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GNU 链接器在 ELF 文件中的节之间产生无用的间距

tag-icon • tag-icon gnu linker elf
GNU 链接器在 ELF 文件中的节之间产生无用的间距

我的目的是将文本部分放置在内存中的特定位置（0x00100000）。

SECTIONS
{
    . = 0x00100000;
    .text :
    {   
        *(.text*)
    }
}

虽然链接器做执行此操作（注意 0x01000000Addr字段）：

$ readelf -S file.elf 
There are 12 section headers, starting at offset 0x104edc:

Section Headers:
  [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al
  [ 0]                   NULL            00000000 000000 000000 00      0   0  0
  [ 1] .text             PROGBITS        00100000 100000 000e66 00  AX  0   0  4
  [ 2] .eh_frame         PROGBITS        00100e68 100e68 000628 00   A  0   0  4
...

它还.text在 ELF 文件中的该部分之前放置了约 1MB 的零（注意该.text部分的偏移量为 1MB）。另一种方式显示：

$ hexdump -C file.elf
00000000  7f 45 4c 46 01 01 01 00  00 00 00 00 00 00 00 00  |.ELF............|
00000010  02 00 03 00 01 00 00 00  0c 00 10 00 34 00 00 00  |............4...|
00000020  dc 4e 10 00 00 00 00 00  34 00 20 00 02 00 28 00  |.N......4. ...(.|
00000030  0c 00 0b 00 01 00 00 00  00 00 00 00 00 00 00 00  |................|
00000040  00 00 00 00 90 14 10 00  96 04 4f 00 07 00 00 00  |..........O.....|
00000050  00 00 20 00 51 e5 74 64  00 00 00 00 00 00 00 00  |.. .Q.td........|
00000060  00 00 00 00 00 00 00 00  00 00 00 00 07 00 00 00  |................|
00000070  10 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000080  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00100000  02 b0 ad 1b 03 00 00 00  fb 4f 52 e4 8b 25 90 04  |.........OR..%..|
00100010  4f 00 50 53 e8 88 00 00  00 fa f4 eb fc 55 89 e5  |O.PS.........U..|
00100020  83 ec 10 c7 45 f8 00 80  0b 00 c7 45 fc 00 00 00  |....E......E....|
00100030  00 eb 24 8b 45 fc 8d 14  00 8b 45 f8 01 d0 8b 4d  |..$.E.....E....M|

如何防止这种情况发生？我是否不正确地使用了位置计数器（“点”符号）语法？

答案1

事实证明，告诉链接器模拟 elf_i386 产生了我正在寻找的输出，尽管我不明白为什么。即，使用以下命令调用链接器：

$ ld -melf_i386 [...]

使用和不使用生成的文件-melf_i386似乎大多相似：

with.elf:    ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), statically linked, not stripped, with debug_info
without.elf: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), statically linked, not stripped, with debug_info

除了它们的大小有很大不同：

$ ls -l *.elf
-rwxr-xr-x 1 user user   10948 May 24 11:56 with.elf
-rwxr-xr-x 1 user user 1055428 May 24 11:56 without.elf

据我所知，输出文件是另外的确切地相同：

$ readelf -S with.elf
There are 12 section headers, starting at offset 0x28e4:

Section Headers:
  [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al
  [ 0]                   NULL            00000000 000000 000000 00      0   0  0
  [ 1] .text             PROGBITS        00100000 001000 000205 00  AX  0   0  4
  [ 2] .eh_frame         PROGBITS        00100208 001208 0000b8 00   A  0   0  4
  [ 3] .bss              NOBITS          001002c0 0012c0 3ef000 00  WA  0   0  4
  [ 4] .debug_info       PROGBITS        00000000 0012c0 0007bf 00      0   0  1
  [ 5] .debug_abbrev     PROGBITS        00000000 001a7f 0002c9 00      0   0  1
  [ 6] .debug_aranges    PROGBITS        00000000 001d48 000060 00      0   0  1
  [ 7] .debug_line       PROGBITS        00000000 001da8 00023c 00      0   0  1
  [ 8] .debug_str        PROGBITS        00000000 001fe4 0004bd 01  MS  0   0  1
  [ 9] .symtab           SYMTAB          00000000 0024a4 000280 10     10  22  4
  [10] .strtab           STRTAB          00000000 002724 00014e 00      0   0  1
  [11] .shstrtab         STRTAB          00000000 002872 000070 00      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),
  p (processor specific)

请注意“偏移”字段略有不同

$ readelf -S without.elf
There are 12 section headers, starting at offset 0x1018e4:

Section Headers:
  [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al
  [ 0]                   NULL            00000000 000000 000000 00      0   0  0
  [ 1] .text             PROGBITS        00100000 100000 000205 00  AX  0   0  4
  [ 2] .eh_frame         PROGBITS        00100208 100208 0000b8 00   A  0   0  4
  [ 3] .bss              NOBITS          001002c0 1002c0 3ef000 00  WA  0   0  4
  [ 4] .debug_info       PROGBITS        00000000 1002c0 0007bf 00      0   0  1
  [ 5] .debug_abbrev     PROGBITS        00000000 100a7f 0002c9 00      0   0  1
  [ 6] .debug_aranges    PROGBITS        00000000 100d48 000060 00      0   0  1
  [ 7] .debug_line       PROGBITS        00000000 100da8 00023c 00      0   0  1
  [ 8] .debug_str        PROGBITS        00000000 100fe4 0004bd 01  MS  0   0  1
  [ 9] .symtab           SYMTAB          00000000 1014a4 000280 10     10  22  4
  [10] .strtab           STRTAB          00000000 101724 00014e 00      0   0  1
  [11] .shstrtab         STRTAB          00000000 101872 000070 00      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),
  p (processor specific)

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