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From: Andy Lutomirski <luto@amacapital.net>
Date: Tue, 7 Oct 2014 14:38:06 -0700
Message-ID: <CALCETrWfZwbGCxnUAg0PnM=tN8MGRQkHrJVC42bVF7sdJKXLmw@mail.gmail.com>
Subject: Re: [PATCH] [RFC] mnt: add ability to clone mntns starting with the
 current root
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>, Andrey Vagin <avagin@openvz.org>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>, Andrey Vagin <avagin@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Cyrill Gorcunov <gorcunov@openvz.org>,
        Pavel Emelyanov <xemul@parallels.com>,
        Serge Hallyn <serge.hallyn@canonical.com>,
        Rob Landley <rob@landley.net>
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On Tue, Oct 7, 2014 at 2:26 PM, Eric W. Biederman <ebiederm@xmission.com> wrote:
> Andy Lutomirski <luto@amacapital.net> writes:
>
>> Why should MNT_LOCKED on submounts be enforced?
>>
>> Is it because, if you retain a reference to the detached tree, then
>> you can see under the submounts?
>
> Yes. MNT_DETACH is a recursive operation that detaches all of the mount
> and all of it's submounts.  Which means you can see under the submounts
> if you have a reference to a detached mount.
>
>> If so, let's fix *that*.  Because
>> otherwise the whole model of pivot_root + detach will break.
>
> I am not certain what you are referring to.  pivot_root doesn't
> manipulate the mount tree so you can see under anything.
>
> What I believe is the appropriate fix is to fail umount2(...,MNT_DETACH)
> if there are any referenced mount points being detached that have a
> locked submount.

Most of the container-using things do, roughly:

Unshare userns and mountns
Mount some new stuff
pivot_root to the new stuff
MNT_DETACH the old.

That last step will almost always fail if you make this change.

--Andy
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