Linux内核--网络协议栈深入分析(四)--套接字内核初始化和创建过程

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/7984238

更多请查看专栏http://blog.csdn.net/column/details/linux-kernel-net.html

作者：闫明

1、系统初始化过程中会调用sock_init函数进行套接字的初始化，主要是进行缓存的初始化

static int __init sock_init(void)
{
	int err;

	 //初始化.sock缓存

	sk_init();

	 //初始化sk_buff缓存
	skb_init();

	 //初始化协议模块缓存

	init_inodecache();
        //注册文件系统类型
	err = register_filesystem(&sock_fs_type);
	if (err)
		goto out_fs;
	sock_mnt = kern_mount(&sock_fs_type);
	if (IS_ERR(sock_mnt)) {
		err = PTR_ERR(sock_mnt);
		goto out_mount;
	}

.........................
out:
	return err;

out_mount:
	unregister_filesystem(&sock_fs_type);
out_fs:
	goto out;
}

2、INET协议族的初始化函数

static int __init inet_init(void)
{
	struct sk_buff *dummy_skb;
	struct inet_protosw *q;
	struct list_head *r;
	int rc = -EINVAL;

	BUILD_BUG_ON(sizeof(struct inet_skb_parm) > sizeof(dummy_skb->cb));

	sysctl_local_reserved_ports = kzalloc(65536 / 8, GFP_KERNEL);
	if (!sysctl_local_reserved_ports)
		goto out;

	//下面注册传输层协议操作集
	rc = proto_register(&tcp_prot, 1);
	if (rc)
		goto out_free_reserved_ports;

	rc = proto_register(&udp_prot, 1);
	if (rc)
		goto out_unregister_tcp_proto;

	rc = proto_register(&raw_prot, 1);
	if (rc)
		goto out_unregister_udp_proto;

	rc = proto_register(&ping_prot, 1);
	if (rc)
		goto out_unregister_raw_proto;

	//注册INET协议族的handler
	(void)sock_register(&inet_family_ops);

.........................

	/*
	 *	Add all the base protocols.
	 */
	//将INET协议族协议数据包接收函数添加到系统中
	if (inet_add_protocol(&icmp_protocol, IPPROTO_ICMP) < 0)
		printk(KERN_CRIT "inet_init: Cannot add ICMP protocol\n");
	if (inet_add_protocol(&udp_protocol, IPPROTO_UDP) < 0)
		printk(KERN_CRIT "inet_init: Cannot add UDP protocol\n");
	if (inet_add_protocol(&tcp_protocol, IPPROTO_TCP) < 0)
		printk(KERN_CRIT "inet_init: Cannot add TCP protocol\n");
#ifdef CONFIG_IP_MULTICAST
	if (inet_add_protocol(&igmp_protocol, IPPROTO_IGMP) < 0)
		printk(KERN_CRIT "inet_init: Cannot add IGMP protocol\n");
#endif

	/* Register the socket-side information for inet_create. */
	for (r = &inetsw[0]; r < &inetsw[SOCK_MAX]; ++r)
		INIT_LIST_HEAD(r);
	//将inetsw_array中的元素按套接字类型注册到inetsw链表数组中
	for (q = inetsw_array; q < &inetsw_array[INETSW_ARRAY_LEN]; ++q)
		inet_register_protosw(q);

	/*
	 *	Set the ARP module up
	 */

	arp_init();

	/*
	 *	Set the IP module up
	 */

	ip_init();

	tcp_v4_init();

	/* Setup TCP slab cache for open requests. */
	tcp_init();

	/* Setup UDP memory threshold */
	udp_init();

	/* Add UDP-Lite (RFC 3828) */
	udplite4_register();

	ping_init();

	/*
	 *	Set the ICMP layer up
	 */

	if (icmp_init() < 0)
		panic("Failed to create the ICMP control socket.\n");

