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

[Andy Lutomirski]

On Thu, Aug 21, 2014 at 6:58 PM, Richard Guy Briggs <rgb at redhat.com> wrote:
> On 14/08/21, Andy Lutomirski wrote:
>> On Aug 20, 2014 8:12 PM, "Richard Guy Briggs" <rgb at redhat.com> 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.

Here's a specific use case for *not* exposing this: Tor.  Ideally, Tor
clients would run in a namespace that does not know about any global
identity.  That means no IP addresses, but it also means no global
namespace serial numbers.

--Andy