Date: Thu, 10 Feb 2011 13:29:27 +0100
From: Petr Uzel <petr.uzel@suse.cz>
To: Chris Samuel <chris@csamuel.org>
Cc: Felix Blanke <felixblanke@gmail.com>, kreijack@inwind.it,
        Hugo Mills <hugo-lkml@carfax.org.uk>, linux-btrfs@vger.kernel.org,
        Linux Kernel <linux-kernel@vger.kernel.org>
Subject: Re: LOOP_GET_STATUS(64) truncates pathnames to 64 chars (was Re:
 Bug in mkfs.btrfs?!)
Message-ID: <20110210122927.GB20488@foxbat.suse.cz>
Mail-Followup-To: Chris Samuel <chris@csamuel.org>,
	Felix Blanke <felixblanke@gmail.com>, kreijack@inwind.it,
	Hugo Mills <hugo-lkml@carfax.org.uk>, linux-btrfs@vger.kernel.org,
	Linux Kernel <linux-kernel@vger.kernel.org>
References: <20110122144513.GA2539@scooter>
 <20110122145222.GB2539@scooter>
 <20110122151124.GC29985@carfax.org.uk>
 <20110122155612.GA3664@scooter>
 <20110123181827.GF29985@carfax.org.uk>
 <4D3CA568.7050506@libero.it>
 <20110124130104.GA7354@scooter>
 <4D3E160F.4050006@csamuel.org>
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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.
>  */
>=20
> On 25/01/11 00:01, Felix Blanke wrote:
>=20
> > 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?
>=20
> It's in the kernel, and there is both LOOP_GET_STATUS (old
> implementation) and LOOP_GET_STATUS64 (new implementation).
>=20
> They return structures called loop_info and loop_info64
> respectively and both are defined in include/linux/loop.h .
>=20
> 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:
>=20
> lstat("/dev/disk/by-id/ata-INTEL_SSDSA2M160G2GC_CVPO939201JX160AGN-par",
> 0x7fffa30b3cf0) =3D -1 ENOENT (No such file or directory)
>=20
> I've CC'd this to the LKML in case they have any feedback on
> this apparent problem with the API.
=20
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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