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Date:   Thu, 23 May 2019 16:40:50 +0200
From:   Jan Kara <jack@suse.cz>
To:     Christian Brauner <christian@brauner.io>
Cc:     Amir Goldstein <amir73il@gmail.com>, Jan Kara <jack@suse.cz>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Matthew Bobrowski <mbobrowski@mbobrowski.org>
Subject: Re: [PATCH] fanotify: remove redundant capable(CAP_SYS_ADMIN)s
Message-ID: <20190523144050.GE2949@quack2.suse.cz>
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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.

								Honza

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