[OE-core] [PATCH v3] wic: ensure generated disk system identifier is non-zero

[Jonathan Liu]

Hi Ed,

On 30 July 2017 at 20:02, Ed Bartosh <ed.bartosh at linux.intel.com> wrote:
> On Sat, Jul 29, 2017 at 12:45:27AM +1000, Jonathan Liu wrote:
>> Zero may be interpreted as no MBR signature present and another
>> partitioning program might install a new MBR signature.
>>
>> Signed-off-by: Jonathan Liu <net147 at gmail.com>
>> ---
>>  scripts/lib/wic/plugins/imager/direct.py | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/scripts/lib/wic/plugins/imager/direct.py b/scripts/lib/wic/plugins/imager/direct.py
>> index f20d8433f1..fe9b688ab0 100644
>> --- a/scripts/lib/wic/plugins/imager/direct.py
>> +++ b/scripts/lib/wic/plugins/imager/direct.py
>> @@ -297,7 +297,7 @@ class PartitionedImage():
>>                            # all partitions (in bytes)
>>          self.ptable_format = ptable_format  # Partition table format
>>          # Disk system identifier
>> -        self.identifier = int.from_bytes(os.urandom(4), 'little')
>> +        self.identifier = int.from_bytes(os.urandom(4), 'little') or 0xffffffff
>>
>
> Can we generate random identifier by calling os.urandom again if
> self.identifier is zero? Something like this, may be?
>
> while True:
>     self.identifier = int.from_bytes(os.urandom(4), 'little')
>     if self.identifier:
>         break
>
> Assigning constant 0xffffffff can potentially create nonunique identifiers.
>

There is no guarantee that urandom will create unique identifiers.
Infinite loop needs a timeout and error in the case it is unable to
generate a suitable identifier. It is only 0xffffffff if it is 0. That
means 0xffffffff is twice as likely to occur (2 in 4294967296 instead
of 1 in 4294967296) so there is tiny bias. Note that this is how it is
implemented for the DISK_SIGNATURE variable in OE and in GNU Parted as
well.

>>          self.partitions = partitions
>>          self.partimages = []
>> --
>> 2.13.2
>>
>> --
>> _______________________________________________
>> Openembedded-core mailing list
>> Openembedded-core at lists.openembedded.org
>> http://lists.openembedded.org/mailman/listinfo/openembedded-core
>
> --
> --
> Regards,
> Ed

Regards,
Jonathan