linux-kernel - Re: [PATCH] [RFC] mnt: add ability to clone mntns starting with the current root

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Date:	Tue, 7 Oct 2014 14:52:21 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Andrey Vagin <avagin@...nvz.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Andrey Vagin <avagin@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Rob Landley <rob@...dley.net>
Subject: Re: [PATCH] [RFC] mnt: add ability to clone mntns starting with the
 current root

On Tue, Oct 7, 2014 at 2:50 PM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
> Andy Lutomirski <luto@...capital.net> writes:
>
>> On Tue, Oct 7, 2014 at 2:26 PM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
>>> Andy Lutomirski <luto@...capital.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.
>
> I don't think so.
>
> I expect I could add full busy detection of normal umounts and those
> applications would not fail.
>
> What I am proposing is a more targeted version of busy detection that
> looks at each mount in the set that detach will unmount.  For each mount
> if it is busy with non-submount references and it has at least one
> locked submount fail the detach with -EBUSY.
>
> Do you really think we have userspace references to the one or more of the
> mounts under old?
>

I suspect that we have a userspace reference to old itself.

--Andy

> Eric



-- 
Andy Lutomirski
AMA Capital Management, LLC
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