Re: [PATCH V4 3/8] namespaces: expose ns instance serial numbers in proc

On Thu, Aug 21, 2014 at 6:58 PM, Richard Guy Briggs <rgb(a)redhat.com&gt; wrote: > On 14/08/21, Andy Lutomirski wrote: >> On Aug 20, 2014 8:12 PM, "Richard Guy Briggs" <rgb(a)redhat.com&gt; wrote: >>> Expose the namespace instace serial numbers in the proc filesystem at >>> /proc/<pid>/ns/<ns>_snum. The link text gives the serial number in hex. >> >> What's the use case? >> >> I understand the utility of giving unique numbers to the audit code, >> but I don't think this part is necessary for that, and I'd like to >> understand what else will use this before committing to a duplicative >> API like this. > > How does a container manager get those numbers? It could provoke a task > to cause an audit event that emits a NS_INFO message, or it could run a > task in that container to report its namespace serial numbers directly > from its /proc mount. Why does a container manager need them? Is there any reason that keeping them entirely contained within the audit system would be a problem? > > The discussion in this thread touches on the use cases: > https://lkml.org/lkml/2014/4/22/662 > >> Note that this API is thoroughly incompatible with CRIU. If we do >> this, someone will ask for a namespace number namespace, and that way >> lies madness. > > I had a very brief look at CRIU, but not enough to understand the issue. > Others have hinted at this problem. > > Do you have a suggestion of a different approach that would be > compatible with CRIU? > > I'd originally considered some sort of UUID that would be globally > unique, but that would be very hard to devise or guarantee, and besides, > namespaces aren't only used by containers and could be shared in other > ways. Tracking the usage and migration of namespaces should be the task > of an upper layer. > CRIU wants to save the complete state of a namespace and then restore it. For that to work, any information exposed to things in the namespace *cannot* be globally unique or unique per boot, since CRIU needs to arrange for that information to match whatever it was when CRIU saved it.

Also, I think that code running in a namespace has no business even knowing a unique identity of that namespace from the perspective of the host.