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SubjectRe: [PATCH 01/24] Unionfs: Documentation
> In message <20070108111852.ee156a90.akpm@osdl.org>, Andrew Morton writes:
> > On Sun, 7 Jan 2007 23:12:53 -0500
> > "Josef 'Jeff' Sipek" <jsipek@cs.sunysb.edu> wrote:
> >
> > > +Modifying a Unionfs branch directly, while the union is mounted, is
> > > +currently unsupported.
> >
> > Does this mean that if I have /a/b/ and /c/d/ unionised under /mnt/union, I
> > am not allowed to alter anything under /a/b/ and /c/d/? That I may only
> > alter stuff under /mnt/union?
> >
> > If so, that sounds like a significant limitation.
<snip>
> Now, we've discussed a number of possible solutions. Thanks to suggestions
> we got at OLS, we discussed a way to hide the lower namespace, or make it
> readonly, using existing kernel facilities. But my understanding is that
> even it'd work, it'd only address new processes: if an existing process has
> an open fd in a lower branch before we "lock up" the lower branch's name
> space, that process may still be able to make lower-level changes.
> Detecting such processes may not be easy. What to do with them, once
> detected, is also unclear. We welcome suggestions.
Yes, making fs readonly at VFS level would not work for already opened
files. But you if you create new union, you could lock down the
filesystems you are unioning (via s_umount semaphore), go through lists
of all open fd's on those filesystems and check whether they are open
for write or not. If some fd is open for writing, you simply fail to
create the union (and it's upto user to solve the problem). Otherwise
you mark filesystems as RO and safely proceed with creating the union.
I guess you must have come up with this solution. So what is the problem
with it?

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