浅析Arm Linux操作系统调用流程详细解析

    系统调用是os操作系统提供的服务,用户程序通过各种系统调用,来引用内核提供的各种服务,系统调用的执行让用户程序陷入内核,该陷入动作由swi软中断完成.

At91rm9200处理器对应的linux2.4.19内核系统调用对应的软中断定义如下:

#if defined(__thumb__)         //thumb模式
#define __syscall(name)/
    "push    {r7}/n/t" /
    "mov    r7, #" __sys1(__NR_##name) "/n/t"  /
    "swi    0/n/t" /
    "pop    {r7}"
#else                        //arm模式
#define __syscall(name) "swi/t" __sys1(__NR_##name) "/n/t"
#endif

#define __sys2(x) #x
#define __sys1(x) __sys2(x)
#define __NR_SYSCALL_BASE    0x900000                        //此为OS_NUMBER << 20运算值
#define __NR_open            (__NR_SYSCALL_BASE+ 5)    //0x900005

举一个例子来说:open系统调用,库函数最终会调用__syscall (open),宏展开之后为swi #__NR_open,即,swi #0x900005触发中断,中断号0x900005存放在[lr,#-4]地址中,处理器跳转到arch/arm/kernel/entry- common.S中vector_swi读取[lr,#-4]地址中的中断号,之后查询arch/arm/kernel/entry-common.S中的sys_call_table系统调用表,该表内容在arch/arm/kernel/calls.S中定义,__NR_open在表中对应的顺序号为

__syscall_start:

...
.long    SYMBOL_NAME(sys_open)                                               //第5个
...
将sys_call_table[5]中内容传给pc,系统进入sys_open函数,处理实质的open动作

注:用到的一些函数数据所在文件,如下所示
arch/arm/kernel/calls.S声明了系统调用函数
include/asm-arm/unistd.h定义了系统调用的调用号规则

vector_swi定义在arch/arm/kernel/entry-common.S
vector_IRQ定义在arch/arm/kernel/entry-armv.S
vector_FIQ定义在arch/arm/kernel/entry-armv.S

arch/arm/kernel/entry-common.S中对sys_call_table进行了定义:
    .type    sys_call_table, #object
ENTRY(sys_call_table)
#include "calls.S"                                                                                //将calls.S中的内容顺序链接到这里

源程序：

ENTRY(vector_swi)
    save_user_regs
    zero_fp
    get_scno                                        //将[lr,#-4]中的中断号转储到scno(r7)
    arm710_bug_check scno, ip
#ifdef CONFIG_ALIGNMENT_TRAP
    ldr    ip, __cr_alignment
    ldr    ip, [ip]
    mcr    p15, 0, ip, c1, c0                       @ update control register
#endif
    enable_irq ip

    str    r4, [sp, #-S_OFF]!                       @ push fifth arg

    get_current_task tsk
    ldr    ip, [tsk, #TSK_PTRACE]                   @ check for syscall tracing
    bic    scno, scno, #0xff000000                  @ mask off SWI op-code
//#define OS_NUMBER    9[entry-header.S]
//所以对于上面示例中open系统调用号scno=0x900005
//eor scno,scno,#0x900000
//之后scno=0x05
    eor    scno, scno, #OS_NUMBER << 20             @ check OS number
//sys_call_table项为calls.S的内容
    adr    tbl, sys_call_table                      @ load syscall table pointer
    tst    ip, #PT_TRACESYS                         @ are we tracing syscalls?
    bne    __sys_trace

    adrsvc    al, lr, ret_fast_syscall              @ return address
    cmp    scno, #NR_syscalls                       @ check upper syscall limit
//执行sys_open函数
    ldrcc    pc, [tbl, scno, lsl #2]                @ call sys_* routine
    add    r1, sp, #S_OFF
2:  mov    why, #0                                  @ no longer a real syscall
    cmp    scno, #ARMSWI_OFFSET
    eor    r0, scno, #OS_NUMBER << 20               @ put OS number back
    bcs    SYMBOL_NAME(arm_syscall)   
    b    SYMBOL_NAME(sys_ni_syscall)                @ not private func
    /*
     * This is the really slow path. We're going to be doing
     * context switches, and waiting for our parent to respond.
     */
__sys_trace:
    add    r1, sp, #S_OFF
    mov    r0, #0                                   @ trace entry [IP = 0]
    bl    SYMBOL_NAME(syscall_trace)
/*
//2007-07-01 gliethttp [entry-header.S]
//Like adr, but force SVC mode (if required)
  .macro adrsvc, cond, reg, label
     adr/cond /reg, /label
  .endm
//对应反汇编:
//add lr, pc, #16 ; lr = __sys_trace_return
*/
    adrsvc    al, lr, __sys_trace_return            @ return address
    add    r1, sp, #S_R0 + S_OFF                    @ pointer to regs
    cmp    scno, #NR_syscalls                       @ check upper syscall limit
    ldmccia    r1, {r0 - r3}                        @ have to reload r0 - r3
    ldrcc    pc, [tbl, scno, lsl #2]                @ call sys_* routine
    b    2b

__sys_trace_return:
    str    r0, [sp, #S_R0 + S_OFF]!                 @ save returned r0
    mov    r1, sp
    mov    r0, #1                                   @ trace exit [IP = 1]
    bl    SYMBOL_NAME(syscall_trace)
    b    ret_disable_irq

    .align    5
#ifdef CONFIG_ALIGNMENT_TRAP
    .type    __cr_alignment, #object
__cr_alignment:
    .word    SYMBOL_NAME(cr_alignment)
#endif

    .type    sys_call_table, #object
ENTRY(sys_call_table)
#include "calls.S"

来源：赛迪网    作者：skid