Re: [Ksummit-discuss] [TECH TOPIC] Linux kernel SMB server (CIFSD)

[Jeff Layton <jlayton@kernel.org>]

On Wed, 2019-07-24 at 13:50 +0300, Amir Goldstein wrote:
> On Wed, Jul 24, 2019 at 11:03 AM Sergey Senozhatsky
> <sergey.senozhatsky.work@gmail.com> wrote:
> > On (07/24/19 14:52), Sergey Senozhatsky wrote:
> > > On (05/30/19 01:59), Theodore Ts'o wrote:
> > > > From: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> > [..]
> > > To extend the topic with some discussion points:
> > > 
> > > - We are facing a rather familiar problem. Basically, SMB2 CHANGE_NOTIFY
> > >   [1] is something what fsnotify normally does, except that we can't use
> > >   it in cifsd kernel module. I see that NFS guys had [2] same issues some
> > >   time ago.
> > > 
> > >   So the question is - how to do fs notify style monitoring in a kernel
> > >   module (nfsd, cifsd)? Any chance the kernel can start exporting fsnotify
> > >   symbols?
> 
> I don't see a problem with exporting those symbols.
> As a matter of fact, I have a plan to use them also from overlayfs.
> At the time that Jeff posted his patches, there was no active fsnotify
> maintainer.
> Jeff, did you abandon this effort for nfsd?
> 

(cc'ing Trond)

Trond has been driving those patches more recently. He posted a revised
set of them recently, and they still use fsnotify to detect unlink
activity:

https://www.spinics.net/lists/linux-nfs/msg73692.html

> > I even looked at LSM hooks, because they do what we need - invoke
> > creat, open, truncate, chmod, chown, unlink, callbacks. But, first,
> > I do realize that LSM was not meant to be used as some sort of fsnotify
> > replacement, and, second, LSM symbols are not exported anyway :)
> > 
> 
> fsnotify could actually be called from LSM hooks, see:
> https://github.com/amir73il/linux/commits/fsnotify_dirent_perm
> 
> It may or may not be clear to you, but what you get extra from fsnotify
> compared to using bare LSM hooks is:
> - Minimal performance overhead for a non-marked object
> - Ability to subscribe/unsubscribe to certain events dynamically
> - Manage multiple subscriber instances (groups), like one per SMB session
> - Queuing infrastructure for async events if you need it
-- 
Jeff Layton <jlayton@kernel.org>