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

[Kevin Wolf <kwolf@redhat.com>]

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 '='.

>> +    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?
This whole geometry calculation code in vvfat is way too convoluted to
understand when you haven't looked at it for some months...

Kevin