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

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Date:	Tue, 07 Oct 2014 14:26:00 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Andrey Vagin <avagin@...nvz.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"linux-kernel\@vger.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

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.

> Also, damn it, we need change_the_ns_root instead of pivot_root.  I
> doubt that any container programs actually want to keep the old root
> attached after pivot_root.

Shrug.  Except for chroot_fs_refs() pivot_root is a cheap.

I'm not particularly in favor of merging pivot_root and umount2.  The
number of weird cases in the current api are high.  A merged piece of
code would just make them higher.

I am hoping that one more round of bug fixing will at least get the bugs
for having unprivileged mounts fixed in the current API.

Eric
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