Re: [PATCH 01/24] Unionfs: Documentation

[Shaya Potter <spotter@cs.columbia.edu>]

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.