断点改变程序流程

问题描述

限时删除！！

我试图分析和反转一个Objective-C程序。

我通过手动更改某些操作码对可执行文件进行了少量修改。然而，当我测试修改过的软件时，我得到了

没关系，我想我碰到了一些我不应该做的事情。为了分析错误，我启动了 gdb myprogram 。这里发生了一些事情（对我来说很奇怪）：如果我没有放置任何断点，程序会收到SIGKILL，而如果我试图在收到信号之前几行放置一个断点，似乎没有任何事情发生，程序似乎可以正常工作很好。

从这里我的问题是：断点是否改变程序流程？

如果答案是否定的，那么我想我写的信息不足以解决问题，所以请问，如果您有一些提示或建议将我指向正确的方向，我将不胜感激。

我使用MacOS 10.7.4和 gdb 6.3.50（Apple版本gdb-1752）。假设我无法访问源代码。

解决方案

改变代码工作方式的断点的头号原因是种族条件。它基本上是这样的：

 没有断点：$ b​​ $ b发出一些异步请求
做一些响应
错误，因为请求尚未响应
 
带有断点：$ b​​ $ b发送一些异步请求
等待用户继续
响应等待继续时到达
做出回应
的事情OK！ 
  

I'm trying to analyze and reverse a Objective-C program I have.

I made few modifications to the executable by changing some opcodes by hand. When I test the modified software, however, I get

That's fine, I think I touched something I should not. I launched then gdb myprogram in order to analyze the error. Here something (strange to me) happened: if I do not put any breakpoint the program receives SIGKILL, while if I try to put a breakpoint few lines before the one in which I receive the signal nothing seems to happen and the program seems to work fine.

From here my question: does a breakpoint change the program flow?
If the answer is no, then I imagine the informations I wrote are not enough to solve so please ask, I would appreciate if you have some tips or suggestions to point me to the right direction.

I'm using MacOS 10.7.4 and gdb 6.3.50 (Apple version gdb-1752). Assume I don't have access to the source code.

解决方案

The number one cause for breakpoints altering how the code works is race conditions. It basically goes like this:

Without breakpoints:
    make some asynchronous request
    do something with response
    ERROR because request hasn't responded yet

With breakpoints:
    send some asynchronous request
    wait for user to continue
    response arrived while waiting for the continue
    do something with response
    OK!

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