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

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Date:   Mon, 20 Feb 2017 16:48:55 -0800
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     "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@...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 Tue, 2017-02-07 at 01:24 +0900, J. R. Okajima wrote:
> James Bottomley:
> > 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.
> 
> I think it serious.
> Reusing the backend fs' inum is a good approach which Amir wrote.
> Based on this, I'd suggest you to support the hardlinks.

I realised as I was trimming down the vestigial inode properties in the
patch that actually shiftfs does use the i_ino from the underlying for
userspace.  The reason why is that it comes from the getattr call in
stat and that's fully what the underlying filesystem returns (including
the inode number).

James