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

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Message-ID: <1486497908.2488.64.camel@HansenPartnership.com>
Date:   Tue, 07 Feb 2017 12:05:08 -0800
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Amir Goldstein <amir73il@...il.com>,
        Djalal Harouni <tixxdz@...il.com>, Chris Mason <clm@...com>,
        Theodore Tso <tytso@....edu>,
        Josh Triplett <josh@...htriplett.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        Seth Forshee <seth.forshee@...onical.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        LSM List <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>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Serge E. Hallyn" <serge@...lyn.com>, Phil Estes <estesp@...il.com>
Subject: Re: [RFC 1/1] shiftfs: uid/gid shifting bind mount

On Tue, 2017-02-07 at 11:49 -0800, Christoph Hellwig wrote:
> On Tue, Feb 07, 2017 at 11:02:03AM -0800, James Bottomley wrote:
> > >   Another option would be to require something like a project as
> > > used 
> > > for project quotas as the root.  This would also be conveniant as
> > > it 
> > > could storge the used remapping tables.
> > 
> > So this would be like the current project quota except set on a
> > subtree?  I could see it being done that way but I don't see what
> > advantage it has over using flags in the subtree itself (the
> > mapping is
> > known based on the mount namespace, so there's really only a single
> > bit
> > of information to store).
> 
> projects (which are the underling concept for project quotas) are
> per-subtree in practice - the flag is set on an inode and then
> all directories and files underneath inherit the project ID,
> hardlinking outside a project is prohinited.

OK, this is what I don't understand: how is something that's inode
based limited to be per-subtree?  The way I've seen the VFS operate it
seems that any given inode (and indeed dentry) can appear in many
subtrees so how do I limit them to just one?

James