[PATCH net-next 9/9] net: dsa: tag_ocelot: call only the relevant portion of __skb_vlan_pop() on TX

[Vladimir Oltean]

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
$ # needed to work around an apparent DSA RX filtering bug
$ ip link set $eth promisc on

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@xxxxxxx>
---
 include/linux/if_vlan.h | 20 ++++++++++++++++++++
 net/core/skbuff.c       |  8 +-------
 net/dsa/tag_ocelot.c    |  2 +-
 3 files changed, 22 insertions(+), 8 deletions(-)

diff --git a/include/linux/if_vlan.h b/include/linux/if_vlan.h
index 3698f2b391cd..4d54814143a8 100644
--- a/include/linux/if_vlan.h
+++ b/include/linux/if_vlan.h
@@ -685,6 +685,26 @@ static inline void vlan_set_encap_proto(struct sk_buff *skb,
 		skb->protocol = htons(ETH_P_802_2);
 }
 
+/**
+ * vlan_remove_tag - remove outer VLAN tag from payload
+ * @skb: skbuff to remove tag from
+ *
+ * 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
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 050a875d09c5..0a7c058d4849 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -5939,7 +5939,6 @@ EXPORT_SYMBOL(skb_ensure_writable);
  */
 int __skb_vlan_pop(struct sk_buff *skb, u16 *vlan_tci)
 {
-	struct vlan_hdr *vhdr;
 	int offset = skb->data - skb_mac_header(skb);
 	int err;
 
@@ -5955,13 +5954,8 @@ int __skb_vlan_pop(struct sk_buff *skb, u16 *vlan_tci)
 
 	skb_postpull_rcsum(skb, skb->data + (2 * ETH_ALEN), VLAN_HLEN);
 
-	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);
-	__skb_pull(skb, VLAN_HLEN);
+	vlan_remove_tag(skb, vlan_tci);
 
-	vlan_set_encap_proto(skb, vhdr);
 	skb->mac_header += VLAN_HLEN;
 
 	if (skb_network_offset(skb) < ETH_HLEN)
diff --git a/net/dsa/tag_ocelot.c b/net/dsa/tag_ocelot.c
index 73ee09de1a3a..20bf7074d5a6 100644
--- a/net/dsa/tag_ocelot.c
+++ b/net/dsa/tag_ocelot.c
@@ -30,7 +30,7 @@ static void ocelot_xmit_get_vlan_info(struct sk_buff *skb, struct dsa_port *dp,
 	br_vlan_get_proto(br, &proto);
 
 	if (ntohs(hdr->h_vlan_proto) == proto) {
-		__skb_vlan_pop(skb, &tci);
+		vlan_remove_tag(skb, &tci);
 		*vlan_tci = tci;
 	} else {
 		rcu_read_lock();
-- 
2.34.1