Re: grub-probe for nested BSD partition on Linux

[Andrei Borzenkov]

28.02.2017 21:31, Lennart Sorensen пишет:
> On Tue, Feb 28, 2017 at 08:13:53PM +0300, Andrei Borzenkov wrote:
>> Sorry? vda7 is 256M, how can you suddenly pretend it is 2G?
>>
>> 10:~ # fdisk -l /dev/vda
>> Disk /dev/vda: 5 GiB, 5368709120 bytes, 10485760 sectors
>> Units: sectors of 1 * 512 = 512 bytes
>> Sector size (logical/physical): 512 bytes / 512 bytes
>> I/O size (minimum/optimal): 512 bytes / 512 bytes
>> Disklabel type: dos
>> Disk identifier: 0x882b18da
>>
>> Device     Boot   Start      End Sectors  Size Id Type
>> /dev/vda1          2048  2099199 2097152    1G 83 Linux
>> /dev/vda2       2099200  6293503 4194304    2G a6 OpenBSD
>> /dev/vda3       6293504 10485759 4192256    2G  5 Extended
>> /dev/vda5       6295552  6819839  524288  256M 83 Linux
>> /dev/vda6       6821888  7346175  524288  256M 83 Linux
>> 10:~ # fdisk -l /dev/vda2
>> Disk /dev/vda2: 2 GiB, 2147483648 bytes, 4194304 sectors
>> Geometry: 16 heads, 63 sectors/track, 10402 cylinders
>> Units: sectors of 1 * 512 = 512 bytes
>> Sector size (logical/physical): 512 bytes / 512 bytes
>> I/O size (minimum/optimal): 512 bytes / 512 bytes
>> Disklabel type: bsd
>>
>> Slice   Start      End  Sectors  Size Type     Fsize Bsize Cpg
>> a     2099200  2623488   524289  256M boot         0     0   0
>> b     2099200  2623488   524289  256M unused       0     0   0
>> c     2099200  6293503  4194304    2G unused       0     0   0
>> d           0 10485215 10485216    5G unused       0     0   0
>>
>> Partition table entries are not in disk order.
> 
> Well that does look mostly sane for an ancient BSD version.
> 

In case I was not clear.

BSD partition a is represented in Linux as /dev/vda7. grub-probe -t
compatibility_hints -d /dev/vda7 returns hd0,msdos2 which is obviously
wrong, because it refers to 2GB area on disk while partition /dev/vda7
is just 256M. I would expect either hd0,msdos2,openbsd1 or nothing
depending on whether such nested partition is supported.

> Slice a is the rootfs for BSD
> 
> Slice b should traditionally be swap, but appears to be unused here,
> and for some reason uses the same part of the disk as slice a, which
> does look odd.  Maybe since the type is unknown, it doesn't matter.
> 
> Slice c is the whole partition containing the slices (vda2)
> 
> Slice d is the whole disk (vda), which apparently hasn't been used in
> BSD for a couple of decades now, if wikipedia can be trusted.
> 
> So if linux only bothers to count the partition with a type that isn't
> unused, then it probably makes sense.  That appears to be what the
> partition parsing code does.
> 
> The start being the same as the start of vda2 does seem odd given it
> appears that you are supposed to leave space at the start (1 track
> recommended, or 64 sectors), and it appears the disklabel is at sector
> 1 (0 being the first), so it doesn't seem like having your filesystem
> start there would be safe unless the filesystem has reserved space at
> the start for such things.
> 

I just pressed 'b' in Linux fdisk and only change two partition sizes. I
have no idea how fdisk choses partition start. Could be a bug in fdisk.