如何确定此地址来自64位macOS应用程序的位置 [英] How to determine where this address comes from on 64-bit macOS application

问题描述

因此，我目前正在调试一个问题，并试图弄清楚这是怎么发生的.

So I'm currently debugging an issue, and trying to figure out how this could happen.

这是obj-c运行时中称为objc_msgsend()

Here is the assembly for a method in the obj-c runtime, called objc_msgsend()

libobjc.A.dylib`objc_msgSend:
    0x7fff9084a0c0 <+0>:   testq  %rdi, %rdi
    0x7fff9084a0c3 <+3>:   je     0x7fff9084a140            ; <+128>
    0x7fff9084a0c6 <+6>:   testb  $0x1, %dil
    0x7fff9084a0ca <+10>:  jne    0x7fff9084a14b            ; <+139>
    0x7fff9084a0cd <+13>:  movabsq $0x7ffffffffff8, %r11
    0x7fff9084a0d7 <+23>:  andq   (%rdi), %r11
    0x7fff9084a0da <+26>:  movq   %rsi, %r10
    0x7fff9084a0dd <+29>:  andl   0x18(%r11), %r10d

我正在使用Xcode的lldb查看寄存器和地址.

I'm using Xcode's lldb to view the registers and addresses.

这是我第一次检出偏移量为+0(预期)的寄存器时得到的有趣输出:

Here is the interesting output I get, when I first check out the registers at offset +0 (expected):

(lldb) register read
r11 = 0x00007fff74a940f0  (void *)0x00007fff74a94118: NSObject

偏移+13后(预期):

(lldb) register read
r11 = 0x00007ffffffffff8

偏移+23后(未预期):

(lldb) register read
r11 = 0x0000000100761138  (void *)0x0000000100761160: GTMOAuth2WindowController

然后如果我po此时的寄存器:

And then if I po the registers at this point:

(lldb) po $rdi
<GTMOAuth2WindowController: 0x6100001c2850>

(lldb) po &$rdi
0x000000010bc2b3b8

(lldb) po $r11
GTMOAuth2WindowController

(lldb) po &$r11
0x000000010bc2b3b8

所以这是我迷路的地方；在偏移+23之后，当我register read时该地址是什么? 0x0000000100761138.我希望它具有0x6100001c2850，即从+23

So here's where I'm lost; after offset +23, what is that address when I register read? 0x0000000100761138. I would've expected it to have 0x6100001c2850, the location of the object from the dereference at +23

如果我po $r11它打印出了类名(这是预期的，因为我们正在查看isa属性)，并且如果我为指针打印了内存中的位置，则它与地址不匹配在register read中，它与%rdi的地址匹配(预期).

If I po $r11 it prints out the class name (which is expected because we're looking at the isa property), and if I print the location in memory for the pointer, it doesn't match the address in register read, it matches the address of %rdi (expected).

推荐答案

在%r11之后的%r11中的地址是表示您GTMOAuth2WindowController类的类对象的地址.

That address in %r11 after <+23>, 0x0000000100761138, is the address of the class object that represents your GTMOAuth2WindowController class.

在编译时(使用@interface和@implementation)定义类时，在运行时会有一个特殊的对象表示该类.实际上，它被称为类对象"，就像您创建的所有对象一样，它是真实的对象.这意味着类对象本身可以响应消息.当您在<+23>之后说po %r11时，调试器将description消息发送到类对象.类对象的description方法以NSString的形式返回类的名称，因此调试器将打印该类的名称.

When you define a class at compile time (with @interface and @implementation), there's a special object at runtime that represents the class. In fact it's called a "class object" and is a real object just like all the objects you created. This means that the class object can itself respond to messages. When you said po %r11 after <+23>, the debugger sent the description message to the class object. The class object's description method returns the name of the class as an NSString, so the debugger printed the name of the class.

您可以在此处了解有关类对象的更多信息.该页面上的图片链接在Chrome中已损坏，但您可以单击它以查看pdf.

You can learn more about class objects here. The image link on that page is broken in Chrome but you can click it to see a pdf.

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