ideas about a unioning file system

Erez Zadok ezk at
Fri Jun 20 09:57:32 PDT 2003

In message <3EF31B05.5070004 at>, Michael Sierchio writes:
> Poul-Henning Kamp wrote:
> > The majority of the problems we have in this area can be nailed down
> > to only two issues:
> > 	1. The very tricky semantics of rename(2)
> > 	2. read/write/mmap coherency for stacked filesystems.
> This is of particular interest to me.  While Poul-Henning has
> done some very interesting work on encryption at the GEOM
> level, I'm trying to work with ACL filesystems and stacking
> crypto filesystems that manage keys with one per file.

Yup, coherency is certainly a difficult problem.  We have only begun to look
at it, and implemented some simple solutions (for NCryptfs).  We'll be
exploring something more like Heidemann's work (SOSP'95) in the future.

> Imagine the coherency problems when pages don't match up
> because of IVs for CBC block mode encryption, and imagine
> the headache for (un-)locking pages at different levels.

Those of you doing crypto f/s, might be interested in reading our
paper on NCryptfs, which was just presented at Usenix:

> Making it easy for FIST and cryptfs (Erez Zadok's stuff) would
> be very helpful.  (I.e. gee, I'd rather not have to do this
> on Linux).

BTW, we've completed a port of the fist templates to 4.x (all versions) and
5.0.  We're planning to release a new fistgen RSN.


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