netdev - Re: [PATCH net] net: bridge: locally receive all multicast packets if IFF

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Message-ID: <7932cd23-571e-4646-b5dd-467ec8106695@nbd.name>
Date: Mon, 17 Feb 2025 13:58:44 +0100
From: Felix Fietkau <nbd@....name>
To: Nikolay Aleksandrov <razor@...ckwall.org>, netdev@...r.kernel.org,
 Roopa Prabhu <roopa@...dia.com>, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>
Cc: bridge@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net: bridge: locally receive all multicast packets if
 IFF_ALLMULTI is set

On 17.02.25 12:54, Nikolay Aleksandrov wrote:
> On 2/17/25 13:26, Felix Fietkau wrote:
>> If multicast snooping is enabled, multicast packets may not always end up on
>> the local bridge interface, if the host is not a member of the multicast
>> group. Similar to how IFF_PROMISC allows all packets to be received locally,
>> let IFF_ALLMULTI allow all multicast packets to be received.
>> 
>> Signed-off-by: Felix Fietkau <nbd@....name>
>> ---
>>  net/bridge/br_input.c | 2 ++
>>  1 file changed, 2 insertions(+)
>> 
>> diff --git a/net/bridge/br_input.c b/net/bridge/br_input.c
>> index 232133a0fd21..7fa2da6985b5 100644
>> --- a/net/bridge/br_input.c
>> +++ b/net/bridge/br_input.c
>> @@ -155,6 +155,8 @@ int br_handle_frame_finish(struct net *net, struct sock *sk, struct sk_buff *skb
>>  			pkt_type = BR_PKT_MULTICAST;
>>  			if (br_multicast_rcv(&brmctx, &pmctx, vlan, skb, vid))
>>  				goto drop;
>> +			if (br->dev->flags & IFF_ALLMULTI)
>> +				local_rcv = true;
>>  		}
>>  	}
>>  
> 
> This doesn't look like a bug fix, IMO it should be for net-next.
> 
> Also you might miss a mcast stat increase, see the multicast code
> below, the only case that this would cover is the missing "else"
> branch of:
>                         if ((mdst && mdst->host_joined) ||
>                              br_multicast_is_router(brmctx, skb)) {
>                                  local_rcv = true;
>                                  DEV_STATS_INC(br->dev, multicast);
>                          }
> 
> So I'd suggest to augment the condition and include this ALLMULTI check there,
> maybe with a comment to mention that all other cases are covered by the current
> code so people are not surprised.
Will do, thanks.

> By the way what is the motivation for supporting this flag? I mean you can
> make the bridge mcast router and it will receive all mcast anyway.

OpenWrt uses a user space daemon for DHCPv6/RA/NDP handling, and in 
relay mode it sets the ALLMULTI flag in order to receive all relevant 
queries on the network.
This works for normal network interfaces and non-snooping bridges, but 
not snooping bridges (unless, as you pointed out, multicast routing is 
enabled).

- Felix