linux-kernel - Re: [PATCH v2 00/28] user_namespace: introduce fsid mappings

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Message-ID: <1581973568.24289.6.camel@HansenPartnership.com>
Date:   Mon, 17 Feb 2020 13:06:08 -0800
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Christian Brauner <christian.brauner@...ntu.com>,
        Stéphane Graber <stgraber@...ntu.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Aleksa Sarai <cyphar@...har.com>, Jann Horn <jannh@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        containers@...ts.linux-foundation.org, smbarber@...omium.org,
        Seth Forshee <seth.forshee@...onical.com>,
        linux-security-module@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Alexey Dobriyan <adobriyan@...il.com>
Subject: Re: [PATCH v2 00/28] user_namespace: introduce fsid mappings

On Fri, 2020-02-14 at 19:35 +0100, Christian Brauner wrote:
[...]
> People not as familiar with user namespaces might not be aware that
> fsid mappings already exist. Right now, fsid mappings are always
> identical to id mappings. Specifically, the kernel will lookup fsuids
> in the uid mappings and fsgids in the gid mappings of the relevant
> user namespace.

This isn't actually entirely true: today we have the superblock user
namespace, which can be used for fsid remapping on filesystems that
support it (currently f2fs and fuse).  Since this is a single shift,
how is it going to play with s_user_ns?  Do you have to understand the
superblock mapping to use this shift, or are we simply using this to
replace s_user_ns?

James