Subject: Re: [PATCH net-next 17/27] OVS: remove assumptions about VLAN_TAG_PRESENT bit

From: Jiri Benc <[email-masked]>
Date: Tue, 13 Dec 2016 11:40:10 +0100

On Tue, 13 Dec 2016 01:12:38 +0100 (CET), Michał Mirosław wrote:
> @@ -850,20 +848,11 @@ static int validate_vlan_from_nlattrs(const struct sw_flow_match *match,
>  		return -EINVAL;
>  	}
>  
> -	if (a[OVS_KEY_ATTR_VLAN])
> -		tci = nla_get_be16(a[OVS_KEY_ATTR_VLAN]);
> -
> -	if (!(tci & htons(VLAN_TAG_PRESENT))) {
> -		if (tci) {
> -			OVS_NLERR(log, "%s TCI does not have VLAN_TAG_PRESENT bit set.",
> -				  (inner) ? "C-VLAN" : "VLAN");
> -			return -EINVAL;
> -		} else if (nla_len(a[OVS_KEY_ATTR_ENCAP])) {
> -			/* Corner case for truncated VLAN header. */
> -			OVS_NLERR(log, "Truncated %s header has non-zero encap attribute.",
> -				  (inner) ? "C-VLAN" : "VLAN");
> -			return -EINVAL;
> -		}
> +	if (!a[OVS_KEY_ATTR_VLAN] && nla_len(a[OVS_KEY_ATTR_ENCAP])) {
> +		/* Corner case for truncated VLAN header. */
> +		OVS_NLERR(log, "Truncated %s header has non-zero encap attribute.",
> +			(inner) ? "C-VLAN" : "VLAN");
> +		return -EINVAL;

This looks wrong. The zero value of nla_get_be16(a[OVS_KEY_ATTR_VLAN])
together with empty a[OVS_KEY_ATTR_ENCAP] means truncated VLAN header
and this needs to be preserved.

In other words, you need to check also !nla_get_be16(a[OVS_KEY_ATTR_VLAN])
here.

Overall, this whole patch looks very suspicious from the very
beginning. The xors used are strong indication that something is not
right.

And indeed, you're breaking uAPI compatibility. Previously, the
VLAG_TAG_PRESENT bit set in OVS_KEY_ATTR_VLAN caused the flow to match
on packets with VLAN tag present. After this patch, it causes the flow
to match only on those packets that have a certain value of
VLAN_CFI_MASK in their VLAN tag (I haven't bother deciphering what
value is checked after all these xors, it's as well possible that it
will also match on packets _without_ any VLAN header).

You have to preserve the old meaning of VLAN_TAG_PRESENT in
OVS_KEY_ATTR_VLAN. When doing flow lookups, VLAN_TAG_PRESENT must match
on and only on packets that have VLAN tag present, irrespective of their
VLAN_CFI_MASK.

If you want to introduce support for lookups on VLAN_CFI_MASK, you'll
have to do that by introducing a new netlink attribute.

 Jiri
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