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Date:   Tue, 14 Jan 2020 04:39:24 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Ian Kent <raven@themaw.net>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Aleksa Sarai <cyphar@cyphar.com>,
        David Howells <dhowells@redhat.com>,
        Eric Biederman <ebiederm@xmission.com>,
        stable <stable@vger.kernel.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Serge Hallyn <serge@hallyn.com>, dev@opencontainers.org,
        Linux Containers <containers@lists.linux-foundation.org>,
        Linux API <linux-api@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RFC 0/1] mount: universally disallow mounting over
 symlinks
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On Tue, Jan 14, 2020 at 08:25:19AM +0800, Ian Kent wrote:

> This isn't right.
> 
> There's actually nothing stopping a user from using a direct map
> entry that's a multi-mount without an actual mount at its root.
> So there could be directories created under these, it's just not
> usually done.
> 
> I'm pretty sure I don't check and disallow this.

IDGI...  How the hell will that work in v5?  Who will set _any_
traps outside the one in root in that scenario?  autofs_lookup()
won't (there it's conditional upon indirect mount).  Neither
will autofs_dir_mkdir() (conditional upon version being less
than 5).  Who will, then?

Confused...