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

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Date:	Tue, 8 Jan 2013 21:22:08 +0100
From:	Jiri Pirko <jiri@...nulli.us>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Daniel Borkmann <dborkman@...hat.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	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 09:04:52PM CET, eric.dumazet@...il.com wrote:
>On Tue, 2013-01-08 at 19:51 +0100, Daniel Borkmann 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;
>> +	}
>>  
>>  	rcu_read_lock();
>>  	list_for_each_entry_rcu(ptype, &ptype_all, list) {
>
>This patch is wrong (it adds a leak), and not needed.
>
>If a packet has no vlan_tci, its for a good reason.
>
>We want sniffer see the packet content as is.


The issue is that for exmaple in af_packet the function packet_rcv()
expects vlan_tci to be filled out as that is on RX path ensured by
__netif_receive_skb(). However on TX path, dev_queue_xmit_nit() is
called with vlan_tci cleaned in case the device does not have TX vlan
accel enabled. This patch is trying to fix this difference.

>
>
>
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