Switch语句与链接—计算机系统基础

实验内容：修改二进制可重定位目标文件“phase1.o”中相关节的内容（注意不允许修改.text节和重定位节的内容），使其与main.o模块如下链接后运行时输出目标字符串“123456789”

gcc -no-pie -o linkbomb main.o phase1.o
./linkbomb
目标字符串

实验步骤：

1. 使用objdump工具获得目标文件的汇编代码，使用readelf工具获得其重定位记录。

2.结合汇编代码和重定位信息，推断目标文件的汇编代码中各函数的功能作用，定位出其中负责输出的函数。

3.构造调用输出函数的指令代码。

4.使用构造的调用输出函数的指令代码，替换目标文件中的do_phase过程中的nop指令，实现目标字符串的输出。

实验验证：

1.查看反汇编代码

objdump -d phase1.o > phase1.s
cat phase1.s

00000028 <do_phase>:
  28:   55                      push   %ebp
  29:   89 e5                   mov    %esp,%ebp
  2b:   83 ec 28                sub    $0x28,%esp
  2e:   c7 45 e6 47 43 5a 56    movl   $0x565a4347,-0x1a(%ebp)
  35:   c7 45 ea 49 50 48 58    movl   $0x58485049,-0x16(%ebp)
  3c:   66 c7 45 ee 42 00       movw   $0x42,-0x12(%ebp)
  42:   c7 45 f0 00 00 00 00    movl   $0x0,-0x10(%ebp)
  49:   e9 e0 00 00 00          jmp    12e <do_phase+0x106>
  4e:   8d 55 e6                lea    -0x1a(%ebp),%edx
  51:   8b 45 f0                mov    -0x10(%ebp),%eax
  54:   01 d0                   add    %edx,%eax
  56:   0f b6 00                movzbl (%eax),%eax
  59:   88 45 f7                mov    %al,-0x9(%ebp)
  5c:   0f be 45 f7             movsbl -0x9(%ebp),%eax
  60:   83 e8 41                sub    $0x41,%eax
  63:   83 f8 19                cmp    $0x19,%eax
  66:   0f 87 b0 00 00 00       ja     11c <do_phase+0xf4>
  6c:   8b 04 85 04 00 00 00    mov    0x4(,%eax,4),%eax
  73:   ff e0                   jmp    *%eax
  75:   c6 45 f7 56             movb   $0x56,-0x9(%ebp)
  79:   e9 9e 00 00 00          jmp    11c <do_phase+0xf4>
  7e:   c6 45 f7 39             movb   $0x39,-0x9(%ebp)
  82:   e9 95 00 00 00          jmp    11c <do_phase+0xf4>
  87:   c6 45 f7 6e             movb   $0x6e,-0x9(%ebp)
  8b:   e9 8c 00 00 00          jmp    11c <do_phase+0xf4>
  90:   c6 45 f7 40             movb   $0x40,-0x9(%ebp)
  94:   e9 83 00 00 00          jmp    11c <do_phase+0xf4>
  99:   c6 45 f7 78             movb   $0x78,-0x9(%ebp)
  9d:   eb 7d                   jmp    11c <do_phase+0xf4>
  9f:   c6 45 f7 34             movb   $0x34,-0x9(%ebp)
  a3:   eb 77                   jmp    11c <do_phase+0xf4>
  a5:   c6 45 f7 3f             movb   $0x3f,-0x9(%ebp)
  a9:   eb 71                   jmp    11c <do_phase+0xf4>
  ab:   c6 45 f7 76             movb   $0x76,-0x9(%ebp)
  af:   eb 6b                   jmp    11c <do_phase+0xf4>
  b1:   c6 45 f7 49             movb   $0x49,-0x9(%ebp)
  b5:   eb 65                   jmp    11c <do_phase+0xf4>
  b7:   c6 45 f7 5b             movb   $0x5b,-0x9(%ebp)
  bb:   eb 5f                   jmp    11c <do_phase+0xf4>
  bd:   c6 45 f7 6b             movb   $0x6b,-0x9(%ebp)
  c1:   eb 59                   jmp    11c <do_phase+0xf4>
  c3:   c6 45 f7 6b             movb   $0x6b,-0x9(%ebp)
