netdev - Re: Socket receives packet to multicast group to which it was not joined since kernel 3.13.10-1

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Message-ID: <20150526174113.GA17751@sbohrermbp13-local.rgmadvisors.com>
Date:	Tue, 26 May 2015 12:41:13 -0500
From:	Shawn Bohrer <shawn.bohrer@...il.com>
To:	Oliver Graff <oliver.e.graff@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Socket receives packet to multicast group to which it was not
 joined since kernel 3.13.10-1

On Sun, May 24, 2015 at 04:55:40AM +0000, Oliver Graff wrote:
> Shawn Bohrer <shawn.bohrer <at> gmail.com> writes:
> 
> > 
> > On Tue, May 13, 2014 at 04:36:41PM -0500, Shawn Bohrer wrote:
> 
> > > If I did "break" something here it appears to be because with
> > > ip_early_demux we only call ip_check_mc_rcu() once on the initial
> > > packet, and subsequent packets destined for that socket simply need to
> > > pass the __udp_is_mcast_sock() test.  With ip_early_demux disaled we
> > > call ip_check_mc_rcu() for every packet.
> > 
> > So is the solution to effectively call ip_check_mc_rcu() inside
> > udp_v4_early_demux()?
> > 
> > --
> > Shawn
> > 
> (See earlier parts of thread here: 
> http://netdev.vger.kernel.narkive.com/mwurLsMT/socket-receives-packet-to-multicast-
> group-to-which-it-was-not-joined-since-kernel-3-13-10-1)
> 
> Was a bug report filed for this / was this fixed? 
> I looked at the diffs from the referenced commit and the current version of 
> udp_v4_early_demux and the changes appear to be 
> minimal,
> it doesn't look like ip_check_mc_rcu was added. Is that all that's needed?

Hi Oliver,

I haven't tested but looking over the code I would say that this has
not been fixed.  Is this issue impacting you?

I think calling ip_check_mc_rcu (you might need some of the code from
ip_route_input_noref too) might "fix" the issue, though I have not
tried.  My concern is that early demux is a performance optimization
and I don't know how much this would impact multicast receive
performance.

As Eric mentioned in the earlier email if you don't care about
performance you can simply disable early demux:

echo 0 >/proc/sys/net/ipv4/ip_early_demux

--
Shawn
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