netdev - Re: Sending undersized ARP packets with VXLAN L3 interface

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Date:	Wed, 27 Aug 2014 11:42:09 -0700
From:	Stephen Hemminger <stephen@...workplumber.org>
To:	Vlad Yasevich <vyasevich@...il.com>
Cc:	Cong Wang <cwang@...pensource.com>,
	Martin Rusko <martin.rusko@...il.com>,
	netdev <netdev@...r.kernel.org>
Subject: Re: Sending undersized ARP packets with VXLAN L3 interface

On Wed, 27 Aug 2014 13:52:03 -0400
Vlad Yasevich <vyasevich@...il.com> wrote:

> On 08/27/2014 01:28 PM, Cong Wang wrote:
> > On Wed, Aug 27, 2014 at 10:06 AM, Martin Rusko <martin.rusko@...il.com> 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 <vyasevich@...il.com>
> 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 <martin.rusko@...il.com>
> Signed-off-by: Vladislav Yasevich <vyasevich@...il.com>
> ---
>  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.
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