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@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;
 		}
 	}
 
-- 
2.47.1

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@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

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@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