Linux Ethernet Bridging - May 2022 - [Bridge] [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 <andrew at 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 at 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;
	}

On Thu, May 05, 2022 at 04:07:20PM -0700, Stephen Hemminger
wrote:
> On Fri,  6 May 2022 00:59:04 +0200
> Andrew Lunn <andrew at 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 at 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;
> 	}
This is not direct bridging of bridges. There is a vlan interface in
the middle. And even if it is not supposed to work, it does work, it
is being used, and it regressed. This fixes the regression.

   Andrew