Date: Tue, 27 Apr 2010 13:58:48 +0300 From: Gleb Kurtsou <gleb.kurtsou@gmail.com> To: =?utf-8?B?THVrw6HFoQ==?= Czerner <czerner.lukas@gmail.com> Cc: freebsd-hackers@freebsd.org Subject: Re: How to change vnode operations ? Message-ID: <20100427105848.GA2003@tops> In-Reply-To: <alpine.DEB.1.10.1004241215530.7101@a04-0215a.kn.vutbr.cz> References: <alpine.DEB.1.10.1004221559270.7101@a04-0215a.kn.vutbr.cz> <20100422191849.GA9895@tops> <alpine.DEB.1.10.1004230756190.7101@a04-0215a.kn.vutbr.cz> <20100423092257.GA2446@tops> <alpine.DEB.1.10.1004241215530.7101@a04-0215a.kn.vutbr.cz>
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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@gmail.com> > > To: Lukáš Czerner <czerner.lukas@gmail.com> > > Cc: freebsd-hackers@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@gmail.com> > > > > To: Lukáš Czerner <czerner.lukas@gmail.com> > > > > Cc: freebsd-hackers@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. > > Thanks again! > -Lukas.
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