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Date:   Sun, 14 Apr 2019 20:26:01 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Amir Goldstein <amir73il@gmail.com>
Cc:     Jan Kara <jack@suse.cz>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        LSM List <linux-security-module@vger.kernel.org>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        James Morris <jmorris@namei.org>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Matthew Bobrowski <mbobrowski@mbobrowski.org>,
        Kentaro Takeda <takedakn@nttdata.co.jp>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        John Johansen <john.johansen@canonical.com>
Subject: Re: fanotify and LSM path hooks
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On Sun, Apr 14, 2019 at 09:51:38PM +0300, Amir Goldstein wrote:

> But the truth is I would much rather that users have a way to mark
> a subtree root and ask fanotify for events under that subtree.
> As a matter of fact, I have some private POC patches that allow users to
> setup a mark on a "subtree root" dentry, which really marks the super block
> and keep a reference to the dentry. Than every event on that super block
> is filtered with is_subdir() against the marked dentry.

And that is_subdir() is protected by what, exactly?  And what happens
if you have many such dentries?

Or, for that matter, what happens if that dentry gets invalidated?