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

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Date:   Tue, 7 Feb 2017 10:39:48 +0100
From:   Djalal Harouni <tixxdz@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     James Bottomley <James.Bottomley@...senpartnership.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 FS Devel <linux-fsdevel@...r.kernel.org>,
        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

Hi,

On Tue, Feb 7, 2017 at 10:19 AM, Christoph Hellwig <hch@...radead.org> wrote:
> On Sat, Feb 04, 2017 at 11:19:32AM -0800, James Bottomley wrote:
>> This allows any subtree to be uid/gid shifted and bound elsewhere.  It
>> does this by operating simlarly to overlayfs.  Its primary use is for
>> shifting the underlying uids of filesystems used to support
>> unpriviliged (uid shifted) containers.  The usual use case here is
>> that the container is operating with an uid shifted unprivileged root
>> but sometimes needs to make use of or work with a filesystem image
>> that has root at real uid 0.
>>
>> The mechanism is to allow any subordinate mount namespace to mount a
>> shiftfs filesystem (by marking it FS_USERNS_MOUNT) but only allowing
>> it to mount marked subtrees (using the -o mark option as root).  Once
>> mounted, the subtree is mapped via the super block user namespace so
>> that the interior ids of the mounting user namespace are the ids
>> written to the filesystem.
>
> Please move this into VFS instead of a stackable fs.  We might need
> addtional parameters to getattr/setattr to specify the ID translation,
> but that's why better than a horrible hack like this.

I proposed an RFC months ago which implements all of this at the VFS
layer [1], I received some feedback especially from Dave Chinner,
however I failed to fix my bugs and improve it not enough resources...

The problems discussed here about a new filesystem: inodes numbers,
quota and many other things where all noted in that thread and
previous threads about shiftfs. We are turning this to a heavy problem
compared to all other namespaces... other namespaces integrate
perfectly with other subsystems and the rest of layers, there is no
special treatment...

Christoph, for the getattr/setattr it won't work since internally the
resolved path may point to a different mount context where we do not
want the ID translation, and we may end up using the wrong vfsmount. A
simple getattr/setattr won't work unless there are bigger changes
too...

[1] https://lkml.org/lkml/2016/5/4/411

-- 
tixxdz