Re: [PATCH net v2] net: usb: lan78xx: disable VLAN filter in promiscuous mode

[Nicolai Buchwitz <nb@tipi-net.de>]

Hi Enrico

On 1.7.2026 15:09, Enrico Pozzobon via B4 Relay wrote:
> From: Enrico Pozzobon <enrico.pozzobon@dissecto.com>
> 
> The hardware VLAN filter (RFE_CTL_VLAN_FILTER_) drops VLAN-tagged 
> frames
> whose VID has not been registered via lan78xx_vlan_rx_add_vid(). It is
> left enabled in promiscuous mode, so packet capture (e.g. tcpdump or
> Wireshark) does not see tagged frames for unregistered VIDs.
> 
> Clear the filter while the interface is promiscuous and restore it from
> NETIF_F_HW_VLAN_CTAG_FILTER otherwise. Enforce the same condition in
> lan78xx_set_features() so netdev_update_features() cannot re-enable the
> filter while promiscuous.
> 
> Fixes: 55d7de9de6c3 ("Microchip's LAN7800 family USB 2/3 to 10/100/1000 
> Ethernet device driver")
> Signed-off-by: Enrico Pozzobon <enrico.pozzobon@dissecto.com>
> ---
> Currently, on microchip lan7801, enabling promiscuous mode does not
> result in VLAN tagged packets being captured. This patch fixes this,
> forcing the RFE_CTL_VLAN_FILTER_ flag to be off when promiscuous mode 
> is
> enabled.
> ---
> Changes in v2:
> - moved VLAN filter logic into lan78xx_update_vlan_filter()
> - Link to v1: 
> https://patch.msgid.link/20260630-lan78xx-vlan-promisc-v1-1-fbf0f903bd8f@dissecto.com
> 
> To: Thangaraj Samynathan <Thangaraj.S@microchip.com>
> To: Rengarajan Sundararajan <Rengarajan.S@microchip.com>
> To: UNGLinuxDriver@microchip.com
> To: Andrew Lunn <andrew+netdev@lunn.ch>
> To: "David S. Miller" <davem@davemloft.net>
> To: Eric Dumazet <edumazet@google.com>
> To: Jakub Kicinski <kuba@kernel.org>
> To: Paolo Abeni <pabeni@redhat.com>
> To: Woojung.Huh@microchip.com
> Cc: netdev@vger.kernel.org
> Cc: linux-usb@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> ---
>  drivers/net/usb/lan78xx.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/usb/lan78xx.c b/drivers/net/usb/lan78xx.c
> index c4cebacabcb5..cb782d81d84f 100644
> --- a/drivers/net/usb/lan78xx.c
> +++ b/drivers/net/usb/lan78xx.c
> @@ -1499,6 +1499,17 @@ static void 
> lan78xx_deferred_multicast_write(struct work_struct *param)
>  	return;
>  }
> 
> +static void lan78xx_update_vlan_filter(struct lan78xx_priv *pdata,
> +				       struct net_device *netdev,
> +				       netdev_features_t features)
> +{
> +	if ((features & NETIF_F_HW_VLAN_CTAG_FILTER) &&
> +	    !(netdev->flags & IFF_PROMISC))
> +		pdata->rfe_ctl |= RFE_CTL_VLAN_FILTER_;
> +	else
> +		pdata->rfe_ctl &= ~RFE_CTL_VLAN_FILTER_;
> +}
> +
>  static void lan78xx_set_multicast(struct net_device *netdev)
>  {
>  	struct lan78xx_net *dev = netdev_priv(netdev);
> @@ -1533,6 +1544,8 @@ static void lan78xx_set_multicast(struct 
> net_device *netdev)
>  		}
>  	}
> 
> +	lan78xx_update_vlan_filter(pdata, dev->net, dev->net->features);
> +
>  	if (netdev_mc_count(dev->net)) {
>  		struct netdev_hw_addr *ha;
>  		int i;
> @@ -3074,10 +3087,7 @@ static int lan78xx_set_features(struct 
> net_device *netdev,
>  	else
>  		pdata->rfe_ctl &= ~RFE_CTL_VLAN_STRIP_;
> 
> -	if (features & NETIF_F_HW_VLAN_CTAG_FILTER)
> -		pdata->rfe_ctl |= RFE_CTL_VLAN_FILTER_;
> -	else
> -		pdata->rfe_ctl &= ~RFE_CTL_VLAN_FILTER_;
> +	lan78xx_update_vlan_filter(pdata, netdev, features);
> 
>  	spin_unlock_irqrestore(&pdata->rfe_ctl_lock, flags);
> 
> 
> ---
> base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
> change-id: 20260623-lan78xx-vlan-promisc-83af8a48a7ec
> 
> Best regards,
> --
> Enrico Pozzobon <enrico.pozzobon@dissecto.com>

AFAIU the findings from Sashiko [1] on v1 are either false positives or 
pre-existing
issues unrelated to this change, so LGTM.

@Thangaraj / maintainers:
The mails to Rengarajan are bouncing since a few series - does the 
MAINTAINERS entry need an update?

Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>

[1] 
https://sashiko.dev/#/patchset/20260630-lan78xx-vlan-promisc-v1-1-fbf0f903bd8f%40dissecto.com

Thanks
Nicolai