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Date:   Sun, 16 Feb 2020 17:40:46 +0100
From:   Christian Brauner <christian.brauner@ubuntu.com>
To:     Florian Weimer <fw@deneb.enyo.de>
Cc:     =?utf-8?B?U3TDqXBoYW5l?= Graber <stgraber@ubuntu.com>,
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        smbarber@chromium.org, Seth Forshee <seth.forshee@canonical.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Serge Hallyn <serge@hallyn.com>,
        James Morris <jmorris@namei.org>,
        Kees Cook <keescook@chromium.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Phil Estes <estesp@gmail.com>, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org,
        containers@lists.linux-foundation.org,
        linux-security-module@vger.kernel.org, linux-api@vger.kernel.org
Subject: Re: [PATCH v2 00/28] user_namespace: introduce fsid mappings
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On Sun, Feb 16, 2020 at 04:55:49PM +0100, Florian Weimer wrote:
> * Christian Brauner:
> 
> > With fsid mappings we can solve this by writing an id mapping of 0
> > 100000 100000 and an fsid mapping of 0 300000 100000. On filesystem
> > access the kernel will now lookup the mapping for 300000 in the fsid
> > mapping tables of the user namespace. And since such a mapping exists,
> > the corresponding files will have correct ownership.
> 
> I'm worried that this is a bit of a management nightmare because the
> data about the mapping does not live within the file system (it's
> externally determined, static, but crucial to the interpretation of
> file system content).  I expect that many organizations have

Iiuc, that's already the case with user namespaces right now e.g. when
you have an on-disk mapping that doesn't match your user namespace
mapping.

> centralized allocation of user IDs, but centralized allocation of the
> static mapping does not appear feasible.

I thought we're working on this right now with the new nss
infrastructure to register id mappings aka the shadow discussion we've
been having.

> 
> Have you considered a more complex design, where untranslated nested
> user IDs are store in a file attribute (or something like that)?  This

That doesn't sound like it would be feasible especially in the nesting
case wrt. to performance.

Christian