linux-kernel - Re: [PATCH 0/7] overlay filesystem v9

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Message-ID: <87sjsak2jt.fsf@tucsk.pomaz.szeredi.hu>
Date:	Thu, 19 May 2011 19:44:54 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Andy Whitcroft <apw@...onical.com>
Cc:	viro@...IV.linux.org.uk, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	nbd@...nwrt.org, neilb@...e.de, hramrach@...trum.cz,
	jordipujolp@...il.com
Subject: Re: [PATCH 0/7] overlay filesystem v9

Andy Whitcroft <apw@...onical.com> writes:

> On Tue, May 17, 2011 at 02:30:45PM +0200, Miklos Szeredi wrote:
>
>> Here's an updated version of the overlay filesystem.
>> 
>> Git tree is here:
>> 
>>   git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/vfs.git overlayfs.v9
>
> Ok I pulled this into the Ubuntu kernel and made an Ubuntu Live CD
> for testing.  Overall it worked pretty well, no hangs, no crashes,
> performance seemed reasonable.  We hit one issue with hard links which
> fail to be possible on overlayfs mounts when the Yama LSM (out of tree)
> is enabled.  This module applies more aggressive checks on hard-links
> preventing links to files you cannot read.

Great, thanks for testing.

> The bug seems to be related to the way we handle user and group owners for
> the overlayfs inodes, which we do not initialise (and they remain as 0,0).
> While these ownerships are never exposed to userspace they are exposed
> to the LSM layer, and the LSM module checks the wrong owner and fails
> to allow the links.

Hmm.

> From what I can see it is completly reasonable to initialise the ownership
> fields in the overlayfs inode from the underlying inode, or for new files
> ones initialise it in the normal way based on the containing directory.
> Now I am nothing like a filesystems expert but looking at what other
> filesystems do I think the patch below is sufficient, but certainly it
> needs some sanity checking.  At least it fixes all the issues I see
> here.

Forgot to attach the patch?

>
> With this in place I have been able to boot a Natty release live CD
> image, and upgrade it in place to the latest and greatest of everything.
> Very nice.  Thanks for your work continuing work on overlayfs.  It is
> starting to look good enough to merge.
>
> Comments?

I'd be happier if the LSM module were fixed not to peek inside the inode
structure.  Some filesystems (notably network ones) do not guarantee i_*
to be up-to-date, relying on these is not a good idea.

Thanks,
Miklos
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