Question

可执行文件由不同用途的段（section/segment）组成。

• .text : 二进制机器码。
• .data : 有初始化值的全局变量。
• .bss : 无初始化值的全局变量。
• .rodata : 只读数据。

查看段相关信息。

$ readelf -S ./test

Section Headers:
  [Nr] Name        Type       Address       Offset
     Size        EntSize      Flags  Link  Info  Align
  [ 0]           NULL       0000000000000000  00000000
     0000000000000000  0000000000000000       0   0   0
  [ 1] .text       PROGBITS     0000000000401000  00001000
     0000000000000029  0000000000000000  AX     0   0   16
  [ 2] .roata      PROGBITS     0000000000402000  00002000
     000000000000000e  0000000000000000   A     0   0   1
  [ 3] .stab       PROGBITS     0000000000000000  00002010
     000000000000009c  0000000000000014       4   0   4
  [ 4] .stabstr      STRTAB       0000000000000000  000020ac
     0000000000000008  0000000000000000       0   0   1
  [ 5] .symtab       SYMTAB       0000000000000000  000020b8
     0000000000000138  0000000000000018       6   9   8
  [ 6] .strtab       STRTAB       0000000000000000  000021f0
     0000000000000030  0000000000000000       0   0   1
  [ 7] .shstrtab     STRTAB       0000000000000000  00002220
     0000000000000037  0000000000000000       0   0   1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
  L (link order), O (extra OS processing required), G (group), T (TLS),
  C (compressed), x (unknown), o (OS specific), E (exclude),
  l (large), p (processor specific)

$ readelf -l ./test

Program Headers:
  Type       Offset       VirtAddr       PhysAddr
         FileSiz      MemSiz        Flags  Align
  LOAD       0x0000000000000000 0x0000000000400000 0x0000000000400000
         0x00000000000000e8 0x00000000000000e8  R    0x1000
  LOAD       0x0000000000001000 0x0000000000401000 0x0000000000401000
         0x0000000000000029 0x0000000000000029  R E  0x1000
  LOAD       0x0000000000002000 0x0000000000402000 0x0000000000402000
         0x000000000000000e 0x000000000000000e  R    0x1000

 Section to Segment mapping:
  Segment Sections...
   00   
   01   .text 
   02   .roata 

注意，段名区分大小写。错误拼写可能导致段无法被赋予正确的权限属性，造成程序头映射错误、反汇编故障、调试故障等。如果将源码中的 .text 改成 .TEXT 。

$ readelf -S ./test

Section Headers:
  [Nr] Name        Type       Address       Offset
     Size        EntSize      Flags  Link  Info  Align
  [ 2] .TEXT       PROGBITS     000000000040100e  0000100e
     0000000000000029  0000000000000000   A     0   0   1
                      # 没有 X !!!

$ readelf -l ./test

Program Headers:
  Type       Offset       VirtAddr       PhysAddr
         FileSiz      MemSiz        Flags  Align
  LOAD       0x0000000000000000 0x0000000000400000 0x0000000000400000
         0x0000000000001037 0x0000000000001037  R    0x1000

 Section to Segment mapping:
  Segment Sections...
   00   .roata .TEXT          # 和 rodata 混在一起。

$ objdump -d -M intel ./test

./test:   file format elf64-x86-64    # 找不到代码段。

$ gdb ./test

Reading symbols from ./test...
Can't find any code sections in symbol file