David Miller <da...@davemloft.net> writes:
> From: Vitaly Kuznetsov <vkuzn...@redhat.com>
> Date: Fri, 16 Sep 2016 12:59:14 +0200
>
>> @@ -595,6 +596,19 @@ static int xennet_start_xmit(struct sk_buff *skb,
>> struct net_device *dev)
>> offset = offset_in_page(skb->data);
>> len = skb_headlen(skb);
>>
>> + /* The first req should be at least ETH_HLEN size or the packet will be
>> + * dropped by netback.
>> + */
>> + if (unlikely(PAGE_SIZE - offset < ETH_HLEN)) {
>> + nskb = skb_copy(skb, GFP_ATOMIC);
>> + if (!nskb)
>> + goto drop;
>> + dev_kfree_skb_any(skb);
>> + skb = nