Re: [PATCH net] net: vlan: fix VLAN 0 refcount imbalance of toggling filtering during runtime

[dongchenchen (A)]

> On Thu, Jun 26, 2025 at 11:41:45AM +0800, dongchenchen (A) wrote:
> As I understand it, there are two issues:
>
> 1. VID 0 is deleted when it shouldn't. This leads to the crashes you
> shared.
>
> 2. VID 0 is not deleted when it should. This leads to memory leaks like
> [1] with a reproducer such as [2].
>
> AFAICT, your proposed patch solves the second issue, but only partially
> addresses the first issue. The automatic addition of VID 0 is assumed to
> be successful, but it can fail due to hardware issues or memory
> allocation failures. I realize it is not common, but it can happen. If
> you annotate __vlan_vid_add() [3] and inject failures [4], you will see
> that the crashes still happen with your patch.

Hi, Ido
Thanks for your review!

> WDYT about something like [5]? Basically, noting in the VLAN info

This fix [5] can completely solve the problem. I will send it together 
with selftest patch.

> whether VID 0 was automatically added upon NETDEV_UP and based on that
> decide whether it should be deleted upon NETDEV_DOWN, regardless of
> "rx-vlan-filter".

one small additional question: vlan0 will not exist if netdev set rx-vlan-filter after NETDEV_UP.
Will this cause a difference in the processing logic for 8021q packets? 
Best regards Dong Chenchen

> [2]
> #!/bin/bash
>
> ip link add bond1 up type bond mode 0
> ethtool -K bond1 rx-vlan-filter off
> ip link del dev bond1
>
> [3]
> diff --git a/net/8021q/vlan_core.c b/net/8021q/vlan_core.c
> index 9404dd551dfd..6dc25c4877f2 100644
> --- a/net/8021q/vlan_core.c
> +++ b/net/8021q/vlan_core.c
> @@ -314,6 +314,7 @@ static int __vlan_vid_add(struct vlan_info *vlan_info, __be16 proto, u16 vid,
>   	*pvid_info = vid_info;
>   	return 0;
>   }
> +ALLOW_ERROR_INJECTION(__vlan_vid_add, ERRNO);
>   
>   int vlan_vid_add(struct net_device *dev, __be16 proto, u16 vid)
>   {
>
> [4]
> #!/bin/bash
>
> echo __vlan_vid_add > /sys/kernel/debug/fail_function/inject
> printf %#x -12 > /sys/kernel/debug/fail_function/__vlan_vid_add/retval
> echo 100 > /sys/kernel/debug/fail_function/probability
> echo 1 > /sys/kernel/debug/fail_function/times
> echo 1 > /sys/kernel/debug/fail_function/verbose
> ip link add bond1 up type bond mode 0
> ip link add link bond1 name vlan0 type vlan id 0 protocol 802.1q
> ip link set dev bond1 down
> ip link del vlan0
> ip link del dev bond1
>
> [5]
> diff --git a/net/8021q/vlan.c b/net/8021q/vlan.c
> index 06908e37c3d9..9a6df8c1daf9 100644
> --- a/net/8021q/vlan.c
> +++ b/net/8021q/vlan.c
> @@ -357,6 +357,35 @@ static int __vlan_device_event(struct net_device *dev, unsigned long event)
>   	return err;
>   }
>   
> +static void vlan_vid0_add(struct net_device *dev)
> +{
> +	struct vlan_info *vlan_info;
> +	int err;
> +
> +	if (!(dev->features & NETIF_F_HW_VLAN_CTAG_FILTER))
> +		return;
> +
> +	pr_info("adding VLAN 0 to HW filter on device %s\n", dev->name);
> +
> +	err = vlan_vid_add(dev, htons(ETH_P_8021Q), 0);
> +	if (err)
> +		return;
> +
> +	vlan_info = rtnl_dereference(dev->vlan_info);
> +	vlan_info->auto_vid0 = true;
> +}
> +
> +static void vlan_vid0_del(struct net_device *dev)
> +{
> +	struct vlan_info *vlan_info = rtnl_dereference(dev->vlan_info);
> +
> +	if (!vlan_info || !vlan_info->auto_vid0)
> +		return;
> +
> +	vlan_info->auto_vid0 = false;
> +	vlan_vid_del(dev, htons(ETH_P_8021Q), 0);
> +}
> +
>   static int vlan_device_event(struct notifier_block *unused, unsigned long event,
>   			     void *ptr)
>   {
> @@ -378,15 +407,10 @@ static int vlan_device_event(struct notifier_block *unused, unsigned long event,
>   			return notifier_from_errno(err);
>   	}
>   
> -	if ((event == NETDEV_UP) &&
> -	    (dev->features & NETIF_F_HW_VLAN_CTAG_FILTER)) {
> -		pr_info("adding VLAN 0 to HW filter on device %s\n",
> -			dev->name);
> -		vlan_vid_add(dev, htons(ETH_P_8021Q), 0);
> -	}
> -	if (event == NETDEV_DOWN &&
> -	    (dev->features & NETIF_F_HW_VLAN_CTAG_FILTER))
> -		vlan_vid_del(dev, htons(ETH_P_8021Q), 0);
> +	if (event == NETDEV_UP)
> +		vlan_vid0_add(dev);
> +	else if (event == NETDEV_DOWN)
> +		vlan_vid0_del(dev);
>   
>   	vlan_info = rtnl_dereference(dev->vlan_info);
>   	if (!vlan_info)
> diff --git a/net/8021q/vlan.h b/net/8021q/vlan.h
> index 5eaf38875554..c7ffe591d593 100644
> --- a/net/8021q/vlan.h
> +++ b/net/8021q/vlan.h
> @@ -33,6 +33,7 @@ struct vlan_info {
>   	struct vlan_group	grp;
>   	struct list_head	vid_list;
>   	unsigned int		nr_vids;
> +	bool			auto_vid0;
>   	struct rcu_head		rcu;
>   };