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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).

    Honza
    --
    Jan Kara <jack@suse.cz>
    SuSE CR Labs
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