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Biederman" Subject: Re: [PATCH 00/34] fs: idmapped mounts Message-ID: <20201029161231.GA108315@cisco> References: <20201029003252.2128653-1-christian.brauner@ubuntu.com> <87pn51ghju.fsf@x220.int.ebiederm.org> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <87pn51ghju.fsf@x220.int.ebiederm.org> Cc: Andy Lutomirski , Mimi Zohar , James Bottomley , Andreas Dilger , Miklos Szeredi , smbarber@chromium.org, Christoph Hellwig , Casey Schaufler , linux-ext4@vger.kernel.org, Mrunal Patel , Kees Cook , Arnd Bergmann , Jann Horn , selinux@vger.kernel.org, Josh Triplett , Seth Forshee , Alexander Viro , Lennart Poettering , OGAWA Hirofumi , Geoffrey Thomas , David Howells , John Johansen , Theodore Tso , Dmitry Kasatkin , containers@lists.linux-foundation.org, Jonathan Corbet , linux-unionfs@vger.kernel.org, linux-security-module@vger.kernel.org, linux-audit@redhat.com, linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org, Alban Crequy , linux-integrity@vger.kernel.org, Stephen Smalley , Todd Kjos X-BeenThere: containers@lists.linux-foundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Linux Containers List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: containers-bounces@lists.linux-foundation.org Sender: "Containers" Hi Eric, On Thu, Oct 29, 2020 at 10:47:49AM -0500, Eric W. Biederman wrote: > Christian Brauner 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. > 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, as does our application at Cisco, and presumably others. Several reasons: * the chown is slow * the chown requires somewhere to write the delta in metadata (e.g. an overlay workdir, or an LV or something), and there are N copies of this delta, one for each container. * it means we need to have a +w filesystem at some point during execution. * it's ugly :). Conceptually, the kernel solves the uid shifting problem for us for most other kernel subsystems (including in a limited way fscaps) by configuring a user namespace. It feels like we should be able to do the same with the VFS. Tycho _______________________________________________ Containers mailing list Containers@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/containers