今天真是不幸，写好的文章竟然在编辑器里被错误冲刷掉了，本来已经完成了本章节的内容，就要在最后关头出现了一个IE错误，现在是重写的，请朋友们见谅。我是无名小卒，请转载的朋友注明出处，谢谢。昨天我们看到了inet_create()函数我们今天继续。
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static int inet_create(struct net *net, struct socket *sock, int protocol)
{
struct sock *sk;
struct list_head *p;
struct inet_protosw *answer;
struct inet_sock *inet;
struct proto *answer_prot;
unsigned char answer_flags;
char answer_no_check;
int try_loading_module = 0;
int err;
if (sock->type != SOCK_RAW &&
sock->type != SOCK_DGRAM &&
!inet_ehash_secret)
build_ehash_secret();
sock->state = SS_UNCONNECTED;
/* Look for the requested type/protocol pair. */
answer = NULL;
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这个函数首先是检查是不是原始的socket和udp的socket，并且判断是否已经有了加密字符如果没有就会调用build_ehash_secret来分配一个
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void build_ehash_secret(void)
{
u32 rnd;
do {
get_random_bytes(&rnd, sizeof(rnd));
} while (rnd == 0);
spin_lock_bh(&inetsw_lock);
if (!inet_ehash_secret)
inet_ehash_secret = rnd;
spin_unlock_bh(&inetsw_lock);
}
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get_random_bytes是取得一个随机数即“熵”，取得后赋值给加密字符使用。然后上面的函数中将socket设置为未连接状态。我们继续往下看
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lookup_protocol:
err = -ESOCKTNOSUPPORT;
rcu_read_lock();
list_for_each_rcu(p, &inetsw[sock->type]) {
answer = list_entry(p, struct inet_protosw, list);
/* Check the non-wild match. */
if (protocol == answer->protocol) {
if (protocol != IPPROTO_IP)
break;
} else {
/* Check for the two wild cases. */
if (IPPROTO_IP == protocol) {
protocol = answer->protocol;
break;
}
if (IPPROTO_IP == answer->protocol)
break;
}
err = -EPROTONOSUPPORT;
answer = NULL;
}
if (unlikely(answer == NULL)) {
if (try_loading_module < 2) {
rcu_read_unlock();
/*
* Be more specific, e.g. net-pf-2-proto-132-type-1
* (net-pf-PF_INET-proto-IPPROTO_SCTP-type-SOCK_STREAM)
*/
if (++try_loading_module == 1)
request_module("net-pf-%d-proto-%d-type-%d",
PF_INET, protocol, sock->type);
/*
* Fall back to generic, e.g. net-pf-2-proto-132
* (net-pf-PF_INET-proto-IPPROTO_SCTP)
*/
else
request_module("net-pf-%d-proto-%d",
PF_INET, protocol);
goto lookup_protocol;
} else
goto out_rcu_unlock;
}
err = -EPERM;
if (answer->capability > 0 && !capable(answer->capability))
goto out_rcu_unlock;
err = -EAFNOSUPPORT;
if (!inet_netns_ok(net, protocol))
goto out_rcu_unlock;
sock->ops = answer->ops;
answer_prot = answer->prot;
answer_no_check = answer->no_check;
answer_flags = answer->flags;
rcu_read_unlock();
BUG_TRAP(answer_prot->slab != NULL);
err = -ENOBUFS;
sk = sk_alloc(net, PF_INET, GFP_KERNEL, answer_prot);
if (sk == NULL)
goto out;
err = 0;
sk->sk_no_check = answer_no_check;
if (INET_PROTOSW_REUSE & answer_flags)
sk->sk_reuse = 1;
inet = inet_sk(sk);
inet->is_icsk = (INET_PROTOSW_ICSK & answer_flags) != 0;
if (SOCK_RAW == sock->type) {
inet->num = protocol;
if (IPPROTO_RAW == protocol)
inet->hdrincl = 1;
}
if (ipv4_config.no_pmtu_disc)
inet->pmtudisc = IP_PMTUDISC_DONT;
else
inet->pmtudisc = IP_PMTUDISC_WANT;
inet->id = 0;
sock_init_data(sock, sk);
sk->sk_destruct = inet_sock_destruct;
sk->sk_family = PF_INET;
sk->sk_protocol = protocol;
sk->sk_backlog_rcv = sk->sk_prot->backlog_rcv;
inet->uc_ttl = -1;
inet->mc_loop = 1;
inet->mc_ttl = 1;
inet->mc_index = 0;
inet->mc_list = NULL;
sk_refcnt_debug_inc(sk);
if (inet->num) {
/* It assumes that any protocol which allows
* the user to assign a number at socket
* creation time automatically
* shares.
