linux汇编之——(6)反汇编条件控制

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C语言的条件控制语句，在汇编里说白了就是通过对ZF零标志位进行判断，然后由跳转指令来实现逻辑分支。

1 条件语句if

int main(void)

{

int a;

if(a)

return 1;

else

return 0;

}

反汇编代码如下

080483b4 <main>:

80483b4: 55 push %ebp

80483b5: 89 e5 mov %esp,%ebp

80483b7: 83 ec 10 sub $0x10,%esp

80483ba: 83 7d fc 00 cmpl $0x0,-0x4(%ebp)

80483be: 74 07 je 80483c7<main+0x13>

80483c0: b8 01 00 00 00 mov $0x1,%eax

80483c5: eb 05 jmp 80483cc<main+0x18>

80483c7: b8 00 00 00 00 mov $0x0,%eax

80483cc: c9 leave

80483cd: c3 ret

80483ce: 90 nop

80483cf: 90 nop

代码简析：cmpl比较a值是否为0，如果状态寄存器的zero标志位为0则je到地址80483c7；如果不为0则正常退出是eax为1然后jmp跳转地址到80483cc

2 条件语句case

int main(void)

{

int a;

switch(a)

{

case 0:

return 0;

default:

return 1;

}

反汇编objdump

080483b4 <main>:

80483b4: 55 push %ebp

80483b5: 89 e5 mov %esp,%ebp

80483b7: 83 ec 10 sub $0x10,%esp

80483ba: 8b 45 fc mov -0x4(%ebp),%eax

80483bd: 85 c0 test %eax,%eax

80483bf: 75 07 jne 80483c8<main+0x14>

80483c1: b8 00 00 00 00 mov $0x0,%eax

80483c6: eb 05 jmp 80483cd<main+0x19>

80483c8: b8 01 00 00 00 mov $0x1,%eax

80483cd: c9 leave

80483ce: c3 ret

三个case 条件

int main(void)

{

inta=1;

switch(a)

{

case 0:

return 0;

case 1:

return -1;

default:

return 1;

}

反汇编objdump

080483b4 <main>:

80483b4: 55 push %ebp

80483b5: 89 e5 mov %esp,%ebp

80483b7: 83 ec 10 sub $0x10,%esp

80483ba: c7 45 fc 01 00 00 00 movl $0x1,-0x4(%ebp)

80483c1: 8b 45 fc mov -0x4(%ebp),%eax

80483c4: 85 c0 test %eax,%eax

80483c6: 74 07 je 80483cf<main+0x1b>

80483c8: 83 f8 01 cmp $0x1,%eax

80483cb: 74 09 je 80483d6<main+0x22>

80483cd: eb 0e jmp 80483dd<main+0x29>

80483cf: b8 00 00 00 00 mov $0x0,%eax

80483d4: eb 0c jmp 80483e2<main+0x2e>

80483d6: b8 ff ff ff ff mov $0xffffffff,%eax

80483db: eb 05 jmp 80483e2<main+0x2e>

80483dd: b8 01 00 00 00 mov $0x1,%eax

80483e2: c9 leave

80483e3: c3 ret

3 Case语句有无break汇编比较

有break语句

int main(void)

{

int a;

switch(a)

{

case 0:

a =1;

break;

default:

a =0;

break;

}

returna;

}

有break语句，反汇编objdump

080483b4 <main>:

80483b4: 55 push %ebp

80483b5: 89 e5 mov %esp,%ebp

80483b7: 83 ec 10 sub $0x10,%esp

80483ba: 8b 45 fc mov -0x4(%ebp),%eax

80483bd: 85 c0 test %eax,%eax

80483bf: 75 09 jne 80483ca<main+0x16>

80483c1: c7 45 fc 01 00 00 00 movl $0x1,-0x4(%ebp)

80483c8: eb 07 jmp 80483d1<main+0x1d>

80483ca: c7 45 fc 00 00 00 00 movl $0x0,-0x4(%ebp)

80483d1: 8b 45 fc mov -0x4(%ebp),%eax

80483d4: c9 leave

80483d5: c3 ret

无break语句

int main(void)

{

int a;

switch(a)

{

case 0:

a =1;

default:

a =0;

}

returna;

}

无break语句，反汇编objdump

080483b4 <main>:

80483b4: 55 push %ebp

80483b5: 89 e5 mov %esp,%ebp

80483b7: 83 ec 10 sub $0x10,%esp

80483ba: 8b 45 fc mov -0x4(%ebp),%eax

80483bd: 85 c0 test %eax,%eax

80483bf: 75 07 jne 80483c8<main+0x14>

80483c1: c7 45 fc 01 00 00 00 movl $0x1,-0x4(%ebp)

80483c8: c7 45 fc 00 00 00 00 movl $0x0,-0x4(%ebp)

80483cf: 8b 45 fc mov -0x4(%ebp),%eax

80483d2: c9 leave

80483d3: c3 ret

从上面对比可以看出缺少break的语句，汇编缺少了一条跳转到出口的语句

80483c8: eb07 jmp 80483d1<main+0x1d>