Question

所以我正在阅读利用剥削的艺术，在书中，他们在C代码中使用了strcpy()函数：

1   #include <stdio.h>
2   #include <string.h>
3   
4       int main() {
5           char str_a[20];
6   
7           strcpy(str_a, "Hello, world!\n");
8           printf(str_a);
9       }

然后，他们继续编译其源代码，并使用gdb对其进行分析。他在第6行，strcpy函数和第8行上设置断点，但是在strcpy上设置断点时，它读取以下内容：

(gdb) break strcpy
Function "strcpy" not defined.
Make breakpoint pending on future shared library load? (y or [n]) y

我知道这是因为尚未加载库，所以询问他是否要将其作为挂起的断点。然后他运行程序并继续执行断点：

一切对他都很好，但是当我尝试在计算机上重新创建它时，我得到以下信息：

frinto@kali:~/Documents/theclang/programs/helloworld$ gcc -m32 -g -o char_array char_array.c 
frinto@kali:~/Documents/theclang/programs/helloworld$ gdb -q char_array
Reading symbols from char_array...done.
(gdb) list
1   #include <stdio.h>
2   #include <string.h>
3   
4       int main() {
5           char str_a[20];
6   
7           strcpy(str_a, "Hello, world!\n");
8           printf(str_a);
9       }
(gdb) break 6
Breakpoint 1 at 0x11b6: file char_array.c, line 6.
(gdb) break strcpy
Function "strcpy" not defined.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 2 (strcpy) pending.
(gdb) break 8
Breakpoint 3 at 0x11d7: file char_array.c, line 8.
(gdb) run
Starting program: /home/frinto/Documents/theclang/programs/helloworld/char_array 

Breakpoint 1, main () at char_array.c:7
7           strcpy(str_a, "Hello, world!\n");
(gdb) cont
Continuing.

Breakpoint 3, main () at char_array.c:8
8           printf(str_a);
(gdb) cont
Continuing.
Hello, world!
[Inferior 1 (process 4021) exited normally]
(gdb)

注意它是如何完全跳过strcpy断点的吗？好吧，我问了我的一个朋友，这里的问题是什么，他告诉我在编译时缺少参数-fno-builtin。我对此参数做了一些最小化的google搜索，我真正理解的是，它允许您在内置函数上设置断点。因此，我使用-fno-builtin参数编译了程序，然后尝试再次重新创建它：

frinto@kali:~/Documents/theclang/programs/helloworld$ gcc -m32 -fno-builtin -g -o char_array char_array.c 
frinto@kali:~/Documents/theclang/programs/helloworld$ gdb -q char_array
Reading symbols from char_array...done.
(gdb) list
1   #include <stdio.h>
2   #include <string.h>
3   
4       int main() {
5           char str_a[20];
6   
7           strcpy(str_a, "Hello, world!\n");
8           printf(str_a);
9       }
(gdb) break 6
Breakpoint 1 at 0x11c6: file char_array.c, line 6.
(gdb) break strcpy
Breakpoint 2 at 0x1040
(gdb) break 8
Breakpoint 3 at 0x11dc: file char_array.c, line 8.
(gdb) run
Starting program: /home/frinto/Documents/theclang/programs/helloworld/char_array 

Breakpoint 1, main () at char_array.c:7
7           strcpy(str_a, "Hello, world!\n");
(gdb) cont
Continuing.

Breakpoint 2, 0xf7e510b0 in ?? () from /lib/i386-linux-gnu/libc.so.6
(gdb) cont
Continuing.

Breakpoint 3, main () at char_array.c:8
8           printf(str_a);
(gdb) cont
Continuing.
Hello, world!
[Inferior 1 (process 3969) exited normally]
(gdb)

现在可以了！我有三个问题：

-fno-builtin参数到底在做什么？
为什么它显示问号而不是其中的strcpy函数

Breakpoint 2, 0xf7e510b0 in ?? () from /lib/i386-linux-gnu/libc.so.6

当我使用strcpy参数时，为什么不要求将-fno-builtin断点设置为未决？

对不起，我只是想确保所有内容都被理解。

Answer 1

来自man gcc

-fno-builtin
-fno-builtin-function

   Don't recognize built-in functions that do not begin with
   __builtin_ as prefix.  GCC normally generates special code to
   handle certain built-in functions more efficiently; for
   instance, calls to "alloca" may become single instructions
   which adjust the stack directly, and calls to "memcpy" may
   become inline copy loops.  The resulting code is often both
   smaller and faster, but since the function calls no longer
   appear as such, you cannot set a breakpoint on those calls, nor
   can you change the behavior of the functions by linking with a
   different library.  In addition, when a function is recognized
   as a built-in function, GCC may use information about that
   function to warn about problems with calls to that function, or
   to generate more efficient code, even if the resulting code
   still contains calls to that function.  For example, warnings
   are given with -Wformat for bad calls to "printf" when "printf"
   is built in and "strlen" is known not to modify global memory.

   With the -fno-builtin-function option only the built-in
   function function is disabled.  function must not begin with
   __builtin_.  If a function is named that is not built-in in
   this version of GCC, this option is ignored.  There is no
   corresponding -fbuiltin-function option; if you wish to enable
   built-in functions selectively when using -fno-builtin or
   -ffreestanding, you may define macros such as:

           #define abs(n)          __builtin_abs ((n))
           #define strcpy(d, s)    __builtin_strcpy ((d), (s))

函数内置函数允许通过内联函数来生成更快的代码，但是如手册中所述

you cannot set a breakpoint on those calls

内联函数意味着，与其生成函数调用，不如将其效果替换为编译器直接插入的代码。这样可以节省函数调用，并且可以更有效地优化它，并且通常可以大大提高性能。

但是，内联函数不再存在于代码中。调试器断点是通过使用某些软件陷阱替换特定地址处的指令或使用特定硬件来识别何时到达断点地址来实现的。但是由于该功能不再存在，因此没有地址与之关联，也无法断点它。

挂起断点是在某些代码上设置断点的一种方法，这些代码稍后将由程序动态加载。使用-fno_builtin，可以直接使用strcpy，并且可以由gdb直接设置bp。

请注意，调试需要-g标志生成的可执行文件中的特定信息。通常，像libc这样的系统库都没有嵌入这些信息，并且在这些库中输入函数时，gdb通过??指示缺少调试信息。

-fno-builtin在这里到底在做什么？

1 个答案: