[英]Reverse engineer optimized c code from assembly
The point of this problem is to reverse engineer c code that was made after running the compiler with level 2 optimization. 这个问题的关键是对使用2级优化运行编译器后生成的c代码进行逆向工程。 The original c code is as follows (computes the greatest common divisor): 原始c代码如下(计算最大公约数):
int gcd(int a, int b){
int returnValue = 0;
if (a != 0 && b != 0){
int r;
int flag = 0;
while (flag == 0){
r = a % b;
if (r ==0){
flag = 1;
} else {
a = b;
b = r;
}
}
returnValue = b;
}
return(returnValue);
}
when I ran the optimized compile I ran this from the command line: 当我运行优化编译时,我从命令行运行它:
gcc -O2 -S Problem04b.c
to get the assembly file for this optimized code 获取此优化代码的程序集文件
.gcd:
.LFB12:
.cfi_startproc
testl %esi, %esi
je .L2
testl %edi, %edi
je .L2
.L7:
movl %edi, %edx
movl %edi, %eax
movl %esi, %edi
sarl $31, %edx
idivl %esi
testl %edx, %edx
jne .L9
movl %esi, %eax
ret
.p2align 4,,10
.p2align 3
.L2:
xorl %esi, %esi
movl %esi, %eax
ret
.p2align 4,,10
.p2align 3
.L9:
movl %edx, %esi
jmp .L7
.cfi_endproc
I need to convert this assembly code back to c code here is where I am at right now: 我需要将此汇编代码转换回c代码,这是我现在所处的位置:
int gcd(int a int b){
/*
testl %esi %esi
sets zero flag if a is 0 (ZF) but doesn't store anything
*/
if (a == 0){
/*
xorl %esi %esi
sets the value of a variable to 0. More compact than movl
*/
int returnValue = 0;
/*
movl %esi %eax
ret
return the value just assigned
*/
return(returnValue);
}
/*
testl %edi %edi
sets zero flag if b is 0 (ZF) but doesn't store anything
*/
if (b == 0){
/*
xorl %esi %esi
sets the value of a variable to 0. More compact than movl
*/
int returnValue = 0;
/*
movl %esi %eax
ret
return the value just assigned
*/
return(returnValue);
}
do{
int r = b;
int returnValue = b;
}while();
}
Can anyone help me write this back in to c code? 任何人都可以帮我写回c代码吗? I'm pretty much lost. 我很丢失。
First of all, you have the values mixed in your code. 首先,您在代码中混合了值。 %esi
begins with the value b
and %edi
begins with the value a
. %esi
以值b
开头, %edi
以值a
开头。
You can infer from the testl %edx, %edx
line that %edx
is used as the condition variable for the loop beginning with .L7
(if %edx
is different from 0 then control is transferred to the .L9
block and then returned to .L7
). 您可以从testl %edx, %edx
行推断出%edx
用作以.L7
开头的循环的条件变量(如果%edx
与0不同,则控制转移到.L9
块然后返回.L7
)。 We'll refer to %edx
as remainder
in our reverse-engineered code. 我们将%edx
称为反向工程代码中的remainder
。
Let's begin reverse-engineering the main loop: 让我们开始对主循环进行逆向工程:
movl %edi, %edx
Since %edi
stores a
, this is equivalent to initializing the value of remainder
with a
: int remainder = a;
由于%edi
存储a
,这等同于初始化的值remainder
与a
: int remainder = a;
. 。
movl %edi, %eax
Store int temp = a;
Store int temp = a;
movl %esi, %edi
Perform int a = b;
执行int a = b;
(remember that %edi
is a
and %esi
is b
). (记住%edi
是a
而%esi
是b
)。
sarl $31, %edx
This arithmetic shift instruction shifts our remainder
variable 31 bits to the right whilst maintaining the sign of the number. 该算术移位指令将remainder
变量31位向右移位,同时保持数字的符号 。 By shifting 31 bits you're setting remainder
to 0 if it's positive (or zero) and to -1 if it's negative. 通过移位31位,如果它为正(或零),则将remainder
设置为0,如果为负,则将其设置为-1。 So it's equivalent to remainder = (remainder < 0) ? -1 : 0
所以它相当于remainder = (remainder < 0) ? -1 : 0
remainder = (remainder < 0) ? -1 : 0
. remainder = (remainder < 0) ? -1 : 0
。
idivl %esi
Divide %edx:%eax
by %esi
, or in our case, divide remainder * temp
by b
(the variable). 将%edx:%eax
除以%esi
,或者在我们的例子中,将remainder * temp
除以b
(变量)。 The remainder will be stored in %edx
, or in our code, remainder
. 余数将存储在%edx
,或存储在我们的代码中, remainder
。 When combining this with the previous instruction: if remainder < 0
then remainder = -1 * temp % b
, and otherwise remainder = temp % b
. 将此与前一条指令组合时:如果remainder < 0
则remainder = -1 * temp % b
,否则remainder = temp % b
。
testl %edx, %edx
jne .L9
Check to see if remainder
is equal to 0 - if it's not, jump to .L9
. 检查remainder
是否等于0 - 如果不是,则跳转到.L9
。 The code there simply sets b = remainder;
那里的代码只是设置b = remainder;
before returning to .L7
. 在返回.L7
之前。 In order to implement this in C, we'll keep a count
variable that will store the amount of times the loop has iterated. 为了在C中实现这一点,我们将保留一个count
变量,该变量将存储循环迭代的次数。 We'll perform b = remainder
at the beginning of the loop but only after the first iteration, meaning when count != 0
. 我们将在循环开始时执行b = remainder
,但仅在第一次迭代之后执行,这意味着当count != 0
。
We're now ready to build our full C loop: 我们现在准备构建完整的C循环:
int count = 0;
do {
if (count != 0)
b = remainder;
remainder = a;
temp = a;
a = b;
if (remainder < 0){
remainder = -1 * temp % b;
} else {
remainder = temp % b;
}
count++;
} while (remainder != 0)
And after the loop terminates, 循环结束后,
movl %esi, %eax
ret
Will return the GCD that the program computed (in our code it'll be stored in the b
variable). 将返回程序计算的GCD(在我们的代码中它将存储在b
变量中)。
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.