ARM linux系统调用的实现原理

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大家都知道linux的应用程序要想访问内核必须使用系统调用从而实现从usr模式转到svc模式。下面咱们看看它的实现过程。

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"