在socket创建成功之后，调用bind函数以完成对指定地址和端口的绑定工作；
下面详细分析bind相关代码；
/*
* Bind a name to a socket. Nothing much to do here since it's
* the protocol's responsibility to handle the local address.
*
* We move the socket address to kernel space before we call
* the protocol layer (having also checked the address is ok).
*/ SYSCALL_DEFINE3(bind, int, fd, struct sockaddr __user *, umyaddr, int, addrlen)
{
struct socket *sock;
struct sockaddr_storage address;
int err, fput_needed; /* 获取socket ，fput_need标识是否需要减少文件引用计数*/
sock = sockfd_lookup_light(fd, &err, &fput_needed);
if (sock) {
/* 将用户空间地址复制到内核空间 */
err = move_addr_to_kernel(umyaddr, addrlen, &address);
if (err >= ) {
/* 安全模块的bind检查 */
err = security_socket_bind(sock,
(struct sockaddr *)&address,
addrlen);
if (!err)
/* 调用socket的bind操作 */
err = sock->ops->bind(sock,
(struct sockaddr *)
&address, addrlen);
} /* 根据fput_needed决定是否减少引用计数 */
fput_light(sock->file, fput_needed);
}
return err;
}
首先根据传入的socket描述符来获取socket结构；
static struct socket *sockfd_lookup_light(int fd, int *err, int *fput_needed)
{
/* 获取fd结构 */
struct fd f = fdget(fd);
struct socket *sock; *err = -EBADF;
if (f.file) {
/* 从文件的私有数据中获取socket */
sock = sock_from_file(f.file, err);
if (likely(sock)) {
/* 设置是否需要减少引用计数的标志 */
*fput_needed = f.flags;
return sock;
}
fdput(f);
}
return NULL;
}
之后，将用户空间的地址拷贝到内核空间；
/*
* Support routines.
* Move socket addresses back and forth across the kernel/user
* divide and look after the messy bits.
*/ /**
* move_addr_to_kernel - copy a socket address into kernel space
* @uaddr: Address in user space
* @kaddr: Address in kernel space
* @ulen: Length in user space
*
* The address is copied into kernel space. If the provided address is
* too long an error code of -EINVAL is returned. If the copy gives
* invalid addresses -EFAULT is returned. On a success 0 is returned.
*/ /* 复制socket地址到内核空间 */
int move_addr_to_kernel(void __user *uaddr, int ulen, struct sockaddr_storage *kaddr)
{
/* 长度检查 */
if (ulen < || ulen > sizeof(struct sockaddr_storage))
return -EINVAL;
if (ulen == )
return ; /* 从用户空间拷贝数据 */
if (copy_from_user(kaddr, uaddr, ulen))
return -EFAULT; /* 审计信息 */
return audit_sockaddr(ulen, kaddr);
}
最后，来看最核心的函数，sock->ops->bind调用inet_bind(为什么? 具体可以参考本博套接字调用关系那片文章)，inet_bind将会进行一些列检查之后，调用传输层的sk->sk_prot->get_port函数来执行更详细的绑定工作，比如tcp会调用inet_csk_get_port函数；
/* 地址绑定 */
int inet_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
{
struct sockaddr_in *addr = (struct sockaddr_in *)uaddr;
struct sock *sk = sock->sk;
struct inet_sock *inet = inet_sk(sk);
struct net *net = sock_net(sk);
unsigned short snum;
int chk_addr_ret;
u32 tb_id = RT_TABLE_LOCAL;
int err; /* If the socket has its own bind function then use it. (RAW) */
/*
如果传输控制块有自己的bind操作则调用，
目前只有raw实现了自己的bind
*/
if (sk->sk_prot->bind) {
err = sk->sk_prot->bind(sk, uaddr, addr_len);
goto out;
} err = -EINVAL;
/* 地址长度错误 */
if (addr_len < sizeof(struct sockaddr_in))
goto out; /* 如果不是AF_INET协议族 */
if (addr->sin_family != AF_INET) {
/* Compatibility games : accept AF_UNSPEC (mapped to AF_INET)
* only if s_addr is INADDR_ANY.
*/
err = -EAFNOSUPPORT; /* 接受AF_UNSPEC && s_addr=htonl(INADDR_ANY)的情况 */
if (addr->sin_family != AF_UNSPEC ||
addr->sin_addr.s_addr != htonl(INADDR_ANY))
goto out;
} tb_id = l3mdev_fib_table_by_index(net, sk->sk_bound_dev_if) ? : tb_id;
chk_addr_ret = inet_addr_type_table(net, addr->sin_addr.s_addr, tb_id); /* Not specified by any standard per-se, however it breaks too
* many applications when removed. It is unfortunate since
* allowing applications to make a non-local bind solves
* several problems with systems using dynamic addressing.
* (ie. your servers still start up even if your ISDN link
* is temporarily down)
*/
err = -EADDRNOTAVAIL; /* 合法性检查 */
if (!net->ipv4.sysctl_ip_nonlocal_bind &&
!(inet->freebind || inet->transparent) &&
addr->sin_addr.s_addr != htonl(INADDR_ANY) &&
chk_addr_ret != RTN_LOCAL &&
chk_addr_ret != RTN_MULTICAST &&
chk_addr_ret != RTN_BROADCAST)
goto out; /* 源端口 */
snum = ntohs(addr->sin_port);
err = -EACCES; /* 绑定特权端口的权限检查 */
if (snum && snum < inet_prot_sock(net) &&
!ns_capable(net->user_ns, CAP_NET_BIND_SERVICE))
goto out; /* We keep a pair of addresses. rcv_saddr is the one
* used by hash lookups, and saddr is used for transmit.
*
* In the BSD API these are the same except where it
* would be illegal to use them (multicast/broadcast) in
* which case the sending device address is used.
*/
lock_sock(sk); /* Check these errors (active socket, double bind). */
err = -EINVAL; /* 传输控制块的状态不是CLOSE || 存在本地端口 */
if (sk->sk_state != TCP_CLOSE || inet->inet_num)
goto out_release_sock; /* 设置源地址rcv_addr用作hash查找，saddr用作传输 */
inet->inet_rcv_saddr = inet->inet_saddr = addr->sin_addr.s_addr; /* 组播或者广播，使用设备地址 */
if (chk_addr_ret == RTN_MULTICAST || chk_addr_ret == RTN_BROADCAST)
inet->inet_saddr = ; /* Use device */ /* Make sure we are allowed to bind here. */ /*
端口不为0，或者端口为0允许绑定
则使用协议的具体获取端口函数绑定端口
*/
if ((snum || !inet->bind_address_no_port) &&
sk->sk_prot->get_port(sk, snum)) { /* 绑定失败 */
inet->inet_saddr = inet->inet_rcv_saddr = ; /* 端口在使用中 */
err = -EADDRINUSE;
goto out_release_sock;
} /* 传输控制块已经绑定本地地址或端口标志 */
if (inet->inet_rcv_saddr)
sk->sk_userlocks |= SOCK_BINDADDR_LOCK;
if (snum)
sk->sk_userlocks |= SOCK_BINDPORT_LOCK; /* 设置源端口 */
inet->inet_sport = htons(inet->inet_num); /* 设置目的地址和端口默认值 */
inet->inet_daddr = ;
inet->inet_dport = ; /* 设置路由默认值 */
sk_dst_reset(sk);
err = ;
out_release_sock:
release_sock(sk);
out:
return err;
}
EXPORT_SYMBOL(inet_bind);
TCP bind()系统调用实现部分，请阅读<TCP层bind系统调用的实现分析>