[cygwin] DD bug fails to wipe last 48 sectors of a disk

[Brian Inglis]

On 2020-06-23 09:28, Hashim Aziz via Cygwin wrote:
> I hadn't checked with 512 byte block sizes because of the amount of time it would have taken, but sure enough have just finished trying with bs=512 and no block size at all (so dd's default block size, which is either 512 or 1024) and although each wipe took over 24 hours, they did indeed wipe all of the sectors. So it seems that there's a bug with regards to how Cygwin handles the last block when a large (i.e. sane) block size is selected, and that this bug doesn't occur on actual UNIX-based systems.
> 
> ________________________________
> From: Cygwin <cygwin-bounces@cygwin.com> on behalf of Eliot Moss <moss@cs.umass.edu>
> Sent: 20 June 2020 9:26 PM
> To: The Cygwin Mailing List <cygwin@cygwin.com>
> Subject: Re: [cygwin] DD bug fails to wipe last 48 sectors of a disk
> 
> On 6/20/2020 1:31 PM, Hashim Aziz via Cygwin wrote:
>> To reproduce simply run the following command on a drive (obviously, this will irreversibly wipe all data):
>>
>> dd if=/dev/zero of=/dev/sdX bs=4M status=progress
>>
>> Both drives were attached via internal SATA (by way of a PCIE to SATA Host Bus Adapter).
>>
>> Cygwin was running in an elevated window as dd cannot run in Cygwin without administrator access, at least not on Windows 10 and not when dealing with raw disks. I was running Avast the first time I discovered this, and am currently running Windows Defender, so doubt that the AV is the cause of this.
>>
>> The hard drives are a Western Digital WD10PURX-64E5EY0 (Serial: WD-WCC4J6HX189U) and a Kingston SV200S3128G (Serial: 12BA315PKAWK).
>>
>> I just ran DD for Windows 0.6beta3 with variations of the following command:
>>
>> dd.exe if=/dev/zero of=\\.\PHYSICALDRIVEX --progress bs=4M
>>
>> ...and can confirm that the bug also manifests here, but in a slightly different way - irrespective of the disk or block size, it fails to wipe the last 176 sectors of the drive.
> 
> I was going to ask: even with block size 512 bytes?  But I guess you checked that ...

I don't have the facilities to test, and there appear to be *NO* Windows
documentation details on error condition handling, but my suspicion is that Unix
reads and writes fail only *AFTER* reading or writing at the end of the device,
but Windows reads and writes extents may be checked and failed *BEFORE* reading
or writing any data near the end of the device.
If the actual Windows error code returned is generic, Cygwin would need to
pre-check the device size as Windows does, and reduce read and write sizes to
the allowed maximum at the end of the device.

-- 
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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