Subject: Re: Sending undersized ARP packets with VXLAN L3 interface

From: Vlad Yasevich <[email-masked]>
Date: Wed, 27 Aug 2014 14:45:12 -0400

On 08/27/2014 02:42 PM, Stephen Hemminger wrote:
> On Wed, 27 Aug 2014 13:52:03 -0400
> Vlad Yasevich <[email-masked]> wrote:
> 
>> On 08/27/2014 01:28 PM, Cong Wang wrote:
>>> On Wed, Aug 27, 2014 at 10:06 AM, Martin Rusko <[email-masked]> wrote:
>>>>
>>>> I'm wondering, where is the proper place to fix this. Should
>>>> arp_create() function allocate skb big enough to produce ethernet
>>>> frame with at least minimum size? Or is it somewhere in NIC drivers
>>>> where small packets are padded with zeros?
>>>
>>> Drivers do that, for example e1000:
>>>
>>>         /* On PCI/PCI-X HW, if packet size is less than ETH_ZLEN,
>>>          * packets may get corrupted during padding by HW.
>>>          * To WA this issue, pad all small packets manually.
>>>          */
>>>         if (skb->len < ETH_ZLEN) {
>>>                 if (skb_pad(skb, ETH_ZLEN - skb->len))
>>>                         return NETDEV_TX_OK;
>>>                 skb->len = ETH_ZLEN;
>>>                 skb_set_tail_pointer(skb, ETH_ZLEN);
>>>         }
>>
>>
>> I think vxlan needs something like this:
>>
>> From: Vladislav Yasevich <[email-masked]>
>> Date: Wed, 27 Aug 2014 13:39:32 -0400
>> Subject: [PATCH] vxlan: Pad short ethernet frames.
>>
>> If sending short ethernet frames from the vxlan device, pad
>> them to minimum size so they can be forwarded after decapsulation.
>>
>> Reported-by: Martin Rusko <[email-masked]>
>> Signed-off-by: Vladislav Yasevich <[email-masked]>
>> ---
>>  drivers/net/vxlan.c | 8 ++++++++
>>  1 file changed, 8 insertions(+)
>>
>> diff --git a/drivers/net/vxlan.c b/drivers/net/vxlan.c
>> index 1fb7b37..48267d4 100644
>> --- a/drivers/net/vxlan.c
>> +++ b/drivers/net/vxlan.c
>> @@ -1939,6 +1939,14 @@ static netdev_tx_t vxlan_xmit(struct sk_buff *skb, struct
>> net_device *dev)
>>  #endif
>>  	}
>>
>> +	/* Pad short frames so they can be forwarded after decapsulation */
>> +	if (skb->len < ETH_ZLEN) {
>> +		if (skb_pad(skb, ETH_ZLEN - skb->len))
>> +			return NETDEV_TX_OK;
>> +		skb->len = ETH_ZLEN;
>> +		skb_set_tail_pointer(skb, ETH_ZLEN);
>> +	}
>> +
>>  	f = vxlan_find_mac(vxlan, eth->h_dest);
>>  	did_rsc = false;
>>
> 
> No. The short frame is perfectly valid, over the VXLAN.
> The system doing the decap and forwarding should be where any padding is added if necessary.
> 

If that's the case, then Martin is most likely seeing a HW bug on the switch.
I wonder how common such a bug might be?

-vlad