MediaTek and VLAN 802.1ad on ethernet

LGA1150 · September 21, 2020, 6:55am

Probably a bug:

torvalds/linux/blob/ba4f184e126b751d1bffad5897f263108befc780/net/dsa/tag_mtk.c#L36


	unsigned char *dest = eth_hdr(skb)->h_dest;
	bool is_multicast_skb = is_multicast_ether_addr(dest) &&
				!is_broadcast_ether_addr(dest);

	/* Build the special tag after the MAC Source Address. If VLAN header
	 * is present, it's required that VLAN header and special tag is
	 * being combined. Only in this way we can allow the switch can parse
	 * the both special and VLAN tag at the same time and then look up VLAN
	 * table with VID.
	 */
	if (!skb_vlan_tagged(skb)) {
		if (skb_cow_head(skb, MTK_HDR_LEN) < 0)
			return NULL;

		skb_push(skb, MTK_HDR_LEN);
		memmove(skb->data, skb->data + MTK_HDR_LEN, 2 * ETH_ALEN);
		is_vlan_skb = false;
	}

	mtk_tag = skb->data + 2 * ETH_ALEN;

skb_vlan_tagged will return true if the frame is either 802.1ad or 802.1q tagged, but the code assumes it is always 802.1q.