net: bridge: locally receive all multicast packets if IFF_ALLMULTI is set

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.

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 multicast routing is enabled).

Reported-by: Felix Fietkau <[email protected]>
Closes: openwrt/openwrt#15857 (comment)
Signed-off-by: Shengyu Qu <[email protected]>
Reviewed-by: Ido Schimmel <[email protected]>
Acked-by: Nikolay Aleksandrov <[email protected]>
Link: https://patch.msgid.link/OSZPR01MB8434308370ACAFA90A22980798B32@OSZPR01MB8434.jpnprd01.prod.outlook.com
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Headcrabed

net/bridge/br_input.c

@@ -189,7 +189,8 @@ int br_handle_frame_finish(struct net *net, struct sock *sk, struct sk_buff *skb
 		if ((mdst || BR_INPUT_SKB_CB_MROUTERS_ONLY(skb)) &&
 		    br_multicast_querier_exists(brmctx, eth_hdr(skb), mdst)) {
 			if ((mdst && mdst->host_joined) ||
-			    br_multicast_is_router(brmctx, skb)) {
+			    br_multicast_is_router(brmctx, skb) ||
+			    br->dev->flags & IFF_ALLMULTI) {
 				local_rcv = true;
 				DEV_STATS_INC(br->dev, multicast);
 			}