Re: [PATCH net-next v11 1/9] net: dsa: add tag driver for LAN9645X

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] The tag driver drops hardware-accelerated VLAN tags on transmit, leading to switch misclassification and double tagging.
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--- Patch [1]: [PATCH net-next v11 1/9] net: dsa: add tag driver for LAN9645X ---
commit 97f24952991b201e3d0f1948d98e9534914b9db3
Author: Jens Emil Schulz Østergaard <[email protected]>

net: dsa: add tag driver for LAN9645X

This commit introduces a new DSA tag driver for the LAN9645X switch family,
using an NPI port for CPU connection. It supports prepending a 28-byte
internal frame header (IFH) on injection and handling a long prefix on
extraction.

> diff --git a/net/dsa/tag_lan9645x.c b/net/dsa/tag_lan9645x.c
> new file mode 100644
> index 0000000000000..6a33216c7792f
> --- /dev/null
> +++ b/net/dsa/tag_lan9645x.c

[ ... ]

> +static void lan9645x_xmit_get_vlan_info(struct sk_buff *skb,
> +					struct net_device *br,
> +					u32 *vlan_tci, u32 *tag_type)
> +{
> +	struct vlan_ethhdr *hdr;
> +	u16 proto, tci;
> +
> +	if (!br || !br_vlan_enabled(br)) {
> +		*vlan_tci = 0;
> +		*tag_type = LAN9645X_IFH_TAG_TYPE_C;
> +		return;
> +	}
> +
> +	hdr = (struct vlan_ethhdr *)skb_mac_header(skb);
> +	br_vlan_get_proto(br, &proto);
> +
> +	if (skb_headlen(skb) >= VLAN_ETH_HLEN &&
> +	    ntohs(hdr->h_vlan_proto) == proto) {
> +		vlan_remove_tag(skb, &tci);
> +		*vlan_tci = tci;
> +	} else {
> +		rcu_read_lock();
> +		br_vlan_get_pvid_rcu(br, &tci);
> +		rcu_read_unlock();
> +		*vlan_tci = tci;
> +	}

[Severity: High]
Does this code drop hardware-accelerated VLAN tags on transmit?

When the network stack transmits a frame through the DSA user port with a
hardware-accelerated VLAN tag, the tag is stored out-of-band in skb->vlan_tci.

By only checking the inline Ethernet header, ntohs(hdr->h_vlan_proto) will
evaluate to the inner network protocol (like IPv4) instead of ETH_P_8021Q.
This causes the check to fail and fall back to using the bridge PVID instead
of extracting the existing out-of-band VLAN tag.

Because the out-of-band tag is ignored and never removed from the socket
buffer, will the conduit driver eventually insert the original VLAN tag into
the payload, resulting in a double-tagged frame on the wire?

> +
> +	*tag_type = (proto != ETH_P_8021Q) ? LAN9645X_IFH_TAG_TYPE_S :
> +					     LAN9645X_IFH_TAG_TYPE_C;
> +}

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Sashiko AI review · https://sashiko.dev/#/patchset/20260805-dsa_lan9645x_switch_driver_base-v11-0-007ebc983a0a@microchip.com?part=1