netdev - Re: multicasting netlink messages to groups

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Date:	Fri, 06 Jul 2007 12:50:54 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	hadi@...erus.ca
Cc:	netdev <netdev@...r.kernel.org>, Patrick McHardy <kaber@...sh.net>,
	Thomas Graf <tgraf@...g.ch>
Subject: Re: multicasting netlink messages to groups > 31 from userspace

On Thu, 2007-07-05 at 09:53 -0400, jamal wrote:
> This email captures the essence of the thread, so let me start here. 
> I dont know if i read well enough all the details, but i think i have a
> good grasp of the discusion.

:)

> The DoS issue is applicable IMO to any IPC. i.e
> if i have access to sending to you, i can send you many messages and
> fill up your unix socket rcvq etc.

Yeah, good point.

> - In addition to the netlink sock open/close that Patrick mentioned, you
> can listen to generic netlink events on the accounting/task netlink and
> find out when processes are created/destroyed if that is useful. I
> couldnt tell why you needed to know this...

No, I need to know if a specific socket that I was using in the kernel
to communicate with a special userspace process is still open, but the
notifier seems to address that. I haven't tested it yet.

> Yes, this is TheWayItHasBeen(tm).
> Patrick had a patch a while back to allow for subscribing to higher
> groups because bind() has a similar issue in that it allows you only to
> subscribe to the first 32. So it is logical to fix connect, and sendmsg
> in a similar fashion.

Yeah, that's the sock opt.

> Fixing connect IMO would be a good first start because if you do
> connect() to the right group(s) before sendsmg() you could send
> to the correct group.
> 
> > It would of course be possible to add a new sockopt
> > NETLINK_{SET,GET}_DST_GROUP, 
> 
> This maybe your best first starting option IMO. It will be in similar
> spirit to Patricks bind() fix.

Right.

> Iirc, i dont think we can do that anymore. But we can introduce a new
> scheme to allow for this. For example, could you not use CMSG to specify
> an explicit list of groups on sendmsg? IIRC, Patrick used CMSG to report
> on rcv which group a msg was received on. 

I'll have to look into it. Patrick suggested this as well in the reply,
but I think it's not going to be too important to me right now,
depending on the outcome in the wireless discussion.

johannes

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