.........................
	if (init_ipv4_mibs())
		printk(KERN_CRIT "inet_init: Cannot init ipv4 mibs\n");

	ipv4_proc_init();

	ipfrag_init();

	dev_add_pack(&ip_packet_type);

	rc = 0;
out:
	return rc;
out_unregister_raw_proto:
	proto_unregister(&raw_prot);
out_unregister_udp_proto:
	proto_unregister(&udp_prot);
out_unregister_tcp_proto:
	proto_unregister(&tcp_prot);
out_free_reserved_ports:
	kfree(sysctl_local_reserved_ports);
	goto out;
}

上面函数中的inetsw_array的定义中有四个元素：

static struct inet_protosw inetsw_array[] =
{
	{
		.type =       SOCK_STREAM,
		.protocol =   IPPROTO_TCP,
		.prot =       &tcp_prot,
		.ops =        &inet_stream_ops,
		.no_check =   0,
		.flags =      INET_PROTOSW_PERMANENT |
			      INET_PROTOSW_ICSK,
	},

	{
		.type =       SOCK_DGRAM,
		.protocol =   IPPROTO_UDP,
		.prot =       &udp_prot,
		.ops =        &inet_dgram_ops,
		.no_check =   UDP_CSUM_DEFAULT,
		.flags =      INET_PROTOSW_PERMANENT,
       },

       {
		.type =       SOCK_DGRAM,
		.protocol =   IPPROTO_ICMP,
		.prot =       &ping_prot,
		.ops =        &inet_dgram_ops,
		.no_check =   UDP_CSUM_DEFAULT,
		.flags =      INET_PROTOSW_REUSE,
       },

       {
	       .type =       SOCK_RAW,
	       .protocol =   IPPROTO_IP,	/* wild card */
	       .prot =       &raw_prot,
	       .ops =        &inet_sockraw_ops,
	       .no_check =   UDP_CSUM_DEFAULT,
	       .flags =      INET_PROTOSW_REUSE,
       }
};

上面的函数会将这个数组中的元素按照type为索引注册到inetsw指针数组中。

函数2中调用的sock_register函数就是想协议族数组net_families中添加inet协议族的net_proto_family的数据定义，主要是协议族的创建方法inet_create下面是它的实现

int sock_register(const struct net_proto_family *ops)
{
	int err;

	if (ops->family >= NPROTO) {
		printk(KERN_CRIT "protocol %d >= NPROTO(%d)\n", ops->family,
		       NPROTO);
		return -ENOBUFS;
	}

	spin_lock(&net_family_lock);
	if (rcu_dereference_protected(net_families[ops->family],
				      lockdep_is_held(&net_family_lock)))
		err = -EEXIST;
	else {
		RCU_INIT_POINTER(net_families[ops->family], ops);//这里就相当于将ops赋予net_families[ops->families]
		err = 0;
	}
	spin_unlock(&net_family_lock);

	printk(KERN_INFO "NET: Registered protocol family %d\n", ops->family);
	return err;
}

3、套接字的创建

套接字分BSD socket的传输层的socket(struct sock结构，与具体的传输层协议有关)。

3.1、BSD socket的创建

应用程序使用函数socket会产生系统调用，调用sys_socket函数来创建BSD socket：

SYSCALL_DEFINE3(socket, int, family, int, type, int, protocol)
{
	int retval;
	struct socket *sock;
	int flags;

	/* Check the SOCK_* constants for consistency.  */
	BUILD_BUG_ON(SOCK_CLOEXEC != O_CLOEXEC);
	BUILD_BUG_ON((SOCK_MAX | SOCK_TYPE_MASK) != SOCK_TYPE_MASK);
	BUILD_BUG_ON(SOCK_CLOEXEC & SOCK_TYPE_MASK);
	BUILD_BUG_ON(SOCK_NONBLOCK & SOCK_TYPE_MASK);

	flags = type & ~SOCK_TYPE_MASK;
	if (flags & ~(SOCK_CLOEXEC | SOCK_NONBLOCK))
		return -EINVAL;
	type &= SOCK_TYPE_MASK;

