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

From: Michael Kerrisk (man-pages)
Date: Mon Jul 25 2016 - 10:46:39 EST

Hi Eric,

On 07/25/2016 03:18 PM, Eric W. Biederman wrote:
"Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx> 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@xxxxxxxxx> 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)

Hi Andrey,

On 07/14/2016 08:20 PM, Andrey Vagin wrote:


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

Since Linux 4.X, the following ioctl(2) calls are supported for namesâ
pace file descriptors. The correct syntax is:

fd = ioctl(ns_fd, ioctl_type);

where ioctl_type is one of the following:

Returns a file descriptor that refers to an owning user namesâ

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 meanâ

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

EINVAL NS_GET_PARENT was called for a nonhierarchical namespace.

EPERM The requested namespace is outside of the current namespace

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

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?



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


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