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

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Date:   Mon, 06 Feb 2017 16:10:11 -0800
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     "J. Bruce Fields" <bfields@...ldses.org>
Cc:     Theodore Ts'o <tytso@....edu>,
        "J. R. Okajima" <hooanon05g@...il.com>,
        Djalal Harouni <tixxdz@...il.com>, Chris Mason <clm@...com>,
        Josh Triplett <josh@...htriplett.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        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>,
        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 16:52 -0500, J. Bruce Fields wrote:
> On Mon, Feb 06, 2017 at 07:18:16AM -0800, James Bottomley wrote:
> > On Mon, 2017-02-06 at 09:50 -0500, Theodore Ts'o wrote:
> > > On Sun, Feb 05, 2017 at 10:46:23PM -0800, James Bottomley wrote:
> > > > Yes, I know the problem.  However, I believe most current linux
> > > > filesystems no longer guarantee stable, for the lifetime of the
> > > > file, inode numbers.  The usual docker container root is 
> > > > overlayfs, which, similarly doesn't support stable inode 
> > > > numbers.  I see the odd complaint about docker with overlayfs 
> > > > having unstable inode numbers, but none seems to have any
> > > > serious repercussions.
> > > 
> > > Um, no.  Most current linux file systems *do* guarantee stable 
> > > inode numbers.  For one thing, NFS would break horribly if you 
> > > didn't have stable inode numbers.  Never mind applications which 
> > > depend on POSIX semantics.  And you wouldn't be able to save 
> > > games in rogue or nethack, either.  :-)
> > 
> > I believe that's why we have the superblock export operations to
> > manufacture unique filehandles in the absence of inode number
> > stability.
> 
> Where did you hear that?
> 
> I'd expect an NFS client to handle non-unique filehandles
> better than non-unique inode numbers.  I believe our client will -EIO 
> on encountering an inode number change (see
> nfs_check_inode_attributes().)
> 
> See also https://tools.ietf.org/html/rfc5661#section-10.3.4.

Could you clarify your point a bit further, please?  Both the
check_inode_attributes() code and section 10.3.4 are talking about
fileids, which are the things that are constructed in the export_ops
... admittedly a lot of fileid_types are based on inode numbers, but
several aren't.  For those that aren't, I believe NFS doesn't care
about the underlying inode number of the exported file.

James