使用gdb进行写操作

使用gdb调试程序，读写操作是很普遍的事情。其中，读操作包括：

• 读取某个变量的值
• 读取某个内存地址里的内容
• 读取某个寄存器的值

对应地，写操作包括：

• 修改某个变量的值
• 修改某个内存地址里的内容
• 修改某个寄存器的值

本文将首先简单介绍一下读操作，然后重点介绍一下写操作。

1. 读操作

• 读取某个变量的值: p <var>
• 读取某个内存地址里的内容: x /NFU <memaddr>
• 读取某个寄存器的值: info r <register>

2. 写操作

先上个例子// foo.c

 #include <stdio.h>

 typedef enum {false, true} bool;

 static bool g_verbose = ;

 static void
 dump(int a[], int n)
 {
         for (int i = ; i < n; i++)
                 printf("%-2d ", a[i]);
         printf("\n");
 }

 int main(int argc, char *argv[])
 {
         int a[] = {0x1, 0x2, 0x3, 0x4, 0x5};
         int n = sizeof(a) / sizeof(int);

         if (g_verbose) {
                 printf("Now dump a[] ...\n");
                 dump(a, n);
         }

         int m = ;
         for (int i = ; i < n; i++)
                 m += a[i];

         return m;
 }

在上面的代码中，第5行我们定义了一个变量g_verbose, 其值为false。现在，我们将通过gdb在第20行将其修改为true，给出三种方法，贯穿了修改变量的值，修改内存地址内容和修改寄存器。

2.1 修改某个变量的值

set var <name> = <value>

e.g.

$ gcc -g -Wall -std=c99 -o foo foo.c

$ gdb foo
GNU gdb (Ubuntu ..-0ubuntu5~..) ..
...<snip>.......................................................................
(gdb) l main
                      printf("%-2d ", a[i]);
              printf("\n");
      }

      int main(int argc, char *argv[])
      {
              int a[] = {0x1, 0x2, 0x3, 0x4, 0x5};
              int n = sizeof(a) / sizeof(int);

              if (g_verbose) {
(gdb) b
Breakpoint  at 0x8048500: file foo.c, line .
(gdb) r
Starting program: /tmp/foo

Breakpoint , main (argc=, argv=0xbffff054) at foo.c:
              if (g_verbose) {
(gdb) p g_verbose
$ = false
(gdb) set var g_verbose = true
(gdb) p g_verbose
$ = true
(gdb) c
Continuing.
Now dump a[] ...

[Inferior  (process ) exited with code ]
(gdb)

2.2 修改某个内存地址里的内容

set *(unsigned char      *)<memaddr> = <value>  ; write 1 byte
set *(unsigned short     *)<memaddr> = <value>  ; write 2 bytes
set *(unsigned int       *)<memaddr> = <value>  ; write 4 bytes
set *(unsigned long long *)<memaddr> = <value>  ; write 8 bytes

or

set *(char      *)<memaddr> = <value>           ; write 1 byte
set *(short     *)<memaddr> = <value>           ; write 2 bytes
set *(int       *)<memaddr> = <value>           ; write 4 bytes
set *(long long *)<memaddr> = <value>           ; write 8 bytes

e.g.

$ gdb foo
GNU gdb (Ubuntu ..-0ubuntu5~..) ..
...<snip>.......................................................................
(gdb) l main
                      printf("%-2d ", a[i]);
              printf("\n");
      }

      int main(int argc, char *argv[])
      {
              int a[] = {0x1, 0x2, 0x3, 0x4, 0x5};
              int n = sizeof(a) / sizeof(int);

              if (g_verbose) {
(gdb)
                      printf("Now dump a[] ...\n");
                      dump(a, n);
              }

              int m = ;
              for (int i = ; i < n; i++)
                      m += a[i];

              return m;
      }
(gdb) b
Breakpoint  at 0x8048500: file foo.c, line .
(gdb) r
Starting program: /tmp/foo

Breakpoint , main (argc=, argv=0xbffff054) at foo.c:
              if (g_verbose) {
(gdb) #
(gdb)
(gdb) p &g_verbose
$ = (bool *) 0x804a02c <g_verbose>
(gdb) #
(gdb)
(gdb) x /x 0x804a02c
0x804a02c <g_verbose>:  0x00000000
(gdb) #
(gdb)
(gdb) set *(unsigned int *)0x804a02c = 0x1
(gdb) #
(gdb)
(gdb) x /x 0x804a02c
0x804a02c <g_verbose>:  0x00000001
(gdb) #
(gdb)
(gdb) c
Continuing.
Now dump a[] ...

