netdev - vlan_get

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Date:	Thu, 24 Mar 2011 14:11:30 -0500
From:	Andrew Strohman <astrohman@...plexinvestments.com>
To:	netdev@...r.kernel.org
Subject: vlan_get_tag()

Hello,

Lately, I was messing around with the linux kernel code, and I wanted
to look up the Vlan ID for a given sk_buff.   Instead of manually
pulling the ID out of the sk_buff object, I used this in
linux/include/linux/if_vlan.h


/**
 327 * vlan_get_tag - get the VLAN ID from the skb
 328 * @skb: skbuff to query
 329 * @vlan_tci: buffer to store vlaue
 330 *
 331 * Returns error if the skb is not VLAN tagged
 332 */
 333static inline int vlan_get_tag(const struct sk_buff *skb, u16 *vlan_tci)
 334{
 335        if (skb->dev->features & NETIF_F_HW_VLAN_TX) {
 336                return __vlan_hwaccel_get_tag(skb, vlan_tci);
 337        } else {
 338                return __vlan_get_tag(skb, vlan_tci);
 339        }
 340}



Everything worked fine until I change the CoS value of my packets, and
then all the sudden the VLAN IDs that where returned from this
function were wrong.  I believe I see why:

/**
 306 * __vlan_hwaccel_get_tag - get the VLAN ID that is in @skb->cb[]
 307 * @skb: skbuff to query
 308 * @vlan_tci: buffer to store vlaue
 309 *
 310 * Returns error if @skb->vlan_tci is not set correctly
 311 */
 312static inline int __vlan_hwaccel_get_tag(const struct sk_buff *skb,
 313                                         u16 *vlan_tci)
 314{
 315        if (vlan_tx_tag_present(skb)) {
 316                *vlan_tci = vlan_tx_tag_get(skb);
 317                return 0;
 318        } else {
 319                *vlan_tci = 0;
 320                return -EINVAL;
 321        }
 322}


#define vlan_tx_tag_get(__skb)          ((__skb)->vlan_tci & ~VLAN_TAG_PRESENT)



#define VLAN_TAG_PRESENT        VLAN_CFI_MASK
#define VLAN_CFI_MASK           0x1000 /* Canonical Format Indicator */



So, to me it seems like this function really returns the entire tci
with the CFI bit zeroed out, not the VLAN ID.  Perhaps, instead of
doing:  ((__skb)->vlan_tci & ~VLAN_TAG_PRESENT), it should do
((__skb)->vlan_tci & VLAN_VID_MASK)?

Andy
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