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

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Message-ID: <37bf04ee-954e-461f-9e37-210a8c5a790a@blackwall.org>
Date: Mon, 17 Feb 2025 13:54:38 +0200
From: Nikolay Aleksandrov <razor@...ckwall.org>
To: Felix Fietkau <nbd@....name>, 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 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.

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.

Thanks,
 Nik