*/
inet->sport = htons(inet->num);
/* Add to protocol hash chains. */
sk->sk_prot->hash(sk);
}
if (sk->sk_prot->init) {
err = sk->sk_prot->init(sk);
if (err)
sk_common_release(sk);
}
out:
return err;
out_rcu_unlock:
rcu_read_unlock();
goto out;
}
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这段代码看似复杂其实分析起来并不算难，首先上面
list_for_each_rcu(p, &inetsw[sock->type])是一个宏，我们看一下
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#define list_for_each_rcu(pos, head) \
for (pos = rcu_dereference((head)->next); \
prefetch(pos->next), pos != (head); \
pos = rcu_dereference(pos->next))
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这段宏我贴些资料供大家理解，下面这些内容出自
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RCU（Read-Copy Update）通过延迟写操作来提高同步性能，具体请参见第3章。这里只分析具有RCU的链表。
RCU常用来保护读操作占多数的链表与数组。具有RCU的链表的操作函数与普通链表操作函数的区别是在函数名后加上了_rcu，如list_for_each_rcu函数。
函数list_for_each_rcu的功能是遍历一个rcu保护的链表。其中，参数pos表示用来做链表位置计数的&struct list_head结构，参数head表示链表头。只要遍历被rcu_read_lock()保护，使用诸如list_add_rcu()的函数对链表同时访问是安全的。
函数List_for_each_rcu列出如下：
#define list_for_each_rcu(pos, head) \
for (pos = (head)->next, prefetch(pos->next); pos != (head); \
pos = rcu_dereference(pos->next), prefetch(pos->next))
函数rcu_dereference在RCU读临界部分中取出一个RCU保护的指针。在需要内存屏障的体系中进行内存屏障（目前只有Alpha体系需要），函数列出如下：
#define rcu_dereference(p) ({ \
typeof(p) _________p1 = p; \
smp_read_barrier_depends(); \
(_________p1); \
})
在include/asm-i386/system.h中：
#define smp_read_barrier_depends() read_barrier_depends()
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很明显上面的宏就是循环检查inetsw数组找到符合我们socket类型的链头，那么这个数组是什么时候初始化的呢？我们再象上一节那样看一下
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static int __init inet_init(void)
{
。。。。。。
for (q = inetsw_array; q < &inetsw_array[INETSW_ARRAY_LEN]; ++q)
inet_register_protosw(q);
。。。。。。
}
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我们看到他循环用inet_register_protosw()函数处理inetsw_array数组中的元素
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static struct inet_protosw inetsw_array[] =
{
{
.type = SOCK_STREAM,
.protocol = IPPROTO_TCP,
.prot = &tcp_prot,
.ops = &inet_stream_ops,
.capability = -1,
.no_check = 0,
.flags = INET_PROTOSW_PERMANENT |
INET_PROTOSW_ICSK,
},
{
.type = SOCK_DGRAM,
.protocol = IPPROTO_UDP,
.prot = &udp_prot,
.ops = &inet_dgram_ops,
.capability = -1,
.no_check = UDP_CSUM_DEFAULT,
.flags = INET_PROTOSW_PERMANENT,
},
{
.type = SOCK_RAW,
.protocol = IPPROTO_IP, /* wild card */
.prot = &raw_prot,
.ops = &inet_sockraw_ops,
.capability = CAP_NET_RAW,
.no_check = UDP_CSUM_DEFAULT,
.flags = INET_PROTOSW_REUSE,
}
};
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我们结合应用程序的练习看一下
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server_sockfd = socket(AF_INET, SOCK_STREAM, 0);
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这里我们看到在上面的数组中第一个元素就是我们需要的，他是被下面的函数登记到数组中的
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void inet_register_protosw(struct inet_protosw *p)
{
struct list_head *lh;
struct inet_protosw *answer;
int protocol = p->protocol;
struct list_head *last_perm;
spin_lock_bh(&inetsw_lock);
if (p->type >= SOCK_MAX)
goto out_illegal;
/* If we are trying to override a permanent protocol, bail. */
answer = NULL;
last_perm = &inetsw[p->type];
list_for_each(lh, &inetsw[p->type]) {
answer = list_entry(lh, struct inet_protosw, list);
/* Check only the non-wild match. */
if (INET_PROTOSW_PERMANENT & answer->flags) {
if (protocol == answer->protocol)
break;
last_perm = lh;
}
answer = NULL;
}
if (answer)
goto out_permanent;
/* Add the new entry after the last permanent entry if any, so that
* the new entry does not override a permanent entry when matched with
* a wild-card protocol. But it is allowed to override any existing
* non-permanent entry. This means that when we remove this entry, the
* system automatically returns to the old behavior.
