linux-kernel - Re: [PATCH v3 00/25] user_namespace: introduce fsid mappings

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Message-ID: <20200219193558.GA27641@mail.hallyn.com>
Date:   Wed, 19 Feb 2020 13:35:58 -0600
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Stéphane Graber <stgraber@...ntu.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Aleksa Sarai <cyphar@...har.com>, Jann Horn <jannh@...gle.com>,
        smbarber@...omium.org, Seth Forshee <seth.forshee@...onical.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Serge Hallyn <serge@...lyn.com>,
        James Morris <jmorris@...ei.org>,
        Kees Cook <keescook@...omium.org>,
        Jonathan Corbet <corbet@....net>,
        Phil Estes <estesp@...il.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        containers@...ts.linux-foundation.org,
        linux-security-module@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH v3 00/25] user_namespace: introduce fsid mappings

On Tue, Feb 18, 2020 at 03:33:46PM +0100, Christian Brauner wrote:
> 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.

So if I have

/proc/self/uid_map: 0 100000 100000
/proc/self/fsid_map: 1000 1000 1

1. If I read files from the rootfs which have host uid 101000, they
will appear as uid 100 to me?

2. If I read host files with uid 1000, they will appear as uid 1000 to me?

3. If I create a new file, as uid 1000, what will be the inode owning uid?