Date:   Thu, 29 Oct 2020 11:44:33 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Tycho Andersen <tycho@...ho.pizza>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        Lennart Poettering <lennart@...ttering.net>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        David Howells <dhowells@...hat.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        containers@...ts.linux-foundation.org,
        Miklos Szeredi <miklos@...redi.hu>, smbarber@...omium.org,
        Christoph Hellwig <hch@...radead.org>,
        linux-ext4@...r.kernel.org, Mrunal Patel <mpatel@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Arnd Bergmann <arnd@...db.de>, Jann Horn <jannh@...gle.com>,
        selinux@...r.kernel.org, Josh Triplett <josh@...htriplett.org>,
        linux-fsdevel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
        Geoffrey Thomas <geofft@...reload.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        John Johansen <john.johansen@...onical.com>,
        Theodore Tso <tytso@....edu>,
        Seth Forshee <seth.forshee@...onical.com>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Jonathan Corbet <corbet@....net>,
        linux-unionfs@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-audit@...hat.com,
        linux-api@...r.kernel.org,
        Casey Schaufler <casey@...aufler-ca.com>,
        Alban Crequy <alban@...volk.io>,
        linux-integrity@...r.kernel.org, Todd Kjos <tkjos@...gle.com>
Subject: Re: [PATCH 00/34] fs: idmapped mounts

Tycho Andersen <tycho@...ho.pizza> writes:

> Hi Eric,
>
> On Thu, Oct 29, 2020 at 10:47:49AM -0500, Eric W. Biederman 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.
>
> Our use case is to have the same directory exposed to several
> different containers which each have disjoint ID mappings.

Why do the you have disjoint ID mappings for the users that are writing
to disk with the same ID?

>> Reading up on systemd-homed it appears to be a way to have encrypted
>> home directories.  Those home directories can either be encrypted at the
>> fs or at the block level.  Those home directories appear to have the
>> goal of being luggable between systems.  If the systems in question
>> don't have common administration of uids and gids after lugging your
>> encrypted home directory to another system chowning the files is
>> required.
>> 
>> Is that the use case you are looking at removing the need for
>> systemd-homed to avoid chowning after lugging encrypted home directories
>> from one system to another?  Why would it be desirable to avoid the
>> chown?
>
> Not just systemd-homed, but LXD has to do this,

I asked why the same disk users are assigned different kuids and the
only reason I have heard that LXD does this is the RLIMIT_NPROC problem.

Perhaps there is another reason.

In part this is why I am eager to hear peoples use case, and why I was
trying very hard to make certain we get the requirements.

I want the real requirements though and some thought, not just we did
this and it hurts.  Changning the uids on write is a very hard problem,
and not just in implementating it but also in maintaining and
understanding what is going on.

Eric

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