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

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Date:   Sun, 05 Feb 2017 22:46:23 -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 Mon, 2017-02-06 at 12:25 +0900, J. R. Okajima 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.
> 
> shiftfs_new_inode() uses get_next_ino() which means
> - 1st time: inodeA is created and cached, inumA is assigned
> - after using inodeA, it will be discarded from the cache
> - 2nd time: inodeA is looked-up again, and another inode number 
> (inumB)   is assgined.

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.

[...]
> If shiftfs will supports exporting via NFS in the future, the
> consistency of inum will be important too.

If it's a problem, then it's fixable with s_export_op, but I was mostly
thinking that because it's not a problem for overlayfs based
containers, it wouldn't be one for shiftfs based ones, which is why I
didn't implement it.

James