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

[Stephen Hemminger <stephen@networkplumber.org>]

On Fri,  6 May 2022 00:59:04 +0200
Andrew Lunn <andrew@lunn.ch> 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;
	}