How to change vnode operations ?

[Lukáš Czerner]

On Tue, 27 Apr 2010, Gleb Kurtsou wrote:

> Date: Tue, 27 Apr 2010 13:58:48 +0300
> From: Gleb Kurtsou <gleb.kurtsou at gmail.com>
> To: Lukáš Czerner <czerner.lukas at gmail.com>
> Cc: freebsd-hackers at freebsd.org
> Subject: Re: How to change vnode operations ?
> 
> On (24/04/2010 12:26), Lukáš Czerner wrote:
> > On Fri, 23 Apr 2010, Gleb Kurtsou wrote:
> > 
> > > Date: Fri, 23 Apr 2010 12:22:57 +0300
> > > From: Gleb Kurtsou <gleb.kurtsou at gmail.com>
> > > To: Lukáš Czerner <czerner.lukas at gmail.com>
> > > Cc: freebsd-hackers at freebsd.org
> > > Subject: Re: How to change vnode operations ?
> > > 
> > > On (23/04/2010 08:10), Lukáš Czerner wrote:
> > > > On Thu, 22 Apr 2010, Gleb Kurtsou wrote:
> > > > 
> > > > > Date: Thu, 22 Apr 2010 22:18:49 +0300
> > > > > From: Gleb Kurtsou <gleb.kurtsou at gmail.com>
> > > > > To: Lukáš Czerner <czerner.lukas at gmail.com>
> > > > > Cc: freebsd-hackers at freebsd.org
> > > > > Subject: Re: How to change vnode operations ?
> > > > > 
> > > > > On (22/04/2010 16:02), Lukáš Czerner wrote:
> > > > > > Hi all,
> > > > > > 
> > > > > > this may sound a little odd, since I have noticed that there is much
> > > > > > work done to not allow such a thing ($SUBJ). But may be you can help
> > > > > > me and point me to the right direction.
> > > > > > 
> > > > > > I am writing a kernel module with somewhat similar functionality
> > > > > > like nullfs has, BUT it has to have some features which nullfs
> > > > > > itself does not provide :
> > > > > > 
> > > > > >  1. I need the new layer to completely hide underlaying layer so no
> > > > > >     one can bypass it.
> > > > > Is hypothetic 'mount -t mynullfs /a /a' good enough for you? I'm not sure
> > > > > what your goals are but completely finding underlaying filesystem won't
> > > > > be easy because of VFS_GET, getfh and other stuff operating with inode
> > > > > numbers.
> > > > 
> > > > Well, it may be good enough, or not. Thats what I am trying to find
> > > > out. Obviously there are problems, as you mentioned, which will not
> > > > exist when I change the vop_vector of the vnode, but as I thought
> > > > and you mentioned it as well, this is not very clean way.
> > > Why don't you like stacked filesystem approach? It's designed to solve
> > > the problem you are describing if I get it right. Although creating
> > > pefs-like filesystem altering data and names is not so easy within
> > > existing framework.
> > > 
> > > > > >  2. Nullfs allows me to to overlay just one directory, but i want to
> > > > > >     include another directories and/or exclude subdirectories/files.
> > > > > >  3. Nullfs just redirects vnode operations to lower layer, I need to
> > > > > >     catch that operation, do something (for example alter the arguments
> > > > > >     somehow etc..), pass the operation (with possibly altered arguments)
> > > > > >     to the lower layer, get the result and then return the result.
> > > > > I'd suggest to take a look at pefs: http://github.com/glk/pefs
> > > > > It's cryptographic stacked filesystem for FreeBSD. It changes file
> > > > > names, hides directory entries, modifies data from lower layer
> > > > > (encrypts or decrypts), supports mounting on same directory, etc.
> > > > 
> > > > Thats great, thanks! I will look at it.
> > > > 
> > > > > 
> > > > > > The best way to do that (I think) is to change vnode operations of
> > > > > > particular vnodes to point to functions defined in that module. At
> > > > > > this point, I can catch any operations with the vnode and this is
> > > > > > the base of what i want.
> > > > > > 
> > > > > > So my question is. I there any "clean" way to chande vnode
> > > > > > operations ? If not, is there any "not so clean" way ? Anyway I will
> > > > > > appreciate any good idea how to do what I have described.
> > > > > Imho, stacked filesystem is the only right way to do it (see null,
> > > > > unionfs, pefs).
> > > > 
> > > > OK. Thanks for pointing me to the pefs, it is interesting and looks
> > > > like a good start. But I would appreciate more comment on the side
> > > > of the whole idea about changing vnode operations from the kernel
> > > > module. It is a little hacky, but aside this I do not see any bigger
> > > > problems, do you ?
> > > Changing vop_vector is too hackish for me. Basically, changing vnode
> > > operations is what stacked filesystems are about. Vnode operations are
> > > the top of the problem, you would also have to deal with parent lookup,
> > > namecache consistency and buffering, which is going to be complicated.
> > > I.e. you'd have to partially reimplement part of VFS layer.
> > > 
> > > nullfs and unionfs pass vnode vobject (buffering layer) from lower
> > > layer, adding your own vobject to vnode would complicate filesystem
> > > significantly. Besides you won't be able to assign 2 vobjects (original
> > > and your own) to a single vnode if you decide to change operations
> > > vector.
> > 
> > This is exactly how I feel about it and I am glad that you actually
> > confirm that, so thank you. Now I am convinced to use stacked
> > filesystem.
> > 
> > But I have one last question, though. It may be stupid, but when I
> > am looking at your pefs I can not figure out how you manage to mount
> > directory above itself. I am staring at the code and just do not see
> > that. It seems to me, like the check is done before the
> > {pefs|nullfs}_mount is even called. Can you point me out to the
> > right direction ?
> It's nullfs that forbids it. VFS is fine about mounting on top of the
> same directory. As far as I remember lower vnode should be unlocked to
> prevent locking against itself and pefs_lock and pefs_unlock had to be
> adjusted to support it. Do not remember specifics, sorry.
> 
> I do 'pefs mount /home/gleb /home/gleb' since October, it works for me.
> 

I have already figured it out. Thanks again!

-Lukas.