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

[Amir Goldstein <amir73il@gmail.com>]

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?

>
> 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

Thanks,
Amir.