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

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Date:   Mon, 6 Feb 2017 09:38:40 -0600
From:   lkml@...garu.com
To:     James Bottomley <James.Bottomley@...senPartnership.com>
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, 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.  The generic one uses inode numbers, but it doesn't have to.
>  I thought reiserfs (if we can go back that far) was the first
> generally used filesystem that didn't guarantee stable inode numbers,
> so we have a lot of historical precedence.
> 
> Thanks to reiserfs, I thought we also iterated to weak stability
> guarantees for inode numbers which mean no inconsistencies in
> applications that use inode numbers for caching?  It's still not POSIX,
> but I thought it was good enough for most use cases.
> 

Even plain tar extraction is sensitive to directory inode stability:
http://git.savannah.gnu.org/cgit/tar.git/tree/src/extract.c?h=release_1_29#n867

This caused errors on overlayfs if the extraction churned through enough
of the dentry cache to evict the relevant directory (can be forced to
reproduce reliably via drop_caches).

Regards,
Vito Caputo