Linux socket 实现(1)
socket
0. socket 相关的系统调用
socket 的操作，如 socket 、 connect 、 accept 都是系统调用， C 库通过软件中断（不是 CPU 的软中断）进入内核态执行系统调用。
（本文使用的内核版本为 4.6.0 , 84787c572d4）
1. 创建套接字（socket）
套接字虽然也是文件，但是不能用 open 来创建，必须使用 socket 系统调用创建，socket 在内核对应的系统调用是 sys_create ，在 net/socket.c 定义，使用 SYSCALL_DEFINE3 宏定义的。
SYSCALL_DEFINE3(socket, int, family, int, type, int, protocol)
{
...
retval = sock_create(family, type, protocol, &sock);
if (retval < 0)
goto out;
retval = sock_map_fd(sock, flags & (O_CLOEXEC | O_NONBLOCK));
if (retval < 0)
goto out_release;
...
}
主要是两个操作: 创建 socket （ sock_create） 文件和获取 socket 文件描述符 （sock_map_fd）。
1.1. 创建 socket 文件：
int sock_create(int family, int type, int protocol, struct socket **res)
{
return __sock_create(current->nsproxy->net_ns, family, type, protocol, res, 0);
}
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;
/*
* Check protocol is in range
*/
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;
/*
* Allocate the socket and allow the family to set things up. if
* the protocol is 0, the family is instructed to select an appropriate
* default.
*/
sock = sock_alloc();
if (!sock) {
net_warn_ratelimited("socket: no more sockets\n");
return -ENFILE; /* Not exactly a match, but its the
closest posix thing */
}
sock->type = type;
#ifdef CONFIG_MODULES
/* Attempt to load a protocol module if the find failed.
*
* 12/09/1996 Marcin: But! this makes REALLY only sense, if the user
* requested real, full-featured networking support upon configuration.
* Otherwise module support will break!
*/
if (rcu_access_pointer(net_families[family]) == NULL)
request_module("net-pf-%d", family);
#endif
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))
goto out_release;
/* Now protected by module ref count */
rcu_read_unlock();
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;
}
首先获取 socket 结构体和 i 节点:
static struct socket *sock_alloc(void)
{
struct inode *inode;
struct socket *sock;
inode = new_inode_pseudo(sock_mnt->mnt_sb);
if (!inode)
return NULL;
sock = SOCKET_I(inode);
kmemcheck_annotate_bitfield(sock, type);
inode->i_ino = get_next_ino();
inode->i_mode = S_IFSOCK | S_IRWXUGO;
inode->i_uid = current_fsuid();
inode->i_gid = current_fsgid();
inode->i_op = &sockfs_inode_ops;
this_cpu_add(sockets_in_use, 1);
return sock;
}
new_inode_pseudo 获取一个 inode - 其所属的 superblock 不能被 umount，且不支持配额、fsnotify、写回等功能。获取 inode 结构体实际是通过 sock_mnt->mnt_sb->alloc_inode 即 sock_alloc_inode 完成的，其实质就是调用 kmem_cache_alloc 得到一块 socket_alloc 空间。
然后对 inode 进行赋值，要赋给它用户 id 、 组id 、 关联的 sockfs_inode_ops 操作.
最后增加当前 cpu 核的 socket 计数 ， 这个操作是和 CPU 绑定的，含义有二：
this_cpu_add操作只是在执行操作的 CPU 核上执行；`this_cpu_add(sockets_in_use, 1);`sockets_in_use变量每个 CPU 核都有自己的定义：static DEFINE_PER_CPU(int, sockets_in_use);
创建好 inode 结构体之后开始创建 socket 结构体。 pf 是 PF_INET 协议族的结构体（net_families[PF_INET]）， pf->create 创建 socket 是通过 inet_create 实现的，这是在协议栈初始化阶段注册的（(void)sock_register(&inet_family_ops);）.
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;
int try_loading_module = 0;
int err;
if (protocol < 0 || protocol >= IPPROTO_MAX)
return -EINVAL;
sock->state = SS_UNCONNECTED;
/* Look for the requested type/protocol pair. */
lookup_protocol:
err = -ESOCKTNOSUPPORT;
rcu_read_lock();
/*
首先检查是否是 ip 协议
*/
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;
} else {
/* Check for the two wild cases. */
if (IPPROTO_IP == protocol) {
protocol = answer->protocol;
break;
}
if (IPPROTO_IP == answer->protocol)
break;
}
err = -EPROTONOSUPPORT;
}
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 &&
!ns_capable(net->user_ns, CAP_NET_RAW))
goto out_rcu_unlock;
sock->ops = answer->ops;
answer_prot = answer->prot;
answer_flags = answer->flags;
rcu_read_unlock();
WARN_ON(!answer_prot->slab);
err = -ENOBUFS;
sk = sk_alloc(net, PF_INET, GFP_KERNEL, answer_prot, kern);
if (!sk)
goto out;
err = 0;
if (INET_PROTOSW_REUSE & answer_flags)
sk->sk_reuse = SK_CAN_REUSE;
inet = inet_sk(sk);
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 (net->ipv4.sysctl_ip_no_pmtu_disc)
inet->pmtudisc = IP_PMTUDISC_DONT;
else
inet->pmtudisc = IP_PMTUDISC_WANT;
inet->inet_id = 0;
sock_init_data(sock, sk);
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;
inet->rcv_tos = 0;
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. */
err = sk->sk_prot->hash(sk);
if (err) {
sk_common_release(sk);
goto out;
}
}
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;
}
到此 socket 和对应 inode 就创建好了并关联起来，下一步就是创建对应的套接字文件描述符并建立关联。
1.2. 创建套接字文件并建立映射：
static int sock_map_fd(struct socket *sock, int flags)
{
struct file *newfile;
int fd = get_unused_fd_flags(flags);
if (unlikely(fd < 0))
return fd;
newfile = sock_alloc_file(sock, flags, NULL);
if (likely(!IS_ERR(newfile))) {
fd_install(fd, newfile);
return fd;
}
put_unused_fd(fd);
return PTR_ERR(newfile);
}
可以分为三步：
get_unused_fd_flags分配 fd 并标记为 busysock_alloc_file创建文件结构体fd_install将 fd 和文件结构体关联起来
1.3. 总结
应用层调用 socket 创建套接字在内核层要进行的操作可以分为 步：
- 申请分配 socket 结构体；
- 创建 socket 文件；
- 生成 inode；
- 关联 inode 和 socket 文件；
- 得到文件描述符；
- 关联文件描述符和 socket 文件。