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Date:	Wed, 17 Oct 2012 09:03:27 +0100 (BST)
From:	GrahamC <grahamc001uk AT yahoo DOT co DOT uk>
Reply-To:	GrahamC <grahamc001uk AT yahoo DOT co DOT uk>
Subject:	Re: Cygwin for Windows 8
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On Oct 16 Corinna Vinschen wrote:
> On Oct 15 23:55, GrahamC wrote:
> > Another anomaly with disk drive partition devices (/dev/sdX1, /dev/sdX2 etc.) is the access control.
> >  
> > While the whole disk devices (/dev/sdX) can only be accessed when running as administrator, the
> > individual partition devices (/dev/sdX1, /dev/sdX2 etc.) can be both read and written without
> > needing administrator mode.
>
> Access control is not done inside Cygwin.  The permissions on the block
> devices are set by Windows.  If you want to take a closer look,  see
> the mapping of the native namespace into the Cygwin /proc/sys namespace.
> Look at everything under /proc/sys/Device/Harddisk*, but make sure you're
> admin, otherwise you won't see everything.  If you want to take an even
> closer look, I suggest to download the winobj tool from sysinternal.  It
> also provides a detailed ACL view.
>
> Device creation is based on what Windows creates. Cygwin only looks
> which devices exist in the internal native namespace and maps those to
> the equivalent POSIX device names.  If a \Device\HarddiskX\PartitionY
> exists, then /dev/sd<X>Y exists in Cygwin, with <X> being 'a' + X.
>
> As for the missing ~70 sectors, I'm not sure.  How do you know?  Does
> the number of KBs you can read differ by this amount from the KBs given
> in /proc/partitions?  
>
> Maybe that's the missing call to FSCTL_ALLOW_EXTENDED_DASD_IO.  Cygwin's
> block device code only calls this Windows ioctl code for full disks, not
> for partitions on the disk.  I fixed that in CVS, though.  Can you give
> the today's snapshot from http://cygwin.com/snapshots/ a try, please?


OK, the 20121016 snapshot of cygwin1.dll fixed the data truncation 
problem. The number of sectors readable from the /dev/sdXY 
devices now match the partition sizes.
 
My missing partition devices appears to be an incompatibility between
the Linux parted tool and Windows. The former puts a 6 byte disk ID in the
partition sector at position 0x1b8..0x1bd, Windows uses the first 4 bytes only
and if two disks have the same value it doesn't create volumes for the second disk. 
Patching the ID to a unique value in the first 4 bytes made the missing partitions
appear at next boot up. 

Non-administrator read and write access to /dev/sdXY devices appears to 
be limited to the USB flash devices. The WinObj tool shows the 
group INTERACTIVE having read and write access to these. I suppose this 
makes sense from the Windows point of view. WinObj also shows group 
Everyone has read access to the raw disk devices DR0, DR1, DR2 etc., 
but that doesn't seem to make it through to the /dev/sdX devices (not 
that I would want it to).

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