	if (SOCK_NONBLOCK != O_NONBLOCK && (flags & SOCK_NONBLOCK))
		flags = (flags & ~SOCK_NONBLOCK) | O_NONBLOCK;

	retval = sock_create(family, type, protocol, &sock);//调用sock_create创建套接字，参数分别是协议族号、套接字类型，使用的传输层协议、执行要创建的套接字的指针的地址。
	if (retval < 0)
		goto out;

	retval = sock_map_fd(sock, flags & (O_CLOEXEC | O_NONBLOCK));
	if (retval < 0)
		goto out_release;

out:
	/* It may be already another descriptor 8) Not kernel problem. */
	return retval;

out_release:
	sock_release(sock);
	return retval;
}

函数sock_create会调用__sock_create函数进行套接字的创建：

int __sock_create(struct net *net, int family, int type, int protocol,
			 struct socket **res, int kern)
{
	int err;
	struct socket *sock;
	const struct net_proto_family *pf;

	/*
	 *      合法性检查
	 */
	if (family < 0 || family >= NPROTO)
		return -EAFNOSUPPORT;
	if (type < 0 || type >= SOCK_MAX)
		return -EINVAL;

	/* Compatibility.

	   This uglymoron is moved from INET layer to here to avoid
	   deadlock in module load.
	 */
	if (family == PF_INET && type == SOCK_PACKET) {
		static int warned;
		if (!warned) {
			warned = 1;
			printk(KERN_INFO "%s uses obsolete (PF_INET,SOCK_PACKET)\n",
			       current->comm);
		}
		family = PF_PACKET;
	}

	err = security_socket_create(family, type, protocol, kern);
	if (err)
		return err;

	sock = sock_alloc();//分配inode结构并获得对应的socket结构
	if (!sock) {
		if (net_ratelimit())
			printk(KERN_WARNING "socket: no more sockets\n");
		return -ENFILE;	/* Not exactly a match, but its the
				   closest posix thing */
	}

	sock->type = type;

	rcu_read_lock();
	pf = rcu_dereference(net_families[family]);
	err = -EAFNOSUPPORT;
	if (!pf)
		goto out_release;

	/*
	 * We will call the ->create function, that possibly is in a loadable
	 * module, so we have to bump that loadable module refcnt first.
	 */
	if (!try_module_get(pf->owner))//模块检测
		goto out_release;

	/* Now protected by module ref count */
	rcu_read_unlock();

	//这里调用inet_create函数对INET协议族进行创建
	err = pf->create(net, sock, protocol, kern);
	if (err < 0)
		goto out_module_put;

	/*
	 * Now to bump the refcnt of the [loadable] module that owns this
	 * socket at sock_release time we decrement its refcnt.
	 */
	if (!try_module_get(sock->ops->owner))
		goto out_module_busy;

	/*
	 * Now that we're done with the ->create function, the [loadable]
	 * module can have its refcnt decremented
	 */
	module_put(pf->owner);
	err = security_socket_post_create(sock, family, type, protocol, kern);
	if (err)
		goto out_sock_release;
	*res = sock;

	return 0;

out_module_busy:
	err = -EAFNOSUPPORT;
out_module_put:
	sock->ops = NULL;
	module_put(pf->owner);
out_sock_release:
	sock_release(sock);
	return err;

out_release:
	rcu_read_unlock();
	goto out_sock_release;
}

其中的参数protocol的取值如下：

/* Standard well-defined IP protocols.  */
enum {
  IPPROTO_IP = 0,		/* Dummy protocol for TCP		*/
  IPPROTO_ICMP = 1,		/* Internet Control Message Protocol	*/
  IPPROTO_IGMP = 2,		/* Internet Group Management Protocol	*/
  IPPROTO_IPIP = 4,		/* IPIP tunnels (older KA9Q tunnels use 94) */
  IPPROTO_TCP = 6,		/* Transmission Control Protocol	*/
  IPPROTO_EGP = 8,		/* Exterior Gateway Protocol		*/
  IPPROTO_PUP = 12,		/* PUP protocol				*/
  IPPROTO_UDP = 17,		/* User Datagram Protocol		*/
  IPPROTO_IDP = 22,		/* XNS IDP protocol			*/
  IPPROTO_DCCP = 33,		/* Datagram Congestion Control Protocol */
  IPPROTO_RSVP = 46,		/* RSVP protocol			*/
  IPPROTO_GRE = 47,		/* Cisco GRE tunnels (rfc 1701,1702)	*/

