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答案后续问题

tag-icon • tag-icon gpt
答案后续问题

根据这个维基百科文章，GUID 分区表的第二个逻辑块地址 (LBA 1) 是主 GPT 标头。然后在有关标头的部分中指出，标头的前 8 个字节用于“签名”。

这个GPT头签名是什么，它与GPT表/存储设备的功能/操作有什么关联？

我在网上搜索过，但是发现关于这个标题签名的用途的信息很少。

编辑：

我在对 Deltik 的评论中提出了以下后续问题回答，他们非常有帮助地扩展了它以解决以下问题：

  • 如果签名为空白，那么GPT表是否就无法识别呢？
  • 我目前正在检查 Chrome OS GPT，其中签名似乎已设置为“IGNOREME”。这会产生什么影响？
  • 还有其他有效的 GPT 签名可用吗？（除了默认的“EFI PART”）

答案1

签名EFI PART( 0x45 0x46 0x49 0x20 0x50 0x41 0x52 0x54) 只是表示“这是一个 GUID 分区表”。软件通过它知道块设备是以 GPT 布局格式化的。

您可以从十六进制转储的这个片段中看到它：

*
000001f0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 55 aa  |..............U.|
00000200  45 46 49 20 50 41 52 54  00 00 01 00 5c 00 00 00  |EFI PART....\...|
00000210  ea b5 99 a4 00 00 00 00  01 00 00 00 00 00 00 00  |................|
00000220  af 12 9e 3b 00 00 00 00  22 00 00 00 00 00 00 00  |...;....".......|
00000230  8e 12 9e 3b 00 00 00 00  7f 8c d6 05 c8 e8 01 4a  |...;...........J|
00000240  b8 74 6b 37 30 7c bf 15  02 00 00 00 00 00 00 00  |.tk70|..........|
00000250  80 00 00 00 80 00 00 00  10 d3 0e 29 00 00 00 00  |...........)....|
00000260  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*

签名并非 GPT 所独有。文件还具有签名软件可以使用它来识别文件是什么。这是file命令。 （示例file）

答案后续问题

那么，如果这个签名是空白的话，GPT 分区表就无法被识别吗？

至少，gdisk不会再认出它了。看看这个演示：

  1. 为演示创建一个文件（充当块设备）：

    root@demo [/tmp]# truncate -s 1M gpt-demo

  2. 使用 GPT 格式化文件：

    root@demo [/tmp]# sgdisk --clear gpt-demo
Creating new GPT entries.
Warning: The kernel is still using the old partition table.
The new table will be used at the next reboot or after you
run partprobe(8) or kpartx(8)
The operation has completed successfully.

  3. 检查分区表的二进制数据：

    root@demo [/tmp]# hexdump -C gpt-demo
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000001c0  02 00 ee 20 20 00 01 00  00 00 ff 07 00 00 00 00  |...  ...........|
000001d0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000001f0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 55 aa  |..............U.|
00000200  45 46 49 20 50 41 52 54  00 00 01 00 5c 00 00 00  |EFI PART....\...|
00000210  13 14 6d 6a 00 00 00 00  01 00 00 00 00 00 00 00  |..mj............|
00000220  ff 07 00 00 00 00 00 00  22 00 00 00 00 00 00 00  |........".......|
00000230  de 07 00 00 00 00 00 00  be 2c cb 6e 58 d4 e3 46  |.........,.nX..F|
00000240  84 84 3c 83 52 59 5b 58  02 00 00 00 00 00 00 00  |..<.RY[X........|
00000250  80 00 00 00 80 00 00 00  86 d2 54 ab 00 00 00 00  |..........T.....|
00000260  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000ffe00  45 46 49 20 50 41 52 54  00 00 01 00 5c 00 00 00  |EFI PART....\...|
000ffe10  18 3a 1f d9 00 00 00 00  ff 07 00 00 00 00 00 00  |.:..............|
000ffe20  01 00 00 00 00 00 00 00  22 00 00 00 00 00 00 00  |........".......|
000ffe30  de 07 00 00 00 00 00 00  be 2c cb 6e 58 d4 e3 46  |.........,.nX..F|
000ffe40  84 84 3c 83 52 59 5b 58  df 07 00 00 00 00 00 00  |..<.RY[X........|
000ffe50  80 00 00 00 80 00 00 00  86 d2 54 ab 00 00 00 00  |..........T.....|
000ffe60  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00100000

