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

[Markus Armbruster <armbru@redhat.com>]

Kevin Wolf <kwolf@redhat.com> writes:

> 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.

Complication: the partition should end on a cylinder boundary.  Shaving
off an unused tail may well interfere with that.  Let's stick to v1
here.

[...]