如何使用GDB查看存储在堆栈上的变量 [英] How to see variables stored on the stack with GDB
问题描述
我试图找出存储在GDB堆栈某个地方的东西。我有一个声明：
I'm trying to figure out what is stored at a certain place on the stack with GDB. I have a statement:
cmpl $0x176,-0x10(%ebp)
在这个函数中，我将0x176与-0x10（％ebp）进行比较，我想知道是否有办法查看存储在 - 0x10（％ebp）。
In this function I'm comparing 0x176 to the -0x10(%ebp) and I am wondering if there is a way to see what is stored at -0x10(%ebp).
推荐答案
我想知道是否有办法看到什么是存储在-0x10（％ebp）。
I am wondering if there is a way to see what is stored at -0x10(%ebp).
假设您已经使用调试信息编译， info locals 会告诉你当前帧中的所有局部变量。之后， print（char *）& a_local - （char *）$ ebp 会告诉你从 a_local 到％ebp ，您通常可以找出本地接近 0x176 的地方。
Assuming you have compiled with debug info, info locals will tell you about all the local variables in current frame. After that, print (char*)&a_local - (char*)$ebp will tell you the offset from start of a_local to %ebp, and you can usually find out what local is close to 0x176.
此外，如果您的本地人有初始化器，您可以执行 info line NN 来确定哪个汇编指令范围对应于初始化给定本地，然后 disas ADDR0，ADDR1 查看反汇编，并再次了解哪个本地位于哪个偏移处。
Also, if your locals have initializers, you can do info line NN to figure out which assembly instruction range corresponds to initialization of a given local, then disas ADDR0,ADDR1 to see the disassembly, and again understand which local is located at what offset.
另一种选择是 readelf -w a.out ，然后查找这样的条目：
Another alternative is to readelf -w a.out, and look for entries like this:
int foo(int x) { int a = x; int b = x + 1; return b - a; }
<1><25>: Abbrev Number: 2 (DW_TAG_subprogram)
<26> DW_AT_external : 1
<27> DW_AT_name : foo
<2b> DW_AT_decl_file : 1
<2c> DW_AT_decl_line : 1
<2d> DW_AT_prototyped : 1
<2e> DW_AT_type : <0x67>
<32> DW_AT_low_pc : 0x0
<36> DW_AT_high_pc : 0x23
<3a> DW_AT_frame_base : 0x0 (location list)
<3e> DW_AT_sibling : <0x67>
<2><42>: Abbrev Number: 3 (DW_TAG_formal_parameter)
<43> DW_AT_name : x
<45> DW_AT_decl_file : 1
<46> DW_AT_decl_line : 1
<47> DW_AT_type : <0x67>
<4b> DW_AT_location : 2 byte block: 91 0 (DW_OP_fbreg: 0)
<2><4e>: Abbrev Number: 4 (DW_TAG_variable)
<4f> DW_AT_name : a
<51> DW_AT_decl_file : 1
<52> DW_AT_decl_line : 1
<53> DW_AT_type : <0x67>
<57> DW_AT_location : 2 byte block: 91 74 (DW_OP_fbreg: -12)
<2><5a>: Abbrev Number: 4 (DW_TAG_variable)
<5b> DW_AT_name : b
<5d> DW_AT_decl_file : 1
<5e> DW_AT_decl_line : 1
<5f> DW_AT_type : <0x67>
<63> DW_AT_location : 2 byte block: 91 70 (DW_OP_fbreg: -16)
这告诉你 x 存储在 fbreg + 0 ， a c> fbreg-12 和 b at fbreg-16 。现在您只需要检查位置列表，以了解如何从％ebp 中派生 fbreg 。上面代码的列表如下所示：
This tells you that x is stored at fbreg+0, a at fbreg-12, and b at fbreg-16. Now you just need to examine location list to figure out how to derive fbreg from %ebp. The list for above code looks like this:
Contents of the .debug_loc section:
Offset Begin End Expression
00000000 00000000 00000001 (DW_OP_breg4: 4)
00000000 00000001 00000003 (DW_OP_breg4: 8)
00000000 00000003 00000023 (DW_OP_breg5: 8)
00000000 <End of list>
因此，对于大部分正文， fbreg 是％ebp + 8 ，这意味着 a 位于％ebp-4 。反汇编确认：
So for most of the body, fbreg is %ebp+8, which means that a is at %ebp-4. Disassembly confirms:
00000000 <foo>:
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: 83 ec 10 sub $0x10,%esp
6: 8b 45 08 mov 0x8(%ebp),%eax # 'x' => %eax
9: 89 45 fc mov %eax,-0x4(%ebp) # '%eax' => 'a'
... ...
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