Messages in this thread | | | From | (Eric W. Biederman) | Date | Mon, 25 Jul 2016 09:59:43 -0500 | Subject | Re: [PATCH 0/5 RFC] Add an interface to discover relationships between namespaces |
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"Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
> Hi Eric, > > On 07/25/2016 03:18 PM, Eric W. Biederman wrote: >> "Michael Kerrisk (man-pages)" <mtk.manpages@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@gmail.com> wrote: >>>>> Hi Andrey, >>>>> >>>>> >>>>> On 07/21/2016 11:06 PM, Andrew Vagin wrote: >>>>>> [snip] >>>>>> 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] >>> 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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