Re: Bug in mkfs.btrfs?!

From: Felix Blanke <felixblanke@gmail.com>
To: Hugo Mills <hugo-lkml@carfax.org.uk>
Cc: kreijack@inwind.it, linux-btrfs@vger.kernel.org
Subject: Re: Bug in mkfs.btrfs?!
Date: Mon, 24 Jan 2011 14:53:05 +0100	[thread overview]
Message-ID: <20110124135305.GB7354@scooter> (raw)
In-Reply-To: <20110124131341.GA3403@carfax.org.uk>

On 24. January 2011 - 13:13, Hugo Mills wrote:
> Date: Mon, 24 Jan 2011 13:13:41 +0000
> From: Hugo Mills <hugo-lkml@carfax.org.uk>
> To: Felix Blanke <felixblanke@gmail.com>
> Cc: kreijack@inwind.it, Hugo Mills <hugo-lkml@carfax.org.uk>,
>  linux-btrfs@vger.kernel.org
> Subject: Re: Bug in mkfs.btrfs?!
> Mail-Followup-To: Hugo Mills <hugo-lkml@carfax.org.uk>, Felix Blanke
>  <felixblanke@gmail.com>, kreijack@inwind.it, linux-btrfs@vger.kernel.org
> 
> On Mon, Jan 24, 2011 at 02:01:04PM +0100, Felix Blanke wrote:
> > Hi,
> > 
> > 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 an ioctl (number 0x4c03) that works on loop devices, and
> returns information about the loop device. Being an ioctl, it's
> implemented in the kernel. Unfortunately, since it's part of the
> kernel API, the size of the name field is probably fixed for the rest
> of time, and so the bug can't be fixed.
>

That sounds great :/

> > I'm searching for the right location where do report that bug :)
> 
>    linux-kernel mailing list, I think.
> 
> > Btw: I tested it with util-linux-2.19-rc1. The strace still contains
> > the truncated path, and no '*'. Therefore I think that ioctl is from
> > the kernel.
> 
>    Indeed.
> 
>    What I find interesting is that my copy of losetup follows symlinks
> from the /dev/disk/by-id/... path back to the original device node
> (/dev/dm-7 in my test case) before setting up the loop, whereas yours
> seems not to.
> 
>    I think that that's probably the easiest solution to this problem:
> modify losetup to use realpath(3) on the device node it's given.

I dont see where that helps with the problem. If I understand Goffredo correct
mkfs.btrfs is using the ioctl to get the path.

Letting losetup following the link will fix the output of "losetup /dev/loopX", but
it will not fix the truncated path from the ioctl and therefore it will not fix the
problem with mkfs.btrfs.

Felix

> 
>    Hugo.
> 
> -- 
> === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
>   PGP key: 515C238D from wwwkeys.eu.pgp.net or http://www.carfax.org.uk
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