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

[Michael Kerrisk (man-pages)]

Hi Eric,

On 07/25/2016 03:18 PM, Eric W. Biederman wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages at gmail.com> writes:
>
>> Hi Andrey,
>>
>> On 07/22/2016 08:25 PM, Andrey Vagin wrote:
>>> 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.
>>
>> For each of the above, I think it is worth mentioning that the
>> close-on-exec flag is set for the returned file descriptor.
>
> Hmm.  That is an odd default.

Why do you say that? It's pretty common as the default for various
APIs that create new FDs these days. (There's of course a strong argument
that the original UNIX default was a design blunder...)

>>>>>
>>>>> 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.
>>
>> Perhaps add "and the caller does not have CAP_SYS_ADMIN" in the initial
>> user namespace"?
>
> Having looked at that bit of code I don't think capabilities really
> have a role to play.

Yes, I caught up with that now. I await to see how this plays out
in the next patch version.

>>>>> 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;
>>> }
>>
>> So, from my point of view, the important piece that was missing from
>> your commit message was the note to use readlink("/proc/self/fd/%d")
>> on the returned FDs. I think that detail needs to be part of the
>> commit message (and also the man page text). I think it even be
>> helpful to include the above program as part of the commit message:
>> it helps people more quickly grasp the API.
>
> Please, please make the standard way to compare these things fstat.
> That is much less magic than a symlink, and a little more future proof.
> Possibly even kcmp.

As in fstat() to get the st_ino field, right?

Cheers,

Michael

> At some point we will care about migrating a migrating sub-container and we
> may have to have some minor changes.
>
> Eric
>


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/