[PATCH 0/5 RFC] Add an interface to discover relationships between namespaces

[Andrey Vagin]

On Thu, Jul 21, 2016 at 11:48 PM, Michael Kerrisk (man-pages)
<mtk.manpages at gmail.com> wrote:
> Hi Andrey,
>
>
> On 07/21/2016 11:06 PM, Andrew Vagin wrote:
>>
>> On Thu, Jul 21, 2016 at 04:41:12PM +0200, Michael Kerrisk (man-pages)
>> wrote:
>>>
>>> Hi Andrey,
>>>
>>> On 07/14/2016 08:20 PM, Andrey Vagin wrote:
>>
>>
>> <snip>
>>
>>>
>>> Could you add here an of the API in detail: what do these FDs refer to,
>>> and how do you use them to solve the use case? And could you you add
>>> that info to the commit messages please.
>>
>>
>> Hi Michael,
>>
>> A patch for man-pages is attached. It adds the following text to
>> namespaces(7).
>>
>> Since  Linux 4.X, the following ioctl(2) calls are supported for names‐
>> pace file descriptors.  The correct syntax is:
>>
>>       fd = ioctl(ns_fd, ioctl_type);
>>
>> where ioctl_type is one of the following:
>>
>> NS_GET_USERNS
>>       Returns a file descriptor that refers to an owning  user  names‐
>>       pace.
>>
>> NS_GET_PARENT
>>       Returns  a  file  descriptor  that refers to a parent namespace.
>>       This ioctl(2) can be used for pid and user namespaces. For  user
>>       namespaces,  NS_GET_PARENT and NS_GET_USERNS have the same mean‐
>>       ing.
>>
>> In addition to generic ioctl(2) errors, the following specific ones can
>> occur:
>>
>> EINVAL NS_GET_PARENT was called for a nonhierarchical namespace.
>>
>> EPERM  The  requested  namespace  is  outside  of the current namespace
>>       scope.
>>
>> ENOENT ns_fd refers to the init namespace.
>
>
> Thanks for this. But still part of the question remains unanswered.
> How do we (in user-space) use the file descriptors to answer any of
> the questions that this patch series was designed to solve? (This
> info should be in the commit message and the man-pages patch.)

I'm sorry, but I am not sure that I understand what you ask.

Here are the origin questions:
Someone else then asked me a question that led me to wonder about
generally introspecting on the parental relationships between user
namespaces and the association of other namespaces types with user
namespaces. One use would be visualization, in order to understand the
running system. Another would be to answer the question I already
mentioned: what capability does process X have to perform operations
on a resource governed by namespace Y?

Here is an example which shows how we can get the owning namespace
inode number by using these ioctl-s.

$ ls -l /proc/13929/ns/pid
lrwxrwxrwx 1 root root 0 Jul 22 21:03 /proc/13929/ns/pid -> 'pid:[4026532228]'

$ ./nsowner /proc/13929/ns/pid
user:[4026532227]

The owning user namespace for pid:[4026532228] is user:[4026532227].

The nsowner  tool is cimpiled from this code:

int main(int argc, char *argv[])
{
        char buf[128], path[] = "/proc/self/fd/0123456789";
        int ns, uns, ret;

        ns = open(argv[1], O_RDONLY);
        if (ns < 0)
                return 1;

        uns = ioctl(ns, NS_GET_USERNS);
        if (uns < 0)
                return 1;

        snprintf(path, sizeof(path), "/proc/self/fd/%d", uns);
        ret = readlink(path, buf, sizeof(buf) - 1);
        if (ret < 0)
                return 1;
        buf[ret] = 0;

        printf("%s\n", buf);

        return 0;
}

Does this example answer to the origin question? If it isn't, could
you eloborate what you expect to see here.

And I wrote one more example which show all relationships between
namespaces. It enumirates all processes in a system, collects all
namespaces and determins parent and owning namespaces for each of
them, then it constructs a namespace tree and shows it.

Here is a code: https://gist.github.com/avagin/db805f95e15ffb0af7e559dbb8de4418

Here is an example of output for my test system:
[root at fc24 nsfs]# ./nstree
user:[4026531837]
 \__  mnt:[4026532203]
 \__  ipc:[4026531839]
 \__  user:[4026532224]
     \__  user:[4026532226]
         \__  user:[4026532227]
             \__  pid:[4026532228]
     \__  pid:[4026532225]
         \__  pid:[4026532228]
 \__  user:[4026532221]
     \__  pid:[4026532222]
     \__  user:[4026532223]
 \__  mnt:[4026532211]
 \__  uts:[4026531838]
 \__  cgroup:[4026531835]
 \__  pid:[4026531836]
     \__  pid:[4026532225]
         \__  pid:[4026532228]
     \__  pid:[4026532222]
 \__  mnt:[4026531857]
 \__  mnt:[4026531840]
 \__  net:[4026531957]

Thanks,
Andrew

>
> Thanks,
>
> Michael
>
>
>>>> [1] https://lkml.org/lkml/2016/7/6/158
>>>> [2] https://lkml.org/lkml/2016/7/9/101
>>>>
>>>> Cc: "Eric W. Biederman" <ebiederm at xmission.com>
>>>> Cc: James Bottomley <James.Bottomley at HansenPartnership.com>
>>>> Cc: "Michael Kerrisk (man-pages)" <mtk.manpages at gmail.com>
>>>> Cc: "W. Trevor King" <wking at tremily.us>
>>>> Cc: Alexander Viro <viro at zeniv.linux.org.uk>
>>>> Cc: Serge Hallyn <serge.hallyn at canonical.com>
>>>>
>>>> --
>>>> 2.5.5
>>>>
>>>>
>>>
>>>
>>> --
>>> Michael Kerrisk
>>> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
>>> Linux/UNIX System Programming Training: http://man7.org/training/
>
>
>
> --
> Michael Kerrisk
> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
> Linux/UNIX System Programming Training: http://man7.org/training/
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