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Date:   Sat, 16 Jan 2021 01:27:18 +0100
From:   Christian Brauner <>
To:     Theodore Ts'o <>
Cc:     Christoph Hellwig <>,
        Dave Chinner <>,
        "Darrick J. Wong" <>,
        Alexander Viro <>,,
        John Johansen <>,
        James Morris <>,
        Mimi Zohar <>,
        Dmitry Kasatkin <>,
        Stephen Smalley <>,
        Casey Schaufler <>,
        Arnd Bergmann <>,
        Andreas Dilger <>,
        OGAWA Hirofumi <>,
        Geoffrey Thomas <>,
        Mrunal Patel <>,
        Josh Triplett <>,
        Andy Lutomirski <>,
        Alban Crequy <>,
        Tycho Andersen <>,
        David Howells <>,
        James Bottomley <>,
        Seth Forshee <>,
        St?phane Graber <>,
        Linus Torvalds <>,
        Aleksa Sarai <>,
        Lennart Poettering <>,
        "Eric W. Biederman" <>,,
        Phil Estes <>, Serge Hallyn <>,
        Kees Cook <>,
        Todd Kjos <>, Paul Moore <>,
        Jonathan Corbet <>,,,,,,,
Subject: Re: [PATCH v5 00/42] idmapped mounts

On Fri, Jan 15, 2021 at 12:51:20PM -0500, Theodore Ts'o wrote:
> On Fri, Jan 15, 2021 at 04:24:23PM +0000, Christoph Hellwig wrote:
> > 
> > That is what the capabilities are designed for and we already check
> > for them.
> So perhaps I'm confused, but my understanding is that in the
> containers world, capabilities are a lot more complicated.  There is:
> 1) The initial namespace capability set
> 2) The container's user-namespace capability set
> 3) The namespace in which the file system is mounted --- which is
>       "usually, but not necessarily the initial namespace" and
>       presumably could potentially not necessarily be the current
>       container's user name space, is namespaces can be hierarchically
>       arranged.
> Is that correct?  If so, how does this patch set change things (if
> any), and and how does this interact with quota administration
> operations?

The cases you listed are correct. The patchset doesn't change them.
Simply put, the patchset doesn't alter capability checking in any way.

> On a related note, ext4 specifies a "reserved user" or "reserved
> group" which can access the reserved blocks.  If we have a file system
> which is mounted in a namespace running a container which is running
> RHEL or SLES, and in that container, we have a file system mounted (so
> it was not mounted in the initial namespace), with id-mapping --- and
> then there is a further sub-container created with its own user
> sub-namespace further mapping uids/gids --- will the right thing
> happen?  For that matter, how *is* the "right thing" defined?

In short, nothing changes. Whatever happened before happens now.

Specifically s_resuid/s_resgid are superblock mount options and so never
change on a per-mount basis and thus also aren't affected by idmapped

> Sorry if this is a potentially stupid question, but I find user
> namespaces and id and capability mapping to be hopefully confusing for
> my tiny brain.  :-)

No, I really appreciate the questions. :) My brain can most likely
handle less. :)


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