Re: [PATCH RFC] net: bridge: Clear offload_fwd_mark when passing frame up bridge interface.

On Fri,  6 May 2022 00:59:04 +0200
Andrew Lunn [off-list ref] wrote:

It is possible to stack bridges on top of each other. Consider the
following which makes use of an Ethernet switch:

       br1
     /    \
    /      \
   /        \
 br0.11    wlan0
   |
   br0
 /  |  \
p1  p2  p3

br0 is offloaded to the switch. Above br0 is a vlan interface, for
vlan 11. This vlan interface is then a slave of br1. br1 also has
wireless interface as a slave. This setup trunks wireless lan traffic
over the copper network inside a VLAN.

A frame received on p1 which is passed up to the bridge has the
skb->offload_fwd_mark flag set to true, indicating it that the switch
has dealt with forwarding the frame out ports p2 and p3 as
needed. This flag instructs the software bridge it does not need to
pass the frame back down again. However, the flag is not getting reset
when the frame is passed upwards. As a result br1 sees the flag,
wrongly interprets it, and fails to forward the frame to wlan0.

When passing a frame upwards, clear the flag.

RFC because i don't know the bridge code well enough if this is the
correct place to do this, and if there are any side effects, could the
skb be a clone, etc.

Fixes: f1c2eddf4cb6 ("bridge: switchdev: Use an helper to clear forward mark")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>

Bridging of bridges is not supposed to be allowed.
See:

bridge:br_if.c

	/* No bridging of bridges */
	if (dev->netdev_ops->ndo_start_xmit == br_dev_xmit) {
		NL_SET_ERR_MSG(extack,
			       "Can not enslave a bridge to a bridge");
		return -ELOOP;
	}