netdev - Re: BUG?: receiving on a packet socket with .sll

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Date:   Sun, 6 May 2018 21:14:59 +0200
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>
Subject: Re: BUG?: receiving on a packet socket with .sll_protocoll and
 bridging

Hello Willem,

On Sun, May 06, 2018 at 06:58:34PM +0200, Willem de Bruijn wrote:
> On Sat, May 5, 2018 at 10:57 AM, Uwe Kleine-König
> <u.kleine-koenig@...gutronix.de> wrote:
> > 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  ....@...h.......
> >         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.

OK, thanks for finding that place. It would have taken quite some of my
time to find it.

> 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[].

Still I wonder if there is something to fix in the kernel or if this
inconsistency is intended (or at least accepted).

Best regards
Uwe

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