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

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Message-ID: <20170207091924.GA13995@infradead.org>
Date:   Tue, 7 Feb 2017 01:19:24 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     James Bottomley <James.Bottomley@...senPartnership.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 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.