套接字系統調用如何到達sys_socketcall

accept函數為例

系統調用中參數從用戶態向內核態的傳遞是通過寄存器完成的，eax表示系統調用，ebx表示第一個參數，ecx表示第二個參數，edx表示第三個參數（主要針對socke.S）

第一層：accept.s文件

（glibc函數庫）sysdepsunissysvlinuxaccept.S

#define socket accept    //將socket定義為accept
#define _socket _libc_accept
#define NARGS 3           //系統調用的個數
#include < socket.S >     //這幾個套接字函數的通用實現

第二層：socket.S文件

#include < sysdep.h >
#include < sys/socketcall.h >

#define P(a, b) P2(a, b)
#define P2(a, b) a##b

.text
/* The socket-oriented system calls are handled unusally in Linux.
  They are all gated through the single `socketcall' system call number.
  `socketcall' takes two arguments: the first is the subcode, specifying
  which socket function is being called; and the second is a pointer to
  the arguments to the specific function.

  The .S files for the other calls just #define socket and #include this. */

.globl P(__,socket)
ENTRY (P(__,socket))

/* Save registers. */
movl %ebx, %edx

movl $SYS_ify(socketcall), %eax/* System call number in %eax. */
//展開$SYS_iff()宏
/* 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. */
int $0x80

/* Restore registers. */
movl %edx, %ebx

/* %eax is < 0 if there was an error. */
cmpl $-125, %eax
jae syscall_error

/* Successful; return the syscall's value. */
ret

PSEUDO_END (P(__,socket))
c
weak_alias (P(__,socket), socket)

重點看：

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

展開SYS_iff()宏 (glibc函數庫： sysdepsunixsysdep.h)

#ifdef __STDC__
#define SYS_ify(syscall_name) SYS_##syscall_name
#else
#define SYS_ify(syscall_name) SYS_/**/syscall_name
#endif

預處理后為：

movl $SYS_socketcall,%eax
//將系統調用號放入eax中，但是對于網絡套接字所有的接口函數而言，他們的共同入口函數是sys_socketcall

//在內核中，總入口函數sys_socketcall的系統調用號為_NR_socketcall,定義在include/linux/unistd.h
#define __NR_socketcall102

//在glib函數庫中,有如下的預定義
#define SYS_socketcall _NR_socketcall

所以：

movl $SYS_socketcall,%eax
//將sys_socketcall函數對應的系統調用號賦予eax寄存器，從而使套接字系統調用進入到正常的入口函數中

這樣套接字系統調用進入到正確的函數中了。

那么第一個參數是識別系統調用的具體函數的，這個參數在socket.S（glibc庫）中：

movl $P(SOCKOP_,socket), %ebx
//預處理后：
movl $SOCKOP_accept,%ebx//這就完成了第一個參數的傳遞

關于SOCKOP_accept:

查看(glibc庫：sysdepsunixsysvlinuxsocketcall.h)

#ifndef _SYS_SOCKETCALL_H

#define _SYS_SOCKETCALL_H	1

/* Define unique numbers for the operations permitted on socket.  Linux
   uses a single system call for all these functions.  The relevant code
   file is /usr/include/linux/net.h.
   We cannot use a enum here because the values are used in assembler
   code.  */

#define SOCKOP_socket		1
#define SOCKOP_bind		2
#define SOCKOP_connect		3
#define SOCKOP_listen		4
//可以看到SOCKOP_accept對應的值為5
#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

#endif /* _SYS_SOCKETCALL_H */

在linux內核（include/linux/net.h）:

#ifndef _SYS_SOCKETCALL_H

#define _SYS_SOCKETCALL_H	1

/* Define unique numbers for the operations permitted on socket.  Linux
   uses a single system call for all these functions.  The relevant code
   file is /usr/include/linux/net.h.
   We cannot use a enum here because the values are used in assembler
   code.  */

#define SOCKOP_socket		1
#define SOCKOP_bind		2
#define SOCKOP_connect		3
#define SOCKOP_listen		4
//這與glibc庫中的SOCKOP_accept對應的值相同
#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

#endif /* _SYS_SOCKETCALL_H */

第二個參數

在socket.S中：以指針的方式設置了sys_socketcall的第二個參數

lea 4(%esp), %ecx		/* Address of args is 2nd arg.  */

設置完以上的系統調用號還有參數后進入軟中斷：int $0x80，進入了內核態進行處理

第三層：entry.S文件

_system_call:
	pushl %eax			# save orig_eax
	SAVE_ALL
	movl $-ENOSYS,EAX(%esp)
	cmpl $(NR_syscalls),%eax
	jae ret_from_sys_call
    /*對應上面，未調用軟中斷之前，eax寄存器中被初始化為系統調用號，即_NR_socketcall,
       這個調用號別用于在_sys_call_table數組中進行函數指針的尋址，將對應的函數地址對eax寄存器賦值      */
	movl _sys_call_table(,%eax,4),%eax
	testl %eax,%eax
	je ret_from_sys_call
	movl _current,%ebx
	andl $~CF_MASK,EFLAGS(%esp)	# clear carry - assume no errors
	movl $0,errno(%ebx)
	movl %db6,%edx
	movl %edx,dbgreg6(%ebx)  # save current hardware debugging status
	testb $0x20,flags(%ebx)		# PF_TRACESYS
	jne 1f
    /*上面給eax復制后，這里進行調用該函數，到這為止，accept系統調用將請求傳遞給了sys_socketcall,
        然后將socket.S中設置的參數傳遞給sock_accept函數，就完成了應用層接口函數accept到BSD socket層函數的請求傳遞工作
        */
	call *%eax
	movl %eax,EAX(%esp)		# save the return value
	movl errno(%ebx),%edx
	negl %edx
	je ret_from_sys_call
	movl %edx,EAX(%esp)
	orl $(CF_MASK),EFLAGS(%esp)	# set carry to indicate error
	jmp ret_from_sys_call