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

"Michael Kerrisk (man-pages)" [off-list ref] writes:

Hi Eric,

On 07/25/2016 03:18 PM, Eric W. Biederman wrote:

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"Michael Kerrisk (man-pages)" [off-list ref] writes:

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Hi Andrey,

On 07/22/2016 08:25 PM, Andrey Vagin wrote:

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On Thu, Jul 21, 2016 at 11:48 PM, Michael Kerrisk (man-pages)
[off-list ref] wrote:

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Hi Andrey,


On 07/21/2016 11:06 PM, Andrew Vagin wrote:

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[snip]

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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...)

Interesting.  I haven't kept up on that, but it seems reasonable.

[snip]

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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?

Both the st_ino and st_dev fields.

The most likely change to support checkpoint/restart in the future is to
preserve st_ino across migrations and instantiate a different instance
of nsfs to hold the inode numbers from the previous machine.

We would need to handle the preservation carefully or else there is
a chance that two namespace file descriptors (collected from different
sources) with different st_dev and st_ino fields may actuall refer to
the same object.

Which is a long way of saying we have the st_dev field please use it,
it may matter at some point.

Eric
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