[Inferior  (process ) exited with code ]
(gdb) q

2.3 修改某个寄存器的值

set $<register> = <value>

e.g.

$ gdb foo
GNU gdb (Ubuntu ..-0ubuntu5~..) ..
...<snip>.......................................................................
(gdb) display /i $eip
(gdb) set disassembly-flavor intel
(gdb) disas /m main
Dump of assembler code for function main:
      {
   0x080484c7 <+>:     push   ebp
   0x080484c8 <+>:     mov    ebp,esp
   0x080484ca <+>:     and    esp,0xfffffff0
   0x080484cd <+>:     sub    esp,0x30

              int a[] = {0x1, 0x2, 0x3, 0x4, 0x5};
   0x080484d0 <+>:     mov    DWORD PTR [esp+0x1c],0x1
   0x080484d8 <+>:    mov    DWORD PTR [esp+0x20],0x2
   0x080484e0 <+>:    mov    DWORD PTR [esp+0x24],0x3
   0x080484e8 <+>:    mov    DWORD PTR [esp+0x28],0x4
   0x080484f0 <+>:    mov    DWORD PTR [esp+0x2c],0x5

              int n = sizeof(a) / sizeof(int);
   0x080484f8 <+>:    mov    DWORD PTR [esp+0x18],0x5

              if (g_verbose) {
   0x08048500 <+>:    mov    eax,ds:0x804a02c
   0x08048505 <+>:    test   eax,eax
   0x08048507 <+>:    je     0x8048529 <main+>

                      printf("Now dump a[] ...\n");
   0x08048509 <+>:    mov    DWORD PTR [esp],0x80485f6
   0x08048510 <+>:    call   0x8048340 <puts@plt>

                      dump(a, n);
   0x08048515 <+>:    mov    eax,DWORD PTR [esp+0x18]
   0x08048519 <+>:    mov    DWORD PTR [esp+0x4],eax
   0x0804851d <+>:    lea    eax,[esp+0x1c]
   0x08048521 <+>:    mov    DWORD PTR [esp],eax
   0x08048524 <+>:    call   0x804847d <dump>
              }

              int m = ;
   0x08048529 <+>:    mov    DWORD PTR [esp+0x10],0x0

              for (int i = ; i < n; i++)
   0x08048531 <+>:   mov    DWORD PTR [esp+0x14],0x0
   0x08048539 <+>:   jmp    0x804854c <main+>
   0x08048547 <+>:   add    DWORD PTR [esp+0x14],0x1
   0x0804854c <+>:   mov    eax,DWORD PTR [esp+0x14]
   0x08048550 <+>:   cmp    eax,DWORD PTR [esp+0x18]
   0x08048554 <+>:   jl     0x804853b <main+>

                      m += a[i];
   0x0804853b <+>:   mov    eax,DWORD PTR [esp+0x14]
   0x0804853f <+>:   mov    eax,DWORD PTR [esp+eax*+0x1c]
   0x08048543 <+>:   add    DWORD PTR [esp+0x10],eax

              return m;
   0x08048556 <+>:   mov    eax,DWORD PTR [esp+0x10]

      }
   0x0804855a <+>:   leave
   0x0804855b <+>:   ret

End of assembler dump.
(gdb) b
Breakpoint  at 0x8048500: file foo.c, line .
(gdb) r
Starting program: /tmp/foo

Breakpoint , main (argc=, argv=0xbffff054) at foo.c:
              if (g_verbose) {
: x/i $eip
=> 0x8048500 <main+>: mov    eax,ds:0x804a02c
(gdb) #
(gdb)
(gdb) info r eax
eax            0x1
(gdb) ni
0x08048505                    if (g_verbose) {
: x/i $eip
=> 0x8048505 <main+>: test   eax,eax
(gdb) info r eax
eax            0x0
(gdb) #
(gdb) set $eax = 0x1
(gdb) #
(gdb)
(gdb) info r eax
eax            0x1
(gdb) c
Continuing.
Now dump a[] ...

[Inferior  (process ) exited with code ]
(gdb) q

总结： 熟练掌握了gdb的读写操作，对于调试那种release版本的程序bug是相当有帮助的。最近两天，我在一个测试环境中调试一个跟子网掩码有关的bug。TNND可执行程序的符号表已经被删除，而且测试环境中不能clone源代码进行修改后编译（因为源代码保护）。幸运地是，还可以自己安装gdb。 有了gdb, 通过修改某个寄存器的值快速地把debug信息打印了出来，从而有效地缩小了bug存在的代码范围。一句话，gdb很好使，你值得拥有！