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net: dsa: tag_brcm: legacy: fix untagged rx on unbridged ports for bcm63xx

The internal switch on BCM63XX SoCs will unconditionally add 802.1Q VLAN
tags on egress to CPU when 802.1Q mode is enabled. We do this
unconditionally since commit ed409f3bbaa5 ("net: dsa: b53: Configure
VLANs while not filtering").

This is fine for VLAN aware bridges, but for standalone ports and vlan
unaware bridges this means all packets are tagged with the default VID,
which is 0.

While the kernel will treat that like untagged, this can break userspace
applications processing raw packets, expecting untagged traffic, like
STP daemons.

This also breaks several bridge tests, where the tcpdump output then
does not match the expected output anymore.

Since 0 isn't a valid VID, just strip out the VLAN tag if we encounter
it, unless the priority field is set, since that would be a valid tag
again.

Fixes: 964dbf186eaa ("net: dsa: tag_brcm: add support for legacy tags")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://patch.msgid.link/20251027194621.133301-1-jonas.gorski@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

authored by

Jonas Gorski and committed by
Jakub Kicinski 3d18a84e 5a89b27a

+8 -2
+8 -2
net/dsa/tag_brcm.c
··· 224 224 { 225 225 int len = BRCM_LEG_TAG_LEN; 226 226 int source_port; 227 + __be16 *proto; 227 228 u8 *brcm_tag; 228 229 229 230 if (unlikely(!pskb_may_pull(skb, BRCM_LEG_TAG_LEN + VLAN_HLEN))) 230 231 return NULL; 231 232 232 233 brcm_tag = dsa_etype_header_pos_rx(skb); 234 + proto = (__be16 *)(brcm_tag + BRCM_LEG_TAG_LEN); 233 235 234 236 source_port = brcm_tag[5] & BRCM_LEG_PORT_ID; 235 237 ··· 239 237 if (!skb->dev) 240 238 return NULL; 241 239 242 - /* VLAN tag is added by BCM63xx internal switch */ 243 - if (netdev_uses_dsa(skb->dev)) 240 + /* The internal switch in BCM63XX SoCs always tags on egress on the CPU 241 + * port. We use VID 0 internally for untagged traffic, so strip the tag 242 + * if the TCI field is all 0, and keep it otherwise to also retain 243 + * e.g. 802.1p tagged packets. 244 + */ 245 + if (proto[0] == htons(ETH_P_8021Q) && proto[1] == 0) 244 246 len += VLAN_HLEN; 245 247 246 248 /* Remove Broadcom tag and update checksum */