk(sk);
__u8 rcv_wscale;
/* We'll fix this up when we get a response from the other end.
* See tcp_input.c:tcp_rcv_state_process case TCP_SYN_SENT.
*/
tp->tcp_header_len = sizeof(struct tcphdr) +
(sysctl_tcp_timestamps ? TCPOLEN_TSTAMP_ALIGNED : 0);
#ifdef CONFIG_TCP_MD5SIG
if (tp->af_specific->md5_lookup(sk, sk) != NULL)
tp->tcp_header_len += TCPOLEN_MD5SIG_ALIGNED;
#endif
/* If user gave his TCP_MAXSEG, record it to clamp */
if (tp->rx_opt.user_mss)
tp->rx_opt.mss_clamp = tp->rx_opt.user_mss;
tp->max_window = 0;
/* 初始化MTU probe*/
tcp_mtup_init(sk);
/* 设置mss */
tcp_sync_mss(sk, dst_mtu(dst));
if (!tp->window_clamp)
tp->window_clamp = dst_metric(dst, RTAX_WINDOW);
tp->advmss = dst_metric(dst, RTAX_ADVMSS);
if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < tp->advmss)
tp->advmss = tp->rx_opt.user_mss;
tcp_initialize_rcv_mss(sk);
/* 根据接收空间大小初始化一个通告窗口 */
tcp_select_initial_window(tcp_full_space(sk),
tp->advmss - (tp->rx_opt.ts_recent_stamp ? tp->tcp_header_len - sizeof(struct tcphdr) : 0),
&tp->rcv_wnd,
&tp->window_clamp,
sysctl_tcp_window_scaling,
&rcv_wscale);
tp->rx_opt.rcv_wscale = rcv_wscale;
tp->rcv_ssthresh = tp->rcv_wnd;
sk->sk_err = 0;
sock_reset_flag(sk, SOCK_DONE);
tp->snd_wnd = 0;
/* 更新一些滑动窗口的成员*/
tcp_init_wl(tp, tp->write_seq, 0);
tp->snd_una = tp->write_seq;
tp->snd_sml = tp->write_seq;
tp->snd_up = tp->write_seq;
tp->rcv_nxt = 0;
tp->rcv_wup = 0;
tp->copied_seq = 0;
inet_csk(sk)->icsk_rto = TCP_TIMEOUT_INIT;
inet_csk(sk)->icsk_retransmits = 0;
tcp_clear_retrans(tp);
}
skb发送后，connect并没有返回，因为此时连接还没有建立，tcp进入等待状态，此时回到前面的inet_stream_connect函数
在发送syn后进入等待状态
static long inet_wait_for_connect(struct sock *sk, long timeo)
{
DEFINE_WAIT(wait);
/* sk_sleep 保存此INET SOCKET的等待队列 */
prepare_to_wait(sk->sk_sleep, &wait, TASK_INTERRUPTIBLE);
/* Basic assumption: if someone sets sk->sk_err, he _must_
* change state of the socket from TCP_SYN_*.
* Connect() does not allow to get error notifications
* without closing the socket.
*/
/* 定时等待知道状态变化 */
while ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
release_sock(sk);
timeo = schedule_timeout(timeo);
lock_sock(sk);
if (signal_pending(current) || !timeo)
break;
prepare_to_wait(sk->sk_sleep, &wait, TASK_INTERRUPTIBLE);
}
finish_wait(sk->sk_sleep, &wait);
return timeo;
}
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