  IPPROTO_IPV6	 = 41,		/* IPv6-in-IPv4 tunnelling		*/

  IPPROTO_ESP = 50,            /* Encapsulation Security Payload protocol */
  IPPROTO_AH = 51,             /* Authentication Header protocol       */
  IPPROTO_BEETPH = 94,	       /* IP option pseudo header for BEET */
  IPPROTO_PIM    = 103,		/* Protocol Independent Multicast	*/

  IPPROTO_COMP   = 108,                /* Compression Header protocol */
  IPPROTO_SCTP   = 132,		/* Stream Control Transport Protocol	*/
  IPPROTO_UDPLITE = 136,	/* UDP-Lite (RFC 3828)			*/

  IPPROTO_RAW	 = 255,		/* Raw IP packets			*/
  IPPROTO_MAX
};

3.2、INET层socket(inet_socket)和传输层socket(struct sock)创建

函数inet_create完成了上述功能，并初始化了sock的属性值，将socket的sk属性指向sock结构

static int inet_create(struct net *net, struct socket *sock, int protocol,
		       int kern)
{
	struct sock *sk;
	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 (unlikely(!inet_ehash_secret))
		if (sock->type != SOCK_RAW && sock->type != SOCK_DGRAM)
			build_ehash_secret();

	sock->state = SS_UNCONNECTED;

	/* Look for the requested type/protocol pair. */
lookup_protocol:
	err = -ESOCKTNOSUPPORT;
	rcu_read_lock();
	//根据传输层协议的类型创建sock结构
	//遍历inetsw链表
	list_for_each_entry_rcu(answer, &inetsw[sock->type], list) {

		err = 0;
		/* Check the non-wild match. */
		if (protocol == answer->protocol) {
			if (protocol != IPPROTO_IP)
				break;//找到了适配的inetsw[]元素
		} else {
			/* Check for the two wild cases. */
			if (IPPROTO_IP == protocol) {
				protocol = answer->protocol;
				break;
			}
			if (IPPROTO_IP == answer->protocol)
				break;
		}
		err = -EPROTONOSUPPORT;
	}
	//到这里answer指向了合适的inetsw结构，若是TCP协议，answer指向内容如下
	/*
	*	.type =       SOCK_STREAM,
	*	.protocol =   IPPROTO_TCP,
	*	.prot =       &tcp_prot,
	*	.ops =        &inet_stream_ops,
	*	.no_check =   0,
	*	.flags =      INET_PROTOSW_PERMANENT |
	*		      INET_PROTOSW_ICSK,
	*/
	if (unlikely(err)) {
		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 (sock->type == SOCK_RAW && !kern && !capable(CAP_NET_RAW))
		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();

	WARN_ON(answer_prot->slab == NULL);

	err = -ENOBUFS;
	//分配sock结构体内存，这里在inet_init函数初始化好的高速缓冲区中分配内存，然后做一些初始化工作。后面有进一步分析。
	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);//后面有进一步分析，为何可以强制转换？！！
	inet->is_icsk = (INET_PROTOSW_ICSK & answer_flags) != 0;

	inet->nodefrag = 0;

	if (SOCK_RAW == sock->type) {
		inet->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->inet_id = 0;
	//对sk进行初始化设置并将sock中的sk指针指向sk结构
	sock_init_data(sock, sk);

