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

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

On Sat, May 5, 2018 at 10:57 AM, Uwe Kleine-König
<u.kleine-koenig@pengutronix.de> wrote:
> 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?

Packets are redirected to the bridge device in __netif_receive_skb_core
at the rx_handler hook.

This happens after packets are passed to packet types attached to
list ptype_all, which includes packet sockets with protocol ETH_P_ALL.
But before packets are passed to protocol specific packet types (and
sockets) attached to ptype_base[].