netdev - Re: [PATCH net] net: dsa: b53: fix untagged traffic sent via cpu tagged with VID 0

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Message-ID: <20250605202043.ivkjlwtvzi6jqhqx@skbuf>
Date: Thu, 5 Jun 2025 23:20:43 +0300
From: Vladimir Oltean <olteanv@...il.com>
To: Jonas Gorski <jonas.gorski@...il.com>
Cc: Florian Fainelli <florian.fainelli@...adcom.com>,
	Andrew Lunn <andrew@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net: dsa: b53: fix untagged traffic sent via cpu
 tagged with VID 0

Hi Jonas,

On Mon, Jun 02, 2025 at 09:49:14PM +0200, Jonas Gorski wrote:
> When Linux sends out untagged traffic from a port, it will enter the CPU
> port without any VLAN tag, even if the port is a member of a vlan
> filtering bridge with a PVID egress untagged VLAN.
> 
> This makes the CPU port's PVID take effect, and the PVID's VLAN
> table entry controls if the packet will be tagged on egress.
> 
> Since commit 45e9d59d3950 ("net: dsa: b53: do not allow to configure
> VLAN 0") we remove bridged ports from VLAN 0 when joining or leaving a
> VLAN aware bridge. But we also clear the untagged bit, causing untagged
> traffic from the controller to become tagged with VID 0 (and priority
> 0).
> 
> Fix this by not touching the untagged map of VLAN 0. Additionally,
> always keep the CPU port as a member, as the untag map is only effective
> as long as there is at least one member, and we would remove it when
> bridging all ports and leaving no standalone ports.
> 
> Since Linux (and the switch) treats VLAN 0 tagged traffic like untagged,
> the actual impact of this is rather low, but this also prevented earlier
> detection of the issue.
> 
> Fixes: 45e9d59d3950 ("net: dsa: b53: do not allow to configure VLAN 0")
> Signed-off-by: Jonas Gorski <jonas.gorski@...il.com>
> ---

Reviewed-by: Vladimir Oltean <olteanv@...il.com>

However, this situation triggered a bell in my mind. But it looks like
I am about to open a can of worms, so I don't expect that to gate this
patch.

In the configuration below, what will the packet look like on the wire?

$ ip link add br0 type bridge vlan_filtering 1 && ip link set br0 up
$ ip link set lan1 master br0 && ip link set lan1 up
$ bridge vlan add dev br0 vid 2 pvid untagged self
$ bridge vlan add dev lan1 vid 2
$ mausezahn br0 -t ip -b 00:01:02:03:04:05 -c 1 -p 1480

Testing both on sja1105, as well as on veth using software bridging, the
answer should be "tagged with VID 2". However, you make it sounds like
the answer on b53 is "untagged".

Depending on the answer, I can try to make some suggestions what to do
about this.