	//进一步设置sk的其他属性信息
	sk->sk_destruct	   = inet_sock_destruct;
	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_all	= 1;
	inet->mc_index	= 0;
	inet->mc_list	= NULL;

	sk_refcnt_debug_inc(sk);

	if (inet->inet_num) {
		/* It assumes that any protocol which allows
		 * the user to assign a number at socket
		 * creation time automatically
		 * shares.
		 */
		inet->inet_sport = htons(inet->inet_num);
		/* Add to protocol hash chains. */
		sk->sk_prot->hash(sk);//调用inet_hash函数
	}
	
	if (sk->sk_prot->init) {
		err = sk->sk_prot->init(sk);//调用tcp_v4_init_sock函数进行进一步的初始化，由于在函数sk_alloc中一些属性被设置成0了，所以在此调用进行初始化
		if (err)
			sk_common_release(sk);
	}
out:
	return err;
out_rcu_unlock:
	rcu_read_unlock();
	goto out;
}

关于套接字struct sock与struct inet_sock、struct tcp_sock、struct inet_connection_sock等结构之间的关系有待进一步了解。

上篇中已经写过，内核中套接字struct socket、struct sock、struct inet_sock、struct tcp_sock、struct raw_sock、struct udp_sock、struct inet_connection_sock、struct inet_timewait_sock和struct tcp_timewait_sock的关系是：

*struct socket这个是BSD层的socket，应用程序会用过系统调用首先创建该类型套接字，它和具体协议无关。

*struct inet_sock是INET协议族使用的socket结构，可以看成位于INET层，是struct sock的一个扩展。它的第一个属性就是struct sock结构。

*struct sock是与具体传输层协议相关的套接字，所有内核的操作都基于这个套接字。

*struct tcp_sock是TCP协议的套接字表示，它是对struct inet_connection_sock的扩展，其第一个属性就是struct inet_connection_sock inet_conn。

*struct raw_sock是原始类型的套接字表示，ICMP协议就使用这种套接字，其是对struct sock的扩展。

*struct udp_sock是UDP协议套接字表示，其是对struct inet_sock套接字的扩展。

*struct inet_connetction_sock是所有面向连接协议的套接字，是对struct inet_sock套接字扩展。

后面两个是用于控制超时的套接字。

就拿struct inet_sock和struct sock为例来说明，为什么内核中可以直接将sock结构体首地址强制转换成inet_sock的首地址？并且inet_sock的大小要大于sock，直接进行如下强制转换

inet = inet_sk(sk);

static inline struct inet_sock *inet_sk(const struct sock *sk)
{
	return (struct inet_sock *)sk;
}

不会发生内存非法访问吗？！那就是在分配的时候并不只是分配的struct sock结构体大小的存储空间！

可以细看sock结构体分配的代码：

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;
		sk->sk_prot = sk->sk_prot_creator = prot;
		sock_lock_init(sk);
		sock_net_set(sk, get_net(net));
		atomic_set(&sk->sk_wmem_alloc, 1);

		sock_update_classid(sk);
	}

	return sk;
}

紧接着调用sk_prot_alloc函数分配：

static struct sock *sk_prot_alloc(struct proto *prot, gfp_t priority,
		int family)
{
	struct sock *sk;
	struct kmem_cache *slab;

	slab = prot->slab;
	if (slab != NULL) {
		sk = kmem_cache_alloc(slab, priority & ~__GFP_ZERO);
		..............................
	} else
		sk = kmalloc(prot->obj_size, priority);

	.....................

	return sk;
......................
}

上面的代码中首先判断高速缓存中是否可用，如果不可用，直接在内存分配空间，不过大小都是prot->obj_size。

如果是TCP协议中的tcp_prot中指明该属性的大小为.obj_size = sizeof(struct tcp_sock)。

所以，程序中给struct sock指针分配的不是该结构体的实际大小，而是大于其实际大小，以便其扩展套接字的属性占用。
以图例说明tcp_sock是如何从sock强制转换来的：

下篇将分析套接字的绑定、连接等一系列操作的实现。