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

[Markus Armbruster <armbru@redhat.com>]

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?

>>> +    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;

> 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.