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From:   Amir Goldstein <amir73il@gmail.com>
Date:   Thu, 23 May 2019 18:15:39 +0300
Message-ID: <CAOQ4uxhF4WLZRcQcBYGvy+J+cmkUvNLfjGrwvqfBqyfPEHAdhw@mail.gmail.com>
Subject: Re: [PATCH] fanotify: remove redundant capable(CAP_SYS_ADMIN)s
To:     Jan Kara <jack@suse.cz>
Cc:     Christian Brauner <christian@brauner.io>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Matthew Bobrowski <mbobrowski@mbobrowski.org>
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On Thu, May 23, 2019 at 5:40 PM Jan Kara <jack@suse.cz> wrote:
>
> On Thu 23-05-19 15:35:18, Christian Brauner wrote:
> > So let's say the user tells me:
> > - When the "/A/B/C/target" file appears on the host filesystem,
> >   please give me access to "target" in the container at a path I tell
> >   you.
> > What I do right now is listen for the creation of the "target" file.
> > But at the time the user gives me instructions to listen for
> > "/A/B/C/target" only /A might exist and so I currently add a watch on A/
> > and then wait for the creation of B/, then wait for the creation of C/
> > and finally for the creation of "target" (Of course, I also need to
> > handle B/ and C/ being removed again an recreated and so on.). It would
> > be helpful, if I could specify, give me notifications, recursively for
> > e.g. A/ without me having to place extra watches on B/ and C/ when they
> > appear. Maybe that's out of scope...
>
> I see. But this is going to be painful whatever you do. Consider for
> example situation like:
>
> mkdir -p BAR/B/C/
> touch BAR/B/C/target
> mv BAR A
>
> Or even situation where several renames race so that the end result creates
> the name (or does not create it depending on how renames race). And by the
> time you decide A/B/C/target exists, it doesn't need to exist anymore.
> Honestly I don't see how you want to implement *any* solution in a sane
> way. About the most reliable+simple would seem to be stat "A/B/C/target"
> once per second as dumb as it is.
>

Just wanted to point out that while looking at possible solutions for
"path based rules" for fanotify (i.e. subtree filter) I realized that the audit
subsystem already has a quite sophisticated mechanism to maintain
and enforce path based filesystem rules.

I do not love that code at all, I can hardly follow it, but if someone would
have wanted a way to be notified when an object of a given path name
appears or disappears from the namespace, it seems like something in
the kernel is already going to a great deal of effort to do that already.
Or maybe I am misunderstanding what this code does.

Thanks,
Amir.