Linux内核网络协议栈2-socket从库函数到内核

benico 2011-06-14

一、socket()库函数到系统调用,再到内核

1、Linux运行的C库是glibc;

2、socket()调用如下:

1)socket()->__socket():glibc-2.3.6/sysdept/generic/socket.c(weak_alias(name1,name2))

2)__socket():glibc-2.3.6/sysdept/unix/sysv/linux/i386/socket.S

3)ENTER_KERNEL:

movl $SYS_ify(socketcall), %eax	/* System call number in %eax.  */

/* Use ## so `socket' is a separate token that might be #define'd.  */
movl $P(SOCKOP_,socket), %ebx	/* Subcode is first arg to syscall.  */
lea 4(%esp), %ecx		/* Address of args is 2nd arg.  */

        /* Do the system call trap.  */
ENTER_KERNEL
这里,SYS_ify宏定义为:glibc-2.3.6/sysdept/unix/sysv/linux/i386/Sysdept.h
#define SYS_ify(syscall_name)	__NR_##syscall_name;
P宏定义为:glibc-2.3.6/sysdept/unix/sysv/linux/i386/socket.S
#define P(a, b) P2(a, b)
#define P2(a, b) a##b
其中,##为连接符号;
#define __NR_socketcall		102
SOCKOP_socket:glibc-2.3.6/sysdept/unix/sysv/linux/Socketcall.h
#define SOCKOP_socket		1
因此,中断号是102,子中断号是1;
4) int 0x80进入内核:glibc-2.3.6/sysdept/unix/sysv/linux/i386/Sysdept.h:
# define ENTER_KERNEL int $0x80
5) system_call中断入口:kernel/arch/x86/kernel/entry_32.S:
syscall_call:
	call *sys_call_table(,%eax,4)
 

6)进入中断向量表:kernel/arch/x86/kernel/syscall_table_32.S中的102号中断:

.long sys_socketcall
 

7)进入sys_socketcall()函数,根据子中断号以决定走哪个分支:kernel/net/Socket.c:

switch (call) {
	case SYS_SOCKET:
		break;
	case SYS_BIND:
		…...
 

二、socket其他库函数(bind,accept...)

1、对于其他库函数,都是引用上面提到的glibc-2.3.6/sysdept/unix/sysv/linux/i386/socket.S来实现的,如

a)bind.S:

#define	socket	bind
#define	NARGS	3
#define NO_WEAK_ALIAS	1
#include <socket.S>
weak_alias (bind, __bind)
b) accept.S:
#define	socket	accept
#define	__socket __libc_accept
#define	NARGS	3
#define NEED_CANCELLATION
#include <socket.S>
libc_hidden_def (accept)
在各个库函数调用中,设置不同的参数,如socket(用于设置子中断号), NARGS(系统调用的参数个数)等,最终由
movl $P(SOCKOP_,socket), %ebx	/* Subcode is first arg to syscall.  */
来生成最终的子中断号,然后放到ebx寄存器中;2、所有socket系统调用的子中断号参见glibc-2.3.6/sysdept/unix/sysv/linux/Socketcall.h:
#define SOCKOP_socket		1
#define SOCKOP_bind		2
#define SOCKOP_connect		3
#define SOCKOP_listen		4
#define SOCKOP_accept		5
#define SOCKOP_getsockname	6
#define SOCKOP_getpeername	7
#define SOCKOP_socketpair	8
#define SOCKOP_send		9
#define SOCKOP_recv		10
#define SOCKOP_sendto		11
#define SOCKOP_recvfrom		12
#define SOCKOP_shutdown		13
#define SOCKOP_setsockopt	14
#define SOCKOP_getsockopt	15
#define SOCKOP_sendmsg		16
#define SOCKOP_recvmsg		17
 

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