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From: ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman)
Subject: Re: [PATCH] [RFC] mnt: add ability to clone mntns starting with the current root
Date: Tue, 07 Oct 2014 14:50:47 -0700
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Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org> writes:

> On Tue, Oct 7, 2014 at 2:26 PM, Eric W. Biederman <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org> wrote:
>> Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org> 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?

Eric