Re: Improperly nested partitions, help needed!

["Grégoire Sutre" <gregoire.sutre@labri.fr>]

On 09/21/2010 09:42 PM, Svante Signell wrote:

> # file /tmp/sda3.mbr /tmp/sda3.mbr: x86 boot sector; GRand Unified
> Bootloader, stage1 version 0x3, 1st sector stage2 0xdd29b38;
> partition 1: ID=0x83, starthead 239, startsector 63, 35153937
> sectors, extended partition table (last)\011, code offset 0x48

I'm surprised that file is able to tell that this is an extended
partition table.  If we could reliably distinguish MBR partition tables
from EBR ones, then we could prevent GRUB from considering such EBR
partition tables.  But I don't see how a such a distinction could be
made: as far as I know, any EBR partition table could appear in an MBR.

So out of curiosity, I looked a bit deeper at the behavior of the file
command, and I'm afraid that the result of file is not 100% reliable in
this respect: for a partition table with only one used entry, file
detects it as an EBR partition table if the entry is not active, as
shown by this example (one sector in /tmp/sect.bin):

    Device      Boot    Start       End   #sectors  Id  System
/tmp/sect.bin1   *        63  41945714   41945652  83  Linux
/tmp/sect.bin2             0         -          0   0  Empty
/tmp/sect.bin3             0         -          0   0  Empty
/tmp/sect.bin4             0         -          0   0  Empty

$ od -Ax -t x1 /tmp/sect.bin
000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
0001b0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 80 01
0001c0 01 00 83 fe ff ff 3f 00 00 00 34 0a 80 02 00 00
0001d0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
0001f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 aa
000200

$ file /tmp/sect.bin
/tmp/sect.bin: x86 boot sector; partition 1: ID=0x83, active, starthead
1, startsector 63, 41945652 sectors, code offset 0x0


Everything looks good.  But after making the partition inactive,
we get:

$ od -Ax -t x1 /tmp/sect.bin
000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
0001b0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 01
0001c0 01 00 83 fe ff ff 3f 00 00 00 34 0a 80 02 00 00
0001d0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
0001f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 aa
000200

$ file /tmp/sect.bin
/tmp/sect.bin: x86 boot sector; partition 1: ID=0x83, starthead 1,
startsector 63, 41945652 sectors, extended partition table (last)\011,
code offset 0x0

Still, this partition table may well be contained in an MBR.

Grégoire