Re: [PATCH net] net: dsa: fix VLAN 0 filter imbalance when toggling filtering

[Vladimir Oltean <olteanv@gmail.com>]

On Tue, Apr 22, 2025 at 08:49:13PM +0200, Jonas Gorski wrote:
> When a net device has NETIF_F_HW_VLAN_CTAG_FILTER set, the 8021q code
> will add VLAN 0 when enabling the device, and remove it on disabling it
> again.
> 
> But since we are changing NETIF_F_HW_VLAN_CTAG_FILTER during runtime in
> dsa_user_manage_vlan_filtering(), user ports that are already enabled
> may end up with no VLAN 0 configured, or VLAN 0 left configured.
> 
> E.g.the following sequence would leave sw1p1 without VLAN 0 configured:
> 
> $ ip link add br0 type bridge vlan_filtering 1
> $ ip link set br0 up
> $ ip link set sw1p1 up (filtering is 0, so no HW filter added)
> $ ip link set sw1p1 master br0 (filtering gets set to 1, but already up)
> 
> while the following sequence would work:
> 
> $ ip link add br0 type bridge vlan_filtering 1
> $ ip link set br0 up
> $ ip link set sw1p1 master br0 (filtering gets set to 1)
> $ ip link set sw1p1 up (filtering is 1, HW filter is added)
> 
> Likewise, the following sequence would leave sw1p2 with a VLAN 0 filter
> enabled on a vlan_filtering_is_global dsa switch:
> 
> $ ip link add br0 type bridge vlan_filtering 1
> $ ip link set br0 up
> $ ip link set sw1p1 master br0 (filtering set to 1 for all devices)
> $ ip link set sw1p2 up (filtering is 1, so VLAN 0 filter is added)
> $ ip link set sw1p1 nomaster (filtering is reset to 0 again)
> $ ip link set sw1p2 down (VLAN 0 filter is left configured)
> 
> This even causes untagged traffic to break on b53 after undoing the
> bridge (though this is partially caused by b53's own doing).
> 
> Fix this by emulating 8021q's vlan_device_event() behavior when changing
> the NETIF_F_HW_VLAN_CTAG_FILTER flag, including the printk, so that the
> absence of it doesn't become a red herring.
> 
> While vlan_vid_add() has a return value, vlan_device_event() does not
> check its return value, so let us do the same.
> 
> Fixes: 06cfb2df7eb0 ("net: dsa: don't advertise 'rx-vlan-filter' when not needed")
> Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
> ---

Why does the b53 driver depend on VID 0? CONFIG_VLAN_8021Q can be
disabled or be an unloaded module, how does it work in that case?