VFAT i_pos value

[hirofumi@mail.parknet.co.jp (OGAWA Hirofumi)]

Kai Meyer <kai@gnukai.com> writes:

>> The i_pos means directory entry (contains inode information in unix-fs)
>> position,
>>
>>      block number == i_pos / (logical-blocksize / 32)
>>      offset       == i_pos&  (logical-blocksize / 32)
>>
>> the above position's directory entry contains information for
>> problematic file. This is how to use i_pos information.
>>
>> FWIW, in this error case, the cluster chain in FAT table which is
>> pointed by that entry, it has invalid cluster value.
>>
>> Thanks.
>
> If you would verify my math for me, I would appreciate it.
>
> In this case, my logical block size is 4096, because byte 13 of the 8Gb 
> file system is 8, and I take that to be 8 * 512, which is 4096. So:
>
> block_number = 523791 / (4096 / 32) = 4092
> offset = 523791 % (4096 / 32) = 15  // I assume you meant modulo in your 
> original post, and not binary AND.

Whoops, you are right. (I forgot "-1")

> So if the block_number is 4092, I would multiply that by 8 (sectors per 
> logical block) to get the sector number:
> 32736

Right.

> Does the error indicate that sector contains the corrupted data?

No.

> Or is it the sector that contains the information that points to the
> corrupted data?

Right.

The i_pos is pointing a directory entry (include/linux/msdos_fs.h:
struct msdos_dir_entry).

And starthi (if FAT32) and start contain the pointer to next cluster
number. That message was outputted when walking in cluster chain.

If you want to see actual corrupted data, you can check the cluster
chain by pointing from that directory entry.

Thanks.
-- 
OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>