On 12/12/2016 07:18 PM, Eric W. Biederman wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx> writes:
>
>> On 12/11/2016 11:30 PM, Eric W. Biederman wrote:
>>> "Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx> writes:
>>>
>>>> [was: [PATCH 0/4 v3] Add an interface to discover relationships
>>>> between namespaces]
>>>
>>> One small comment below.
>>>
>>>>
>>>> Introspecting namespace relationships
>>>> Since Linux 4.9, two ioctl(2) operations are provided to allow
>>>> introspection of namespace relationships (see user_namespaces(7)
>>>> and pid_namespaces(7)). The form of the calls is:
>>>>
>>>> ioctl(fd, request);
>>>>
>>>> In each case, fd refers to a /proc/[pid]/ns/* file.
>>>>
>>>> NS_GET_USERNS
>>>> Returns a file descriptor that refers to the owning user
>>>> namespace for the namespace referred to by fd.
>>>>
>>>> NS_GET_PARENT
>>>> Returns a file descriptor that refers to the parent names‐
>>>> pace of the namespace referred to by fd. This operation is
>>>> valid only for hierarchical namespaces (i.e., PID and user
>>>> namespaces). For user namespaces, NS_GET_PARENT is synony‐
>>>> mous with NS_GET_USERNS.
>>>>
>>>> In each case, the returned file descriptor is opened with O_RDONLY
>>>> and O_CLOEXEC (close-on-exec).
>>>>
>>>> By applying fstat(2) to the returned file descriptor, one obtains
>>>> a stat structure whose st_ino (inode number) field identifies the
>>>> owning/parent namespace. This inode number can be matched with
>>>> the inode number of another /proc/[pid]/ns/{pid,user} file to
>>>> determine whether that is the owning/parent namespace.
>>>
>>> Like all fstat inode comparisons to be fully accurate you need to
>>> compare both the st_ino and st_dev. I reserve the right for st_dev to
>>> be significant when comparing namespaces. Otherwise I might have to
>>> create a namespace of namespaces someday and that is ugly.
>>>
>>>> Either of these ioctl(2) operations can fail with the following
>>>> error:
>>>>
>>>> EPERM The requested namespace is outside of the caller's names‐
>>>> pace scope. This error can occur if, for example, the own‐
>>>> ing user namespace is an ancestor of the caller's current
>>>> user namespace. It can also occur on attempts to obtain
>>>> the parent of the initial user or PID namespace.
>>>>
>>>> Additionally, the NS_GET_PARENT operation can fail with the fol‐
>>>> lowing error:
>>>>
>>>> EINVAL fd refers to a nonhierarchical namespace.
>>>>
>>>> See the EXAMPLE section for an example of the use of these opera‐
>>>> tions.
>>
>> So, after playing with this a bit, I have a question.
>>
>> I gather that in order to, for example, elaborate the tree of user
>> namespaces on the system, one would use NS_GET_PARENT on each of
>> the /proc/*/ns/user files and match up the results. Right?
>>
>> What happens if one of the parent user namespaces contains no
>> processes? That is, the parent namespace exists by virtue of being
>> pinned because a proc/PID/ns/user file is open or bind mounted.
>> (Chrome seems to do this sort of dance with user namespaces, for
>> example.) How do we find the ancestor of *that* user namespace?
>
> What is returned from NS_GET_USERNS and NS_GET_PARENT is a file
> descriptor, that you can call NS_GET_PARENT on.
Thanks, Eric. While trying to solve the small task I set myself,
and probably confused by past discussions[1], I was overlooking
the obvious.
Cheers,
Michael
[1] https://lkml.org/lkml/2016/7/28/365
--
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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