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

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Date:   Mon, 06 Feb 2017 08:29:43 -0800
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Amir Goldstein <amir73il@...il.com>,
        "J. R. Okajima" <hooanon05g@...il.com>
Cc:     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 Mon, 2017-02-06 at 08:38 +0200, Amir Goldstein wrote:
> On Mon, Feb 6, 2017 at 5:25 AM, J. R. Okajima <hooanon05g@...il.com>
> wrote:
> > James Bottomley:
> > > This allows any subtree to be uid/gid shifted and bound
> > > elsewhere.  It
> >         :::
> > 
> > Interesting.
> > But I am afraid that the inconsistency problem of the inode numbers 
> > will happen.
> > 
> 
> Yet another example that overlayfs already is in the process of 
> solving (it is fixed for stat of merged directory inode).
> In fact, fir the case of single layer overlay (as well as shiftfs) 
> the solution is trivial - preserve underlying inode st_ino/d_ino and 
> use the overlayed fs st_dev.

not sure I follow what st_ino is, do you mean  s_root->d_inode->i_ino?
or did you mean s_dev (which is more traditional)?

The problem with this is there's no way to ensure global uniqueness in
a mapping that goes (ino, ino) -> (ino) (or (s_dev, ino) -> (ino)) and
I believe global uniqueness is more important because the i_ino is used
in the hashed lookups.  Secondly you're not guaranteed that s_root
->d_inode->i_ino is unique ... historically a lot of filesystems use a
well known inode number as the root, that's why filehandles
traditionally used something representing the device and the inode
number (we also have s_dev uniqueness problems for tmpfs which is used
in some overlays).

We can certainly construct a filehandle using an export operations
override that is unique and can be used to lookup the underlying object
(based on the underlying device and inode).

James