Re: [RFC 1/1] shiftfs: uid/gid shifting bind mount

From: Christoph Hellwig <hch@infradead.org>
To: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Djalal Harouni <tixxdz@gmail.com>, Chris Mason <clm@fb.com>,
	Theodore Tso <tytso@mit.edu>,
	Josh Triplett <josh@joshtriplett.org>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Andy Lutomirski <luto@kernel.org>,
	Seth Forshee <seth.forshee@canonical.com>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	Dongsu Park <dongsu@endocode.com>,
	David Herrmann <dh.herrmann@googlemail.com>,
	Miklos Szeredi <mszeredi@redhat.com>,
	Alban Crequy <alban.crequy@gmail.com>,
	Al Viro <viro@ZenIV.linux.org.uk>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	Phil Estes <estesp@gmail.com>
Subject: Re: [RFC 1/1] shiftfs: uid/gid shifting bind mount
Date: Tue, 7 Feb 2017 01:19:24 -0800	[thread overview]
Message-ID: <20170207091924.GA13995@infradead.org> (raw)
In-Reply-To: <1486235972.2484.19.camel@HansenPartnership.com>

On Sat, Feb 04, 2017 at 11:19:32AM -0800, James Bottomley wrote:
> This allows any subtree to be uid/gid shifted and bound elsewhere.  It
> does this by operating simlarly to overlayfs.  Its primary use is for
> shifting the underlying uids of filesystems used to support
> unpriviliged (uid shifted) containers.  The usual use case here is
> that the container is operating with an uid shifted unprivileged root
> but sometimes needs to make use of or work with a filesystem image
> that has root at real uid 0.
> 
> The mechanism is to allow any subordinate mount namespace to mount a
> shiftfs filesystem (by marking it FS_USERNS_MOUNT) but only allowing
> it to mount marked subtrees (using the -o mark option as root).  Once
> mounted, the subtree is mapped via the super block user namespace so
> that the interior ids of the mounting user namespace are the ids
> written to the filesystem.

Please move this into VFS instead of a stackable fs.  We might need
addtional parameters to getattr/setattr to specify the ID translation,
but that's why better than a horrible hack like this.