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

[Serge E. Hallyn]

Quoting Michael Kerrisk (man-pages) (mtk.manpages at gmail.com):
> 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.

Thanks - that had caught my eye but I hadn't had time to look into the
justification for this.  Hiding this kind of thing indeed seems wrong to
me, unless there is a really good justification for it, i.e. a way
to use that info in an exploit.