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Date:   Fri, 30 Oct 2020 02:51:48 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org,
        John Johansen <john.johansen@...onical.com>,
        James Morris <jmorris@...ei.org>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
        Geoffrey Thomas <geofft@...reload.com>,
        Mrunal Patel <mpatel@...hat.com>,
        Josh Triplett <josh@...htriplett.org>,
        Andy Lutomirski <luto@...nel.org>,
        Amir Goldstein <amir73il@...il.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Theodore Tso <tytso@....edu>, Alban Crequy <alban@...volk.io>,
        Tycho Andersen <tycho@...ho.ws>,
        David Howells <dhowells@...hat.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Jann Horn <jannh@...gle.com>,
        Seth Forshee <seth.forshee@...onical.com>,
        St├ęphane Graber <stgraber@...ntu.com>,
        Lennart Poettering <lennart@...ttering.net>,
        smbarber@...omium.org, Phil Estes <estesp@...il.com>,
        Serge Hallyn <serge@...lyn.com>,
        Kees Cook <keescook@...omium.org>,
        Todd Kjos <tkjos@...gle.com>, Jonathan Corbet <corbet@....net>,
        containers@...ts.linux-foundation.org,
        linux-security-module@...r.kernel.org, linux-api@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-unionfs@...r.kernel.org,
        linux-audit@...hat.com, linux-integrity@...r.kernel.org,
        selinux@...r.kernel.org
Subject: Re: [PATCH 00/34] fs: idmapped mounts

On 2020-10-29, Eric W. Biederman <ebiederm@...ssion.com> wrote:
> Christian Brauner <christian.brauner@...ntu.com> writes:
> 
> > Hey everyone,
> >
> > I vanished for a little while to focus on this work here so sorry for
> > not being available by mail for a while.
> >
> > Since quite a long time we have issues with sharing mounts between
> > multiple unprivileged containers with different id mappings, sharing a
> > rootfs between multiple containers with different id mappings, and also
> > sharing regular directories and filesystems between users with different
> > uids and gids. The latter use-cases have become even more important with
> > the availability and adoption of systemd-homed (cf. [1]) to implement
> > portable home directories.
> 
> Can you walk us through the motivating use case?
> 
> As of this year's LPC I had the distinct impression that the primary use
> case for such a feature was due to the RLIMIT_NPROC problem where two
> containers with the same users still wanted different uid mappings to
> the disk because the users were conflicting with each other because of
> the per user rlimits.
> 
> Fixing rlimits is straight forward to implement, and easier to manage
> for implementations and administrators.

This is separate to the question of "isolated user namespaces" and
managing different mappings between containers. This patchset is solving
the same problem that shiftfs solved -- sharing a single directory tree
between containers that have different ID mappings. rlimits (nor any of
the other proposals we discussed at LPC) will help with this problem.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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