Re: overlayfs: issue with a replaced lower squashfs with export-table

From: Vivek Goyal <vgoyal@redhat.com>
To: Amir Goldstein <amir73il@gmail.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>,
	overlayfs <linux-unionfs@vger.kernel.org>,
	Fabian <godi.beat@gmx.net>
Subject: Re: overlayfs: issue with a replaced lower squashfs with export-table
Date: Wed, 8 Jul 2020 10:26:53 -0400	[thread overview]
Message-ID: <20200708142653.GB103536@redhat.com> (raw)
In-Reply-To: <20200708142353.GA103536@redhat.com>

On Wed, Jul 08, 2020 at 10:23:53AM -0400, Vivek Goyal wrote:
> On Wed, Jul 08, 2020 at 11:50:29AM +0300, Amir Goldstein wrote:
> > On Wed, Jul 8, 2020 at 11:37 AM Miklos Szeredi <miklos@szeredi.hu> wrote:
> > >
> > > On Wed, Jul 8, 2020 at 10:31 AM Amir Goldstein <amir73il@gmail.com> wrote:
> > >
> > > >
> > > > 1) is not problematic IMO and the simple patch I posted may be applied
> > > > for fixing the reported issue, but it only solved the special case of null uuid.
> > > > The problem still exists with re-creating lower on xfs/ext4, e.g. by
> > > > rm -rf and unpacking image tar.
> > >
> > > How so?  st_ino may be reused but the fh is guaranteed to be unique.
> > >
> > 
> > Doh! You are right. I was talking nonsense.
> > The only problem would be with re-creating an xfs/ext4 lower image
> > with the same uuid maybe because a basic image is cloned.
> > 
> > In any case, it's a corner of a corner of a corner.
> > I will post the patch to fix null uuid.
> 
> It will also be good if we can bring some clarity to the documentation
> for future references in section "Sharing and copying layers".
> 
> So if IIUC,
> 
> - sharing layers should work with all features of overlayfs.
> 
> - copying layers works only if index and nfs_export is not enabled. Even
>   if index is not enabled, copying layers will change inode number
>   reporting behavior (as origin verification will fail). We probably
>   say something about this.
> 
> - Modifying/recreating lower layer only works when
>   metacopy/index/nfs_export are not enabled at any point of time. This
>   also will change inode number reporting behavior.

Well, this is not entirely true. redirect might be broken if lower layers have
been modified/recreated and that will have issues with directories.

/me is again wondering what's the use case of modifying lower layer
with an existing upper. Is it fair to say, no don't recreate/modify
lower layers and use with existing upper.

Thanks
Vivek