linux-kernel - Re: [PATCH v4 0/2] Add further ioctl() operations for namespace discovery

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Date:   Wed, 25 Jan 2017 15:28:27 +1300
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     "Michael Kerrisk \(man-pages\)" <mtk.manpages@...il.com>
Cc:     "Serge E. Hallyn" <serge@...lyn.com>,
        Linux API <linux-api@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        "linux-fsdevel\@vger.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Andrey Vagin <avagin@...nvz.org>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        "W. Trevor King" <wking@...mily.us>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v4 0/2] Add further ioctl() operations for namespace discovery

ebiederm@...ssion.com (Eric W. Biederman) writes:

> "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> writes:
>
>> Hi Eric,
>>
>> On 25 January 2017 at 14:58, Eric W. Biederman <ebiederm@...ssion.com> wrote:
>>> "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> writes:
>>>
>>>> I would like to write code that discovers the namespace setup on a live
>>>> system.  The NS_GET_PARENT and NS_GET_USERNS ioctl() operations added in
>>>> Linux 4.9 provide much of what I want, but there are still a couple of
>>>> small pieces missing. Those pieces are added with this patch series.
>>>
>>> So it looks like the -EOVERFLOW change broke your example program.
>>> Causing it to abort if -EOVERFLOW is hit.  Do we really want to return
>>> -EOVERFLOW?  Or do you want to fix your program?
>>
>> Bother! Yes, I should have kept the example program in sync. (I
>> overlooked that it was not any more in sync.)
>>
>> So, I want to make sure I understand correctly, before I aswer your
>> question. Suppose we have
>>
>> 1. Outer namespace owned by UID 0
>> 2. Inner namespace owned by UID 1000
>> 3. A UID mapping in the inner namespace that maps '0 1000 1'
>> 4. A processs, X, in the outer namespace with UID 0 (and all caps).
>>
>> That's the case you're meaning, right?
>
> I think so I just noticed you did not handle -EOVERFLOW in
> the one NS_GET_OWNER_UID call.
>
>> So, UID 0 doesn't have a
>> mapping into the inner namespace, but does have all capabilities in
>> that inner namespace, right?
>
> That is correct.

My concern is that the difference between returning -EOVERFLOW and
overflow_uid is primarily about usability.  If you haven't played with
the usability I don't trust that we have made the proper trade off.


Eric