netdev - Re: [PATCH net-next 1/1] rtnetlink: request RTM

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Date:   Tue, 23 Jan 2018 17:50:31 +0100
From:   Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:     Wolfgang Bumiller <w.bumiller@...xmox.com>,
        Jiri Benc <jbenc@...hat.com>
Cc:     Christian Brauner <christian.brauner@...onical.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        davem@...emloft.net, dsahern@...il.com, fw@...len.de,
        daniel@...earbox.net, lucien.xin@...il.com,
        mschiffer@...verse-factory.net, jakub.kicinski@...ronome.com,
        vyasevich@...il.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, stephen@...workplumber.org
Subject: Re: [PATCH net-next 1/1] rtnetlink: request RTM_GETLINK by pid or fd

Le 23/01/2018 à 11:26, Wolfgang Bumiller a écrit :
> On Tue, Jan 23, 2018 at 10:30:09AM +0100, Jiri Benc wrote:
>> On Mon, 22 Jan 2018 23:25:41 +0100, Christian Brauner wrote:
>>> This is not necessarily true in scenarios where I move a network device
>>> via RTM_NEWLINK + IFLA_NET_NS_PID into a network namespace I haven't
>>> created. Here is an example:
>>>
>>> nlmsghdr->nlmsg_flags = NLM_F_REQUEST | NLM_F_ACK;
>>> nlmsghdr->nlmsg_type = RTM_NEWLINK;
>>> /* move to network namespace of pid */
>>> nla_put_u32(nlmsg, IFLA_NET_NS_PID, pid)
>>> /* give interface new name */
>>> nla_put_string(nlmsg, IFLA_IFNAME, ifname)
>>>
>>> The only thing I have is the pid that identifies the network namespace.
>>
>> How do you know the interface did not get renamed in the new netns?
>>
>> This is racy and won't work reliably. You really need to know the
>> netnsid before moving the interface to the netns to be able to do
>> meaningful queries.
> 
> Even if you know the netnsid, do the mentioned watches work for
> nested/child namespaces if eg. a container creates new namespace before
> and/or after the watch was established and moves interfaces to these
> child namespaces, would you just see them disappear, or can you keep
> track of them later on as well?
nsid can be monitored (see ip monitor nsid).

> 
> Even if that works, from what the documentation tells me netlink is an
> unreliable protocol, so if my watcher's socket buffer is full, wouldn't
> I be losing important tracking information?
You can track socket error statistics. In case of error, you can start a dump to
ensure that you have the right view of the system.

> 
> I think one possible solution to tracking interfaces would be to have a
> unique identifier that never changes (even if it's just a simple
> uint64_t incremented whenever an interface is created). But since
> they're not local to the current namespace that may require a lot of
> extra permission checks (but I'm just speculating here...).
It's not possible to have unique identifiers. With CRIU, you need to be able to
reassign all existing ids.