net: dsa: tag_ocelot: call only the relevant portion of __skb_vlan_pop() on TX

ocelot_xmit_get_vlan_info() calls __skb_vlan_pop() as the most
appropriate helper I could find which strips away a VLAN header.
That's all I need it to do, but __skb_vlan_pop() has more logic, which
will become incompatible with the future revert of commit 6d1ccff62780
("net: reset mac header in dev_start_xmit()").

Namely, it performs a sanity check on skb_mac_header(), which will stop
being set after the above revert, so it will return an error instead of
removing the VLAN tag.

ocelot_xmit_get_vlan_info() gets called in 2 circumstances:

(1) the port is under a VLAN-aware bridge and the bridge sends
    VLAN-tagged packets

(2) the port is under a VLAN-aware bridge and somebody else (an 8021q
    upper) sends VLAN-tagged packets (using a VID that isn't in the
    bridge vlan tables)

In case (1), there is actually no bug to defend against, because
br_dev_xmit() calls skb_reset_mac_header() and things continue to work.

However, in case (2), illustrated using the commands below, it can be
seen that our intervention is needed, since __skb_vlan_pop() complains:

$ ip link add br0 type bridge vlan_filtering 1 && ip link set br0 up
$ ip link set $eth master br0 && ip link set $eth up
$ ip link add link $eth name $eth.100 type vlan id 100 && ip link set $eth.100 up
$ ip addr add 192.168.100.1/24 dev $eth.100

I could fend off the checks in __skb_vlan_pop() with some
skb_mac_header_was_set() calls, but seeing how few callers of
__skb_vlan_pop() there are from TX paths, that seems rather
unproductive.

As an alternative solution, extract the bare minimum logic to strip a
VLAN header, and move it to a new helper named vlan_remove_tag(), close
to the definition of vlan_insert_tag(). Document it appropriately and
make ocelot_xmit_get_vlan_info() call this smaller helper instead.

Seeing that it doesn't appear illegal to test skb->protocol in the TX
path, I guess it would be a good for vlan_remove_tag() to also absorb
the vlan_set_encap_proto() function call.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 21 insertions, 0 deletions

diff --git a/include/linux/if_vlan.h b/include/linux/if_vlan.h
index 3698f2b391cd..0f40f379d75c 100644
--- a/include/linux/if_vlan.h
+++ b/include/linux/if_vlan.h
@@ -686,6 +686,27 @@ static inline void vlan_set_encap_proto(struct sk_buff *skb,
 }
 
 /**
+ * vlan_remove_tag - remove outer VLAN tag from payload
+ * @skb: skbuff to remove tag from
+ * @vlan_tci: buffer to store value
+ *
+ * Expects the skb to contain a VLAN tag in the payload, and to have skb->data
+ * pointing at the MAC header.
+ *
+ * Returns a new pointer to skb->data, or NULL on failure to pull.
+ */
+static inline void *vlan_remove_tag(struct sk_buff *skb, u16 *vlan_tci)
+{
+	struct vlan_hdr *vhdr = (struct vlan_hdr *)(skb->data + ETH_HLEN);
+
+	*vlan_tci = ntohs(vhdr->h_vlan_TCI);
+
+	memmove(skb->data + VLAN_HLEN, skb->data, 2 * ETH_ALEN);
+	vlan_set_encap_proto(skb, vhdr);
+	return __skb_pull(skb, VLAN_HLEN);
+}
+
+/**
  * skb_vlan_tagged - check if skb is vlan tagged.
  * @skb: skbuff to query
  *