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SubjectRe: [PATCH 01/24] Unionfs: Documentation
> Josef Sipek wrote:
> >On Wed, Jan 10, 2007 at 05:12:15PM +0100, Jan Kara wrote:
> >> I see :). To me it just sounds as if you want to do remount-read-only
> >>for source filesystems, which is operation we support perfectly fine,
> >>and after that create union mount. But I agree you cannot do quite that
> >>since you need to have write access later from your union mount. So
> >>maybe it's not so easy as I thought.
> >> On the other hand, there was some effort to support read-only
> >> bind-mounts of
> >>read-write filesystems (there were even some patches floating around but
> >>I don't think they got merged) and that should be even closer to what
> >>you'd need...
> >
> >Since the RO flag is per-mount point, how do you guarantee that no one is
> >messing with the fs? (I haven't looked at the patches that do per mount
> >ro flag, but this would require some over-arching ro flag - in the
> >superblock most likely.)
> I thought about it, wrote an email, then cancelled it as it won't work.
> what I thought was that you could a limited unionfs case would be with X
> layers read-only and the top layer read-write, and what you would do is
> dynamically make read only bind mounts for the the X layers and since
> you control the top layer hide it from the system.
> However, read only bind mounts are great if you want a limit a process
> to accessing the files read-only, as they won't have access to the other
> vfs_mounts, but it does nothing for the other vfs_mounts that are using
> that same file system. hence, does us no good.
Right, you'd need to remount read-only all the mountpoints of one
filesystem. But if we had read-only bind-mounts, you could do such things
from userspace. It won't be 100% reliable (as it would be racy) but as a
basic protection against stupidity of admin it should work. And it would
be 100% safe against malicious intentions of average user (who has no
right to create new mountpoints).

Jan Kara <>
SuSE CR Labs
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