*/
list_add_rcu(&p->list, last_perm);
out:
spin_unlock_bh(&inetsw_lock);
synchronize_net();
return;
out_permanent:
printk(KERN_ERR "Attempt to override permanent protocol %d.\n",
protocol);
goto out;
out_illegal:
printk(KERN_ERR
"Ignoring attempt to register invalid socket type %d.\n",
p->type);
goto out;
}
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很明显在上面的循环中找到适合的链头位置，将我们的数组中的元素一一注册登记到数组中。我们这里要看一下inet_protosw结构
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struct inet_protosw {
struct list_head list;
/* These two fields form the lookup key. */
unsigned short type; /* This is the 2nd argument to socket(2). */
unsigned short protocol; /* This is the L4 protocol number. */
struct proto *prot;
const struct proto_ops *ops;
int capability; /* Which (if any) capability do
* we need to use this socket
* interface?
*/
char no_check; /* checksum on rcv/xmit/none? */
unsigned char flags; /* See INET_PROTOSW_* below. */
};
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这个结构是专门用于采用IP协议的socket使用。其内部的变量我们暂时不做分析，也不转译了，我们还是坚持“用时学习”的观念。回到inet_create()函数中，我们已经在inetsw数组中找到了我们的协议类型的链头就会取得其宿主inet_protosw 结构，然后answer = list_entry(p, struct inet_protosw, list);接着函数中对其兼容性进行了检测，最关键的地方是
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sock->ops = answer->ops;
answer_prot = answer->prot;
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这二句首先是为socket的协议操作函数进行了挂钩，我们就要看上面的
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{
.type = SOCK_STREAM,
.protocol = IPPROTO_TCP,
.prot = &tcp_prot,
.ops = &inet_stream_ops,
.capability = -1,
.no_check = 0,
.flags = INET_PROTOSW_PERMANENT |
INET_PROTOSW_ICSK,
},
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结合这个元素的设置我们明白了，上面socket的协议操作函数被设置成了inet_stream_ops()，而answer_prot设置成了tcp_prot结构。这是个struct proto结构，我们不看了，其内容很多，但是这个结构的作用得强调一下，它是专门用于socket的传输层使用的结构，而用于网络传输层的结构由另一个结构体来表示struct inet_proto。接着函数中分配了一个sock结构。我是无名小卒，尽管3月份才写博客其实研究内核很多年了，写这些博客是为了与朋友们共享知识发扬copyleft精神，所以请转载的朋友注明出处。分配函数我们曾经在unix的socket创建过程中谈到过，我们不细细研究这个函数了，不过要注意其内部的关键地方sk = sk_alloc(net, PF_INET, GFP_KERNEL, answer_prot);我们看到他传递了一个answer_prot即我们说的用于socket传输层的钩子函数给sk_alloc。
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struct sock *sk_alloc(struct net *net, int family, gfp_t priority,
struct proto *prot)
{
struct sock *sk;
sk = sk_prot_alloc(prot, priority | __GFP_ZERO, family);
if (sk) {
sk->sk_family = family;
/*
* See comment in struct sock definition to understand
* why we need sk_prot_creator -acme
*/
sk->sk_prot = sk->sk_prot_creator = prot;
sock_lock_init(sk);
sock_net_set(sk, get_net(net));
}
return sk;
}
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注意上面sk->sk_prot = sk->sk_prot_creator = prot;将传输层的钩子函数挂入到了sock中了。再回到inet_create函数中，代码是一些对sock的初始化，注意sk是sock结构，而sock是socket结构。使用inet = inet_sk(sk);使sk赋值给struct inet_sock *inet结构变量，我们在unix的socket中曾经说了关于unix 的socket是unix_sock，而这里用于INET的socket结构inet_sock。然后函数通过sock_init_data(sock, sk);对sk进一步的初始化操作，并让sock和sk挂起钩来，这个函数我们已经在那些unix 的socket创建文章中分析过了，请不明白的朋友们看那里的文章学习。下面函数中最关键的地方
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if (sk->sk_prot->init) {
err = sk->sk_prot->init(sk);
