netdev - Re: [PATCH 1/2] openvswitch: fix the calculation of checksum for vlan header

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Date:	Fri, 22 Feb 2013 17:04:20 -0800
From:	Jesse Gross <jesse@...ira.com>
To:	Cong Wang <amwang@...hat.com>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 1/2] openvswitch: fix the calculation of checksum for vlan header

On Fri, Feb 22, 2013 at 1:32 AM, Cong Wang <amwang@...hat.com> wrote:
> From: Cong Wang <amwang@...hat.com>
>
> In vlan_insert_tag(), we insert a 4-byte VLAN header _after_
> mac header:
>
>         memmove(skb->data, skb->data + VLAN_HLEN, 2 * ETH_ALEN);
>         ...
>         veth->h_vlan_proto = htons(ETH_P_8021Q);
>         ...
>         veth->h_vlan_TCI = htons(vlan_tci);
>
> so after it, we should recompute the checksum to include these 4 bytes.
> skb->data still points to the mac header, therefore VLAN header is at
> (2 * ETH_ALEN = 12) bytes after it, not (ETH_HLEN = 14) bytes.
>
> This can also be observed via tcpdump:
>
>          0x0000:  ffff ffff ffff 5254 005d 6f6e 8100 000a
>          0x0010:  0806 0001 0800 0604 0001 5254 005d 6f6e
>          0x0020:  c0a8 026e 0000 0000 0000 c0a8 0282
>
> Similar for __pop_vlan_tci(), the vlan header we remove is the one
> overwritten in:
>
>         memmove(skb->data + VLAN_HLEN, skb->data, 2 * ETH_ALEN);
>
> Therefore the VLAN_HLEN = 4 bytes after 2 * ETH_ALEN is the part
> we want to sub from checksum.
>
> Cc: David S. Miller <davem@...emloft.net>
> Cc: Jesse Gross <jesse@...ira.com>
> Signed-off-by: Cong Wang <amwang@...hat.com>

Applied, thanks.
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