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

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Date:   Fri, 6 May 2022 08:05:40 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     netdev <netdev@...r.kernel.org>,
        Nikolay Aleksandrov <razor@...ckwall.org>,
        Ido Schimmel <idosch@...lanox.com>,
        Vladimir Oltean <vladimir.oltean@....com>,
        bridge@...ts.linux-foundation.org
Subject: Re: [PATCH RFC] net: bridge: Clear offload_fwd_mark when passing
 frame up bridge interface.

On Fri, 6 May 2022 03:18:22 +0200
Andrew Lunn <andrew@...n.ch> wrote:

> 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@...n.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@...n.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

The problem is that doing this kind of nested bridging screws up
Spanning Tree.