linux-kernel - Re: LOOP_GET_STATUS(64) truncates pathnames to 64 chars (was Re: Bug in mkfs.btrfs?!)

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Date:	Thu, 10 Feb 2011 13:29:27 +0100
From:	Petr Uzel <petr.uzel@...e.cz>
To:	Chris Samuel <chris@...muel.org>
Cc:	Felix Blanke <felixblanke@...il.com>, kreijack@...ind.it,
	Hugo Mills <hugo-lkml@...fax.org.uk>,
	linux-btrfs@...r.kernel.org,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: LOOP_GET_STATUS(64) truncates pathnames to 64 chars (was Re:
 Bug in mkfs.btrfs?!)

On Tue, Jan 25, 2011 at 11:15:11AM +1100, Chris Samuel wrote:
> /*
>  * CC'd to linux-kernel in case they have any feedback on this.
>  *
>  * Long thread, trying to work out why mkfs.btrfs failed to
>  * make a filesystem on an encrypted loopback mount called
>  * /dev/loop2. Cause turned out to be mkfs.btrfs calling
>  * LOOP_GET_STATUS to find out if the block device was mounted
>  * and getting a truncated device name back and so it later
>  * fails when lstat() is called on the truncated device path.
>  *
>  * The long device name for the encrypted loopback mount was
>  * because /dev/disk/by-id/$ID was used when Felix created it
>  * to cope with devices moving around.
>  */
> 
> On 25/01/11 00:01, Felix Blanke wrote:
> 
> > you were talking about the LOOP_GET_STATUS function. I'm not
> > quite sure where does it came from. Is it part of the kernel?
> > Or does it come from the util-linux package?
> 
> It's in the kernel, and there is both LOOP_GET_STATUS (old
> implementation) and LOOP_GET_STATUS64 (new implementation).
> 
> They return structures called loop_info and loop_info64
> respectively and both are defined in include/linux/loop.h .
> 
> Sadly in both cases the lengths of paths are defined to be
> LO_NAME_SIZE which is currently 64 and hence either
> implementation will cause the problematic:
> 
> lstat("/dev/disk/by-id/ata-INTEL_SSDSA2M160G2GC_CVPO939201JX160AGN-par",
> 0x7fffa30b3cf0) = -1 ENOENT (No such file or directory)
> 
> I've CC'd this to the LKML in case they have any feedback on
> this apparent problem with the API.
 
Since 2.6.37, you can get full path to the backing file from sys:
cat /sys/block/loopX/loop/backing_file

See
http://linux.derkeiler.com/Mailing-Lists/Kernel/2010-07/msg10996.html


HTH,

Petr

--
Petr Uzel
IRC: ptr_uzl @ freenode

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