  4. 检查如何gdisk读取分区表：

    root@demo [/tmp]# gdisk gpt-demo
GPT fdisk (gdisk) version 1.0.3

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help): q

  5. 更改 GPT 签名：

    root@demo [/tmp]# sed -i 's/EFI PART/IGNOREME/g' gpt-demo

  6. 验证签名是否已被核实：

    root@demo [/tmp]# hexdump -C gpt-demo
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000001c0  02 00 ee 20 20 00 01 00  00 00 ff 07 00 00 00 00  |...  ...........|
000001d0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000001f0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 55 aa  |..............U.|
00000200  49 47 4e 4f 52 45 4d 45  00 00 01 00 5c 00 00 00  |IGNOREME....\...|
00000210  13 14 6d 6a 00 00 00 00  01 00 00 00 00 00 00 00  |..mj............|
00000220  ff 07 00 00 00 00 00 00  22 00 00 00 00 00 00 00  |........".......|
00000230  de 07 00 00 00 00 00 00  be 2c cb 6e 58 d4 e3 46  |.........,.nX..F|
00000240  84 84 3c 83 52 59 5b 58  02 00 00 00 00 00 00 00  |..<.RY[X........|
00000250  80 00 00 00 80 00 00 00  86 d2 54 ab 00 00 00 00  |..........T.....|
00000260  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000ffe00  49 47 4e 4f 52 45 4d 45  00 00 01 00 5c 00 00 00  |IGNOREME....\...|
000ffe10  18 3a 1f d9 00 00 00 00  ff 07 00 00 00 00 00 00  |.:..............|
000ffe20  01 00 00 00 00 00 00 00  22 00 00 00 00 00 00 00  |........".......|
000ffe30  de 07 00 00 00 00 00 00  be 2c cb 6e 58 d4 e3 46  |.........,.nX..F|
000ffe40  84 84 3c 83 52 59 5b 58  df 07 00 00 00 00 00 00  |..<.RY[X........|
000ffe50  80 00 00 00 80 00 00 00  86 d2 54 ab 00 00 00 00  |..........T.....|
000ffe60  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00100000

  7. 请注意，GUID 分区表 (GPT) 不再被识别gdisk：

    root@demo [/tmp]# gdisk gpt-demo
GPT fdisk (gdisk) version 1.0.3

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: not present

Creating new GPT entries.

Command (? for help): q

我目前正在检查 Chrome OS GPT，其中签名似乎已设置为“IGNOREME”。你知道这会有什么影响吗？

据我从上面的演示所知，如果签名不同于EFI PART，则 GPT 无法被识别。

“签名IGNOREME”是 Chromium 创建的特殊签名。这是我找到的文档IGNOREME：

cgptlib: Add support for IGNOREME GPT signature

This patch makes cgpt aware of a special "IGNOREME" GPT header signature
string that may appear in either the primary or the secondary GPT and
cause cgpt (and other cgptlib clients) to completely ignore that GPT. It
will continue to function correctly for all other purposes (using the
data from the non-ignored GPT), but never write any data back to the
ignored GPT.

BRANCH=None
BUG=chrome-os-partner:52595
TEST=unit tests

Change-Id: I7e53542385ae9d8d24dc25b75e91f4ff4917f66f
Signed-off-by: Julius Werner <[email protected]>
Reviewed-on: https://chromium-review.googlesource.com/340072
Reviewed-by: Nam Nguyen <[email protected]>

您是否检查过磁盘/块设备末尾的辅助 GPT？

还有其他有效的 GPT 签名可用吗？

没有，没有其他签名，EFI PART因为这是UEFI 规范（见第 123 页）。（UEFI 规范版本 2.7（勘误表 A）的存档副本）

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