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Date:	Wed, 04 Jul 2007 17:00:08 +0200
From:	Patrick McHardy <>
To:	Johannes Berg <>
CC:	netdev <>, jamal <>,
	Thomas Graf <>,
	Herbert Xu <>
Subject: Re: multicasting netlink messages to groups > 31 from userspace

Johannes Berg wrote:
> On Wed, 2007-07-04 at 16:48 +0200, Patrick McHardy wrote:
>>Yes, although in both cases you have no guarantee how long its
>>going to take, someone else could be flooding the receive queue.
>>For userspace this is more likely to be a real problem though
>>since the kernel will keep processing the queue as long as packets
>>are in it, while userspace could be scheduled away.
> Right, but in the case of wireless you'll have different problems if
> that happens, namely your wireless card won't be reassociating etc. I
> don't think it'll be a problem in practice.

Not by itself probably but a user could DoS your wireless connectivity
this way.

>>I'm not so sure myself whether netlink is really a good idea for
>>userspace<->userspace communication because of the above reason.
>>IIRC Herbert had the same doubts some time ago, I wonder what
>>made him change his mind.
> Hm. The reason I wanted it initially is that this way we can guarantee
> that userspace programs work in either case and also that we have better
> control over the various APIs.
>>There is a notifier for userspace unicast socket releases, would adding
>>another one for multicast groups help?
> Huh I think that notifier is enough in fact. It'll be called if a
> userspace process closes a socket or such, right? Might get a lot of
> events for generic netlink but that should be acceptable since it'd only
> need to check .pid to start with.

I'm not sure, it would probably also have to be called when userspace
unsubscribes from a group, no?
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