Re: Documenting the ioctl interfaces to discover relationships between namespaces

From: Michael Kerrisk (man-pages)
Date: Wed Dec 14 2016 - 03:33:36 EST

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.
>>>> Returns a file descriptor that refers to the owning user
>>>> namespace for the namespace referred to by fd.
>>>> Returns a file descriptor that refers to the parent namesâ
>>>> 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 synonyâ
>>>> 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 namesâ
>>>> pace scope. This error can occur if, for example, the ownâ
>>>> 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 folâ
>>>> lowing error:
>>>> EINVAL fd refers to a nonhierarchical namespace.
>>>> See the EXAMPLE section for an example of the use of these operaâ
>>>> 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.




Michael Kerrisk
Linux man-pages maintainer;
Linux/UNIX System Programming Training: