TCP/IP协议栈源码图解分析系列10:linux内核协议栈中对于socket相关API的实现

题记：本系列文章的目的是抛开书本从Linux内核源代码的角度详细分析TCP/IP协议栈内核相关技术

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linux内核协议栈中对于socket相关API的实现

首先对于内核中断向量表不是很熟悉的请先参考一下博文：《TCP/IP协议栈源码图解分析系列6:linux 系统调用中断向量表》 URL：http://blog.csdn.net/byhankswang/article/details/9284023

首先应该做的事情

定义好了内核中断向量表之后，需要做的就是当用户层程序陷入到内核态之后，通过内核中断向量表找到了内核中对于该系统调用的实现。补充一下内核中SYSCALL_DEFINE的用法：

SYSCALL_DEFINE3(socket, int, family, int, type, int, protocol){ ….}

<=>SYSCALL_DEFINEX(3,_socket,__VA_ARGS__)

<=>_SYSCALL_DEFINE(3,_socket,__VA_ARGS__)

<=>asmlinkage long sys_socket(int family,int type,int protocol)

SYSCALL_DEFINE* 把内核中断向量表和内核实现完美的衔接了起来。

用户层API内核是如何实现的

以socket相关的套接字编程接口为例（linux 3.9.3）：

socket.c:1382:SYSCALL_DEFINE3(socket, int, family, int, type, int, protocol)
socket.c:1423:SYSCALL_DEFINE4(socketpair, int, family, int, type, int, protocol,
socket.c:1519:SYSCALL_DEFINE3(bind, int, fd, struct sockaddr __user *, umyaddr, int, addrlen)
socket.c:1548:SYSCALL_DEFINE2(listen, int, fd, int, backlog)
socket.c:1581:SYSCALL_DEFINE4(accept4, int, fd, struct sockaddr __user *, upeer_sockaddr,
socket.c:1662:SYSCALL_DEFINE3(accept, int, fd, struct sockaddr __user *, upeer_sockaddr,
socket.c:1680:SYSCALL_DEFINE3(connect, int, fd, struct sockaddr __user *, uservaddr,
socket.c:1712:SYSCALL_DEFINE3(getsockname, int, fd, struct sockaddr __user *, usockaddr,
socket.c:1743:SYSCALL_DEFINE3(getpeername, int, fd, struct sockaddr __user *, usockaddr,
socket.c:1775:SYSCALL_DEFINE6(sendto, int, fd, void __user *, buff, size_t, len,
socket.c:1822:SYSCALL_DEFINE4(send, int, fd, void __user *, buff, size_t, len,
socket.c:1834:SYSCALL_DEFINE6(recvfrom, int, fd, void __user *, ubuf, size_t, size,
socket.c:1890:SYSCALL_DEFINE5(setsockopt, int, fd, int, level, int, optname,
socket.c:1924:SYSCALL_DEFINE5(getsockopt, int, fd, int, level, int, optname,
socket.c:1954:SYSCALL_DEFINE2(shutdown, int, fd, int, how)
socket.c:2096:SYSCALL_DEFINE3(sendmsg, int, fd, struct msghdr __user *, msg, unsigned int, flags)
socket.c:2171:SYSCALL_DEFINE4(sendmmsg, int, fd, struct mmsghdr __user *, mmsg,
socket.c:2269:SYSCALL_DEFINE3(recvmsg, int, fd, struct msghdr __user *, msg,
socket.c:2393:SYSCALL_DEFINE5(recvmmsg, int, fd, struct mmsghdr __user *, mmsg,
socket.c:2435:SYSCALL_DEFINE2(socketcall, int, call, unsigned long __user *, args)

然后我们看相关的源代码以socket和bind为例：

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);
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;
}

SYSCALL_DEFINE3(bind, int, fd, struct sockaddr __user *, umyaddr, int, addrlen)
{
struct socket *sock;
struct sockaddr_storage address;
int err, fput_needed;

sock = sockfd_lookup_light(fd, &err, &fput_needed);
if (sock) {
err = move_addr_to_kernel(umyaddr, addrlen, &address);
if (err >= 0) {
err = security_socket_bind(sock,
  (struct sockaddr *)&address,
  addrlen);
if (!err)
err = sock->ops->bind(sock,
     (struct sockaddr *)
     &address, addrlen);
}
fput_light(sock->file, fput_needed);
}
return err;
}

我们可以看到，只要抓住了主要的脉络，分析内核协议栈是很简单的事情，用侯捷先生的话说“源码在手，了无秘密”。