Re: [PATCH] blockdev: Don't fail on missing start sector

[Stanislav Brabec <sbrabec@suse.cz>]

Karel Zak wrote:
> On Mon, Jun 01, 2020 at 01:35:44AM +0200, Stanislav Brabec wrote:
>> Device mapper devices don't provide start sector.
> for which type of the device/mapping? I'd like to reproduce this
> issue.
kpartx-style partition on multipath.
>> It causes "blockdev --report" error:
>> blockdev: /dev/dm-9: failed to read partition start from sysfs: No such
>> file or directory
>>
>> There is no reliable way to detect a geometry in this case.
>> Report N/A instead.
> I guess the "start" file missing in the sysfs/ in this case, right?
Yes.

Actually, debugging shows:

sysfs_blkdev_get_wholedisk(): This function has a special code for
device mapper: If /sys/dev/block/{major}:{minor}/partition does not
exist, check for /sys/dev/block/{major}:{minor}/dm/uuid, parse the
result and attempt to check the structure. (Split by "-", check, whether
string starts by "part".) It starts with "part1" => Device mapper is
detected. => Partition is detected.

The code returns to report_device(), and because partition is detected,
it unconditionally attempts to read "start" file from
"/sys/dev/block/254:9". This fails.

>>              if (ul_path_read_u64(pc, &start, "start") != 0)
>> -                err(EXIT_FAILURE,
>> -                    _("%s: failed to read partition start from sysfs"),
>> -                    device);
>> +                /* TRANSLATORS: Start sector not available. Max. 10
>> letters. */
>> +                sprintf(start_str, "%10s", _("N/A"));
>>          }
>>          ul_unref_path(pc);
>>      }
>> +    if (start_str[0] == 0)
>> +        sprintf(start_str, "%10ju", start);
>
> Maybe we ca use ul_path_read_buffer() rather than ul_path_read_u64() to keep 
> it as string and to avoid sprintf().
This was in the original code, but your proposal makes a perfect sense.
%10ju is just a space padded original string.

-- 
Best Regards / S pozdravem,

Stanislav Brabec
software developer
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