netdev - Re: [PATCH net 1/2] net: dev_queue_xmit_nit: fix skb->vlan

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Date:	Tue, 8 Jan 2013 21:14:50 +0100
From:	Jiri Pirko <jiri@...nulli.us>
To:	Daniel Borkmann <dborkman@...hat.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Ani Sinha <ani@...stanetworks.com>,
	Jiri Pirko <jpirko@...hat.com>
Subject: Re: [PATCH net 1/2] net: dev_queue_xmit_nit: fix skb->vlan_tci field
 value

Tue, Jan 08, 2013 at 07:51:32PM CET, dborkman@...hat.com wrote:
>VLAN packets that are locally injected through taps will loose their
>skb->vlan_tci value when they pass dev_hard_start_xmit and get looped
>back to a packet sniffer via dev_queue_xmit_nit. Besides others, this
>meta data is used in Linux socket filtering for VLANs. Tested with a
>VLAN ancillary ops filter.
>
>Patch is based on a previous version by Jiri Pirko.
>
>Cc: Eric Dumazet <eric.dumazet@...il.com>
>Cc: Ani Sinha <ani@...stanetworks.com>
>Cc: Jiri Pirko <jpirko@...hat.com>
>Reported-by: Paul Pearce <pearce@...berkeley.edu>
>Signed-off-by: Daniel Borkmann <dborkman@...hat.com>
>---
> net/core/dev.c | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
>
>diff --git a/net/core/dev.c b/net/core/dev.c
>index 515473e..723dcd0 100644
>--- a/net/core/dev.c
>+++ b/net/core/dev.c
>@@ -1775,6 +1775,19 @@ static void dev_queue_xmit_nit(struct sk_buff *skb, struct net_device *dev)
> 	struct packet_type *ptype;
> 	struct sk_buff *skb2 = NULL;
> 	struct packet_type *pt_prev = NULL;
>+	struct ethhdr *ehdr;
>+
>+	/* Network taps could make use of skb->vlan_tci, which got wiped
>+	 * out. Hence, we need to reset it correctly.
>+	 */
>+	skb_reset_mac_header(skb);
>+	ehdr = eth_hdr(skb);
>+
>+	if (ehdr->h_proto == __constant_htons(ETH_P_8021Q)) {
>+		skb2 = vlan_untag(skb);
>+		if (likely(skb2))
>+			skb = skb2;
>+	}

	Hmm, nitpick, I think that better would be to do:
		skb = vlan_untag(skb);
		if (unlikely(!skb))
			return;
	
	I believe that better is to deliver skbs in consistent way and
	to do not deliver at all in case of -ENOMEM

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