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Date:   Tue, 28 Jun 2022 14:56:17 +0200
From:   Jan Kara <jack@suse.cz>
To:     Amir Goldstein <amir73il@gmail.com>
Cc:     guowei du <duguoweisz@gmail.com>,
        Christian Brauner <brauner@kernel.org>,
        Jan Kara <jack@suse.cz>, Matthew Bobrowski <repnop@google.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        duguowei <duguowei@xiaomi.com>
Subject: Re: [PATCH 6/6] fanotify: add current_user_instances node
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On Tue 28-06-22 15:29:08, Amir Goldstein wrote:
> On Tue, Jun 28, 2022 at 2:50 PM guowei du <duguoweisz@gmail.com> wrote:
> >
> > hi, Mr Kara, Mr Brauner,
> >
> > I want to know how many fanotify readers are monitoring the fs event.
> > If userspace daemons monitoring all file system events are too many, maybe there will be an impact on performance.
> 
> I want something else which is more than just the number of groups.
> 
> I want to provide the admin the option to enumerate over all groups and
> list their marks and blocked events.

Listing all groups and marks makes sense to me. Often enough I was
extracting this information from a crashdump :).

Dumping of events may be a bit more challenging (especially as we'd need to
format the events which has some non-trivial implications) so I'm not 100%
convinced about that. I agree it might be useful but I'd have to see the
implementation...

> This would be similar to listing all the fdinfo of anon_inode:[fanotify] fds
> of processes that initialised fanotify groups.
> 
> This enumeration could be done for example in /sys/fs/fanotify/groups/
> 
> My main incentive is not only the enumeration.
> My main incentive is to provide an administrative interface to
> check for any fs operations that are currently blocked by a rogue
> fanotify permission events reader and an easy way for administrators
> to kill those rogue processes (i.e. buggy anti-malware).
> 
> This interface is inspired by the ability to enumerate and abort
> fuse connections for rogue fuse servers.
> 
> I want to do that for the existing permission events as a prerequisite
> to adding new blocking events to be used for implementation of
> hierarchical storage managers, similar the Windows ProjFs [1].
> This was allegedly the intended use case for group class
> FAN_CLASS_PRE_CONTENT (see man page).

Yes, that was the original intent of FAN_CLASS_PRE_CONTENT AFAIK.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR