netdev - Re: [PATCHv3] bridge: disable snooping if there is no querier

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Date:	Mon, 05 Aug 2013 10:41:06 +0200
From:	Paul Bolle <pebolle@...cali.nl>
To:	David Miller <davem@...emloft.net>
Cc:	linus.luessing@....de, bridge@...ts.linux-foundation.org,
	stephen@...workplumber.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, herbert@...dor.apana.org.au,
	amwang@...hat.com, linux@...er-net.org.uk
Subject: Re: [PATCHv3] bridge: disable snooping if there is no querier

On Wed, 2013-07-31 at 17:40 -0700, David Miller wrote:
> > If there is no querier on a link then we won't get periodic reports and
> > therefore won't be able to learn about multicast listeners behind ports,
> > potentially leading to lost multicast packets, especially for multicast
> > listeners that joined before the creation of the bridge.
> > 
> > These lost multicast packets can appear since c5c23260594
> > ("bridge: Add multicast_querier toggle and disable queries by default")
> > in particular.
> > 
> > With this patch we are flooding multicast packets if our querier is
> > disabled and if we didn't detect any other querier.
> > 
> > A grace period of the Maximum Response Delay of the querier is added to
> > give multicast responses enough time to arrive and to be learned from
> > before disabling the flooding behaviour again.
> > 
> > Signed-off-by: Linus Lüssing <linus.luessing@....de>
> 
> Looks good, applied, thanks Linus.

0) This patch is part of v3.11-rc4 as commit b00589af3b0. It introduced
a GCC warning:
    net/bridge/br_multicast.c: In function ‘br_multicast_rcv’:
    net/bridge/br_multicast.c:1081:36: warning: ‘max_delay’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    net/bridge/br_multicast.c:1178:16: note: ‘max_delay’ was declared here

1) Summarized, the code reads:

        unsigned long max_delay;

        if (skb->len == sizeof(*mld))
                max_delay = msecs_to_jiffies(ntohs(mld->mld_maxdelay));
        else if (skb->len >= sizeof(*mld2q))
                max_delay = mld2q->mld2q_mrc ? MLDV2_MRC(ntohs(mld2q->mld2q_mrc)) : 1;

        br_multicast_query_received(br, port, !ipv6_addr_any(&ip6h->saddr),
                                    max_delay);
 
So GCC notices that max_delay is still uninitialized if skb->len is
neither equal to sizeof(*mld) nor equal or bigger than sizeof(*mld2q).
To me it looks GCC is right here. At least, it is not obvious that
max_delay will actually not be used in br_multicast_query_received() if
it still is uninitialized.

2) I'm entirely unfamiliar to this code. So I can't say how this warning
might be silenced.


Paul Bolle

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