BUG?: receiving on a packet socket with .sll_protocoll and bridging

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

Hello,

my eventual goal is to implement MRP and for that I started to program a
bit and stumbled over a problem I don't understand.

For testing purposes I created a veth device pair (veth0 veth1), open a
socket for each of the devices and send packets around between them. In
tcpdump a typical package looks as follows:

10:36:34.755208 ae:a9:da:50:48:db (oui Unknown) > 01:15:e4:00:00:01 (oui Unknown), ethertype Unknown (0x88e3), length 58: 
	0x0000:  0001 0212 8000 aea9 da50 48db 0000 0000  .........PH.....
	0x0010:  0000 0589 40f2 6574 6800 0000 0000 0000  ....@.eth.......
	0x0020:  0000 0100 0a80 3d38 4c5e 0000            ......=8L^..

The socket to receive these packages is opened using:

	#define ETH_P_MRP 0x88e3

	struct sockaddr_ll sa_ll = {
		.sll_family = AF_PACKET,
		.sll_protocol = htons(ETH_P_MRP),
		.sll_ifindex = if_nametoindex("veth0")
	};

	fd = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_MRP));
	bind(fd, (struct sockaddr *)&sa_ll, sizeof(sa_ll));

So far everything works fine and I can receive the packets I send.

If now I add veth0 to a bridge (e.g.

	ip link add br0 type bridge
	ip link set dev veth0 master br0

) and continue to send on veth1 and receive on veth0 I don't receive
the packets any more. The other direction (veth0 sending, veth1
receiving) still works fine. Each of the following changes allow to
receive again:

 a) take veth0 out of the bridge
 b) bind(2) the receiving socket to br0 instead of veth0
 c) use .sll_protocol = htons(ETH_P_ALL) for bind(2)

In the end only c) could be sensible (because I need to know the port
the packet entered the stack and that might well be bridged), but I
wonder why .sll_protocol = htons(ETH_P_MRP) seems to do the right thing
for an unbridged device but not for a bridged one.

Is this a bug or a feature I don't understand?

If someone wants to reproduce this locally, I can simplify my program
and provide it here. I tested this on a Debian 4.15 kernel (x86), but
also with the same symptoms on an arm64 with 4.16 and a dsa switch.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |