退出程序x86 [英] Exit program x86

问题描述

我正在学习x86汇编.我试图了解退出程序"如何在x86上工作.我们有一个代码:

I am learning x86 assembly. I am trying to understand how "exiting program" works on x86. We have a code :

push ebp
mov ebp,esp
//Some stuff here
mov esp, ebp
pop ebp
ret

当处理器执行"ret"指令时:

When processor executes instruction "ret" :

EIP将具有从堆栈中弹出的值，即0. 因此，处理器将转到0地址并尝试执行指令...，其中不包含程序代码/可执行代码. 那么，处理器到底在发生什么呢?是否有条件检查，例如if EIP = 0 -> exit program? Or if ESP out of bounds -> exit program?`处理器如何理解此RET指令是程序的结尾?

EIP will have value, which is popped from stack, in other words 0. so processor will go to 0 address and will try to execute instructions ... which doesn't contain program code/executable code. So, what is really going on with processor? Are there condition check, for example, if EIP = 0 -> exit program? Or if ESP out of bounds -> exit program? `How processor understands that this RET instruction is the end of the program?

推荐答案

main().用任何语言(包括asm)编写main与编写任何其他函数都没有区别.

main() is called from the normal C runtime initialization functions. Writing main in any language, including asm, is no different from writing any other function.

执行从_start开始.如果您编写自己的_start，则没有内容可返回，因此您需要进行_exit(2)或exit_group(2)系统调用.

Execution begins at _start. If you write your own _start, it has nothing to return to, so you need to make an _exit(2) or exit_group(2) system call.

(否则执行时，segfault落在代码末尾，或者如果尝试执行ret，它将在堆栈中弹出一个值到程序计数器(EIP)中，并可能在从该段中提取代码时出现segfault可能无效的地址.)

(Or else segfault when execution falls off the end of your code, or if you try to ret it will pop a value off the stack into the program counter (EIP), and probably segfault on code-fetch from that probably-invalid address.)

当您使用C编译器进行+编译+链接时，它会在CRT(C RunTime)启动代码中进行链接，该启动代码提供了_start，该文件初始化libc，然后调用main. main返回后，调用它的CRT代码运行atexit函数，然后将main的返回值传递给退出系统调用.

When you compile + link with a C compiler, it links in CRT (C RunTime) startup code that provides a _start which initializes libc then calls main. After your main returns, the CRT code that called it runs atexit functions and then passes main's return value to an exit system call.

_start不是函数，它是进程入口点.例如，在Linux下，进入_start的ESP指向的是argc，而不是返回地址. (请参阅i386 System V ABI.)

_start isn't a function, it's the process entry point. Under Linux for example, on entry to _start ESP points at argc, not a return address. (See the i386 System V ABI.)

这个问题从另一个角度提出了问题，但是我对另一个最近问题的回答更加详细.

This question comes at the question from a different angle, but my answer to another recent question goes into more detail.

与往常一样，单步调试器是查看正在发生的情况并测试您的理解的好方法.

As always, single-stepping with a debugger is a good way to see what's going on and test your understanding.

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