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;
}
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。