[PATCH net-next] net/sched: em_meta: Fix 'meta vlan' to correctly recognize zero VID frames

[Shmulik Ladkani <shmulik.ladkani@gmail.com>]

META_COLLECTOR int_vlan_tag() assumes that if the accel tag (vlan_tci)
is zero, then no vlan accel tag is present.

This is incorrect for zero VID vlan accel packets, making the following
match fail:
  tc filter add ... basic match 'meta(vlan mask 0xfff eq 0)' ...

Apparently 'int_vlan_tag' was implemented prior VLAN_TAG_PRESENT was
introduced in 05423b2 "vlan: allow null VLAN ID to be used"
(and at time introduced, the 'vlan_tx_tag_get' call in em_meta was not
 adapted).

Fix, testing skb_vlan_tag_present instead of testing skb_vlan_tag_get's
value.

Fixes: 05423b2413 ("vlan: allow null VLAN ID to be used")
Fixes: 1a31f2042e ("netsched: Allow meta match on vlan tag on receive")

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
---
 net/sched/em_meta.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/net/sched/em_meta.c b/net/sched/em_meta.c
index a309a07ccb..41c80b6c39 100644
--- a/net/sched/em_meta.c
+++ b/net/sched/em_meta.c
@@ -176,11 +176,12 @@ META_COLLECTOR(int_vlan_tag)
 {
 	unsigned short tag;
 
-	tag = skb_vlan_tag_get(skb);
-	if (!tag && __vlan_get_tag(skb, &tag))
-		*err = -1;
-	else
+	if (skb_vlan_tag_present(skb))
+		dst->value = skb_vlan_tag_get(skb);
+	else if (!__vlan_get_tag(skb, &tag))
 		dst->value = tag;
+	else
+		*err = -1;
 }
 
 
-- 
2.7.4