linux-kernel - Re: [RFC v2 PATCH 0/8] VFS:userns: support portable root filesystems

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Date:	Sun, 15 May 2016 11:04:36 -0400
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Djalal Harouni <tixxdz@...il.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Chris Mason <clm@...com>, tytso@....edu,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Josh Triplett <josh@...htriplett.org>,
	Andy Lutomirski <luto@...nel.org>,
	Seth Forshee <seth.forshee@...onical.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	Dongsu Park <dongsu@...ocode.com>,
	David Herrmann <dh.herrmann@...glemail.com>,
	Miklos Szeredi <mszeredi@...hat.com>,
	Alban Crequy <alban.crequy@...il.com>,
	Dave Chinner <david@...morbit.com>
Subject: Re: [RFC v2 PATCH 0/8] VFS:userns: support portable root filesystems

On Sat, 2016-05-14 at 21:21 -0500, Eric W. Biederman wrote:
> James if you could see shiftfs with a different set of merits than 
> what to Djalal is doing I think that would be useful.  As it would 
> allow everyone to concentrate on getting the bugs out of their
> solutions.

Just to reply to this specific point.  Djalal's patches can't actually
work for me because I use subtree based roots rather than whole fs
roots ... it's mostly because I work with image directories, not the
full mounted images themselves.  For stuff I unpack into /home, I could
see having /home on a separate directory and adding the vfs_shift_
flags.  however, I'm not doing (and it would be really unsafe to do)
that for / to get my images that unpack in /var/tmp (like the obs build
roots).

However, half the ugliness of the patch set is that it needs lower
layer FS support because vfs_shift_ are mount flags in the superblock. 
 If they were made subtree flags instead (so MNT_ flags), I think you
could eliminate the need to modify any underlying filesystems and they
would allow us to mark subtrees for shifting.  the mount command would
need modifying to add them (like it was for --shared and --private) so
we'd need an additional --vfs-shift --ufs-shift to mark the subtree but
then the series would work for bind mounting subtrees, which is what I
need.  And they would work for *any* filesystem without modification.

This would probably be the better of both worlds because it will work
for the docker case as well.

James