  c7:   eb 53                   jmp    11c <do_phase+0xf4>
  c9:   c6 45 f7 30             movb   $0x30,-0x9(%ebp)
  cd:   eb 4d                   jmp    11c <do_phase+0xf4>
  cf:   c6 45 f7 65             movb   $0x65,-0x9(%ebp)
  d3:   eb 47                   jmp    11c <do_phase+0xf4>
  d5:   c6 45 f7 67             movb   $0x67,-0x9(%ebp)
  d9:   eb 41                   jmp    11c <do_phase+0xf4>
  db:   c6 45 f7 5a             movb   $0x5a,-0x9(%ebp)
  df:   eb 3b                   jmp    11c <do_phase+0xf4>
  e1:   c6 45 f7 32             movb   $0x32,-0x9(%ebp)
  e5:   eb 35                   jmp    11c <do_phase+0xf4>
  e7:   c6 45 f7 38             movb   $0x38,-0x9(%ebp)
  eb:   eb 2f                   jmp    11c <do_phase+0xf4>
  ed:   c6 45 f7 77             movb   $0x77,-0x9(%ebp)
  f1:   eb 29                   jmp    11c <do_phase+0xf4>
  f3:   c6 45 f7 3f             movb   $0x3f,-0x9(%ebp)
  f7:   eb 23                   jmp    11c <do_phase+0xf4>
  f9:   c6 45 f7 37             movb   $0x37,-0x9(%ebp)
  fd:   eb 1d                   jmp    11c <do_phase+0xf4>
  ff:   c6 45 f7 33             movb   $0x33,-0x9(%ebp)
 103:   eb 17                   jmp    11c <do_phase+0xf4>
 105:   c6 45 f7 35             movb   $0x35,-0x9(%ebp)
 109:   eb 11                   jmp    11c <do_phase+0xf4>
 10b:   c6 45 f7 59             movb   $0x59,-0x9(%ebp)
 10f:   eb 0b                   jmp    11c <do_phase+0xf4>
 111:   c6 45 f7 31             movb   $0x31,-0x9(%ebp)
 115:   eb 05                   jmp    11c <do_phase+0xf4>
 117:   c6 45 f7 36             movb   $0x36,-0x9(%ebp)
 11b:   90                      nop
 11c:   8d 55 dc                lea    -0x24(%ebp),%edx
 11f:   8b 45 f0                mov    -0x10(%ebp),%eax
 122:   01 c2                   add    %eax,%edx
 124:   0f b6 45 f7             movzbl -0x9(%ebp),%eax
 128:   88 02                   mov    %al,(%edx)
 12a:   83 45 f0 01             addl   $0x1,-0x10(%ebp)
 12e:   8b 45 f0                mov    -0x10(%ebp),%eax
 131:   83 f8 08                cmp    $0x8,%eax
 134:   0f 86 14 ff ff ff       jbe    4e <do_phase+0x26>
 13a:   8d 55 dc                lea    -0x24(%ebp),%edx
 13d:   8b 45 f0                mov    -0x10(%ebp),%eax
 140:   01 d0                   add    %edx,%eax
 142:   c6 00 00                movb   $0x0,(%eax)
 145:   83 ec 0c                sub    $0xc,%esp
 148:   8d 45 dc                lea    -0x24(%ebp),%eax
 14b:   50                      push   %eax
 14c:   e8 fc ff ff ff          call   14d <do_phase+0x125>
 151:   83 c4 10                add    $0x10,%esp
 154:   90                      nop
 155:   c9                      leave  
 156:   c3                      ret

类似一个9层循环，思路如下

2.修改跳转表(因为不允许修改.text节和重定位节），存放在.rodata节。