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这个就是调用了我们上面提到的传输层结构中的钩子函数init.我们看到上面是tcp_prot结构变量。所以进入其init函数中
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struct proto tcp_prot = {
.name = "TCP",
.owner = THIS_MODULE,
.close = tcp_close,
.connect = tcp_v4_connect,
.disconnect = tcp_disconnect,
.accept = inet_csk_accept,
.ioctl = tcp_ioctl,
.init = tcp_v4_init_sock,
.destroy = tcp_v4_destroy_sock,
.shutdown = tcp_shutdown,
.setsockopt = tcp_setsockopt,
.getsockopt = tcp_getsockopt,
.recvmsg = tcp_recvmsg,
.backlog_rcv = tcp_v4_do_rcv,
.hash = inet_hash,
.unhash = inet_unhash,
.get_port = inet_csk_get_port,
.enter_memory_pressure = tcp_enter_memory_pressure,
.sockets_allocated = &tcp_sockets_allocated,
.orphan_count = &tcp_orphan_count,
.memory_allocated = &tcp_memory_allocated,
.memory_pressure = &tcp_memory_pressure,
.sysctl_mem = sysctl_tcp_mem,
.sysctl_wmem = sysctl_tcp_wmem,
.sysctl_rmem = sysctl_tcp_rmem,
.max_header = MAX_TCP_HEADER,
.obj_size = sizeof(struct tcp_sock),
.twsk_prot = &tcp_timewait_sock_ops,
.rsk_prot = &tcp_request_sock_ops,
.h.hashinfo = &tcp_hashinfo,
#ifdef CONFIG_COMPAT
.compat_setsockopt = compat_tcp_setsockopt,
.compat_getsockopt = compat_tcp_getsockopt,
#endif
};
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我是无名小卒，转载请注明出处，不要担心结构有多么大，很多朋友在看代码时总是被结构所吓倒，要知道一个好的结构体也不是一天所成更不是一人所完成的，所以我们要针对场景来分析记忆。其实上面我们就只关心一个地方.init = tcp_v4_init_sock,，进入钩子函数
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static int tcp_v4_init_sock(struct sock *sk)
{
struct inet_connection_sock *icsk = inet_csk(sk);
struct tcp_sock *tp = tcp_sk(sk);
skb_queue_head_init(&tp->out_of_order_queue);
tcp_init_xmit_timers(sk);
tcp_prequeue_init(tp);
icsk->icsk_rto = TCP_TIMEOUT_INIT;
tp->mdev = TCP_TIMEOUT_INIT;
/* So many TCP implementations out there (incorrectly) count the
* initial SYN frame in their delayed-ACK and congestion control
* algorithms that we must have the following bandaid to talk
* efficiently to them. -DaveM
*/
tp->snd_cwnd = 2;
/* See draft-stevens-tcpca-spec-01 for discussion of the
* initialization of these values.
*/
tp->snd_ssthresh = 0x7fffffff; /* Infinity */
tp->snd_cwnd_clamp = ~0;
tp->mss_cache = 536;
tp->reordering = sysctl_tcp_reordering;
icsk->icsk_ca_ops = &tcp_init_congestion_ops;
sk->sk_state = TCP_CLOSE;
sk->sk_write_space = sk_stream_write_space;
sock_set_flag(sk, SOCK_USE_WRITE_QUEUE);
icsk->icsk_af_ops = &ipv4_specific;
icsk->icsk_sync_mss = tcp_sync_mss;
#ifdef CONFIG_TCP_MD5SIG
tp->af_specific = &tcp_sock_ipv4_specific;
#endif
sk->sk_sndbuf = sysctl_tcp_wmem[1];
sk->sk_rcvbuf = sysctl_tcp_rmem[1];
atomic_inc(&tcp_sockets_allocated);
return 0;
}
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首先函数出现了一个新的结构体struct tcp_sock，这个结构非常大，但是其作用随着我们的分析会越来越清晰，上面代码中对这个结构变量tp和sock结构变量sk进一步的初始化，对后这个地方的初始化工作我们要经常回顾，例如这里的sk->sk_write_space = sk_stream_write_space等钩子函数的挂入，以及缓冲区的相关设置。最后我们回到上一节提到的sys_socket()函数中执行retval = sock_map_fd(sock)来完成创建过程，sock_map_fd我们在unix的socket中详细分析了，他在文件系统中分配一个文件号，以及file文件指针和目录项dentry结构使其与socket挂上钩，最后返回分配的文件号，至此以后我们可以根据这个文件号对创建的socket进行其他操作了。相关内容看unix的socket创建http://blog.chinaunix.net/u2/64681/showart_1300200.html。
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