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From:   Amir Goldstein <amir73il@gmail.com>
Date:   Tue, 28 Jun 2022 15:29:08 +0300
Message-ID: <CAOQ4uxgtZDihnydqZ04wjm2XCYjui0nnkO0VGzyq-+ERW20pJw@mail.gmail.com>
Subject: Re: [PATCH 6/6] fanotify: add current_user_instances node
To:     guowei du <duguoweisz@gmail.com>
Cc:     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>
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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.

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).

Do you want to implement the first step of enumerating fdinfo
of all groups via /sys/fs/fanotify/groups/?

Jan,

If you have objections to any of the ideas above please shout.
I was going to prepare a roadmap for blocking events and post it
for comments, but this patch triggered a heads up.

Thanks,
Amir.

[1] https://docs.microsoft.com/en-us/windows/win32/projfs/projected-file-system