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Date:   Fri, 30 Oct 2020 02:51:48 +1100
From:   Aleksa Sarai <cyphar@cyphar.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Christian Brauner <christian.brauner@ubuntu.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        linux-fsdevel@vger.kernel.org,
        John Johansen <john.johansen@canonical.com>,
        James Morris <jmorris@namei.org>,
        Mimi Zohar <zohar@linux.ibm.com>,
        Dmitry Kasatkin <dmitry.kasatkin@gmail.com>,
        Stephen Smalley <stephen.smalley.work@gmail.com>,
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        Arnd Bergmann <arnd@arndb.de>,
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        OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>,
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        Amir Goldstein <amir73il@gmail.com>,
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Subject: Re: [PATCH 00/34] fs: idmapped mounts
Message-ID: <20201029155148.5odu4j2kt62ahcxq@yavin.dot.cyphar.com>
References: <20201029003252.2128653-1-christian.brauner@ubuntu.com>
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On 2020-10-29, Eric W. Biederman <ebiederm@xmission.com> wrote:
> Christian Brauner <christian.brauner@ubuntu.com> writes:
>=20
> > 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.
>=20
> Can you walk us through the motivating use case?
>=20
> 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.
>=20
> 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.

--=20
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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