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