Re: [Qemu-devel] [PATCH 03/32] vvfat: Fix partition table

[Kevin Wolf <kwolf@redhat.com>]

Am 05.07.2012 11:23, schrieb Markus Armbruster:
> Kevin Wolf <kwolf@redhat.com> writes:
> 
>> Am 29.06.2012 22:33, schrieb Blue Swirl:
>>> On Fri, Jun 29, 2012 at 3:34 PM, Markus Armbruster <armbru@redhat.com> wrote:
>>>> Unless parameter ":floppy:" is given, vvfat creates a virtual image
>>>> with DOS MBR defining a single partition which holds the FAT file
>>>> system.  The size of the virtual image depends on the width of the
>>>> FAT: 32 MiB (CHS 64, 16, 63) for 12 bit FAT, 504 MiB (CHS 1024, 16,
>>>> 63) for 16 and 32 bit FAT, leaving (64*16-1)*63 = 64449 and
>>>> (1024*16-1)*64 = 1032129 sectors for the partition.
>>>>
>>>> However, it screws up the end of the partition in the MBR:
>>>>
>>>>    FAT width param.  start CHS  end CHS     start LBA  size
>>>>        :32:          0,1,1      1023,14,63       63    1032065
>>>>        :16:          0,1,1      1023,14,55       63    1032057
>>>>        :12:          0,1,1        63,14,55       63      64377
>>>>
>>>> The actual FAT file system nevertheless assumes the partition has
>>>> 1032129 or 64449 sectors.  Oops.
>>>>
>>>> Signed-off-by: Markus Armbruster <armbru@redhat.com>
>>>> ---
>>>>  block/vvfat.c |    7 ++++---
>>>>  1 files changed, 4 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/block/vvfat.c b/block/vvfat.c
>>>> index 0fd3367..62745b5 100644
>>>> --- a/block/vvfat.c
>>>> +++ b/block/vvfat.c
>>>> @@ -394,11 +394,12 @@ static void init_mbr(BDRVVVFATState* s)
>>>>
>>>>     /* LBA is used when partition is outside the CHS geometry */
>>>>     lba = sector2CHS(s->bs, &partition->start_CHS, s->first_sectors_number-1);
>>>> -    lba|= sector2CHS(s->bs, &partition->end_CHS,   s->sector_count);
>>>> +    lba |= sector2CHS(s->bs, &partition->end_CHS, s->bs->total_sectors - 1);
>>>>
>>>>     /*LBA partitions are identified only by start/length_sector_long not by CHS*/
>>>> -    partition->start_sector_long =cpu_to_le32(s->first_sectors_number-1);
>>>> -    partition->length_sector_long=cpu_to_le32(s->sector_count - s->first_sectors_number+1);
>>>> +    partition->start_sector_long  = cpu_to_le32(s->first_sectors_number-1);
>>>
>>> Spaces around '-'. Thanks for fixing the other cases, BTW.
>>
>> For compensation there's an extra space before the '='.
> 
> The original lined up the two '='.  I preserved that.  Not that I care
> for it.  Want me to drop the extra space?

Ah, didn't notice that. I don't mind then.

>>>> +    partition->length_sector_long = cpu_to_le32(s->bs->total_sectors
>>>> +                                                - s->first_sectors_number + 1);
>>
>> Just wondering... This should be the same as s->sector_count, right?
> 
> Hmm.  vvfat_open() assigns:
> 
>     s->sector_count = cyls * heads * secs - (s->first_sectors_number - 1);
>     bs->total_sectors = cyls * heads * secs;
> 
> But it then changes it minds and does:
> 
>     s->sector_count = s->faked_sectors + s->sectors_per_cluster*s->cluster_count;

Which probably means that they differ if some sub-cluster sized space is
left unused at the end of the disk. It's not useful to have this space
included in the partition, but it doesn't hurt either. So I suppose
either way is fine.

>> This whole geometry calculation code in vvfat is way too convoluted to
>> understand when you haven't looked at it for some months...
> 
> Yes, it is.  I was briefly tempted to replace it, but then decided to
> limit my footprint in this buck-crazy block driver to minimal bug fixes.

Heh, quite understandable.

Kevin