linux-kernel - fanotify and namespaces/bind mounts

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Date:	Thu, 29 Oct 2015 15:29:53 +0200
From:	Marko Rauhamaa <marko@...ujo.net>
To:	linux-kernel@...r.kernel.org
Subject: fanotify and namespaces/bind mounts

If I call fanotify_mark(... FAN_MARK_ADD | FAN_MARK_MOUNT ...), I get
notifications on all files on the file system.

Except I don't.

If a process has mounted directories on the file system in a different
namespace, the global namespace experiences file system events but no
fanotify events are generated. A similar limitation applies to bind
mounts.

You can easily see this behavior in action. Do "man fanotify", and copy
the example program to, say, example.c. Then,

    cc -o example example.c
    mkdir -p /mnt/xyz
    mount -o bind /root /mnt/xyz
    ./example /root

The last command prints out fanotify events. It will display events for

    touch /root/xyzzy

but nothing for

    touch /mnt/xyz/zyxxy

or

    unshare -m touch /root/yxyyx

Should the current situation be considered a bug in fanotify? Or is
there another recommended way to monitor file system events
categorigally?

Marko
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