netdev - Re: [PATCH] vlan: allow VLAN ID 0 to be used

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Date:	Mon, 26 Oct 2009 17:32:32 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	benny+usenet@...rsen.dk, gertjan_hofman@...oo.com,
	mcarlson@...adcom.com, netdev@...r.kernel.org, kaber@...sh.net
Subject: Re: [PATCH] vlan: allow VLAN ID 0 to be used

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Mon, 26 Oct 2009 17:13:58 +0100

> [PATCH] vlan: allow VLAN ID 0 to be used
> 
> We currently use a 16 bit field (vlan_tci) to store VLAN ID on a skb.
> 
> 0 value is used a special value, meaning VLAN ID not set.
> This forbids use of VLAN ID 0
> 
> As VLAN ID is 12 bits, we can use high order bit as a flag, and
> allow VLAN ID 0
> 
> Reported-by: Gertjan Hofman <gertjan_hofman@...oo.com>
> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>

This is going to need some more work.

IXGBE is already using the higher bits of ->vlan_tci internally,
your change breaks that.

QLGE explicitly initializes skb->vlan_tci to zero, you'll need to make
sure that's OK.

There is an explicit "if (skb->vlan_tci" (ie. zero vs. non-zero) test
in net/core/dev.c:netif_receive_skb()

net/core/skbuff.c:__copy_skb_header() does a straight copy, you'll
need to make sure that's still OK.

net/packet/af_packet.c:tpacket_rcv() and packet_recvmsg() report the
skb->vlan_tci value to userspace, that's broken now as userspace
doesn't expect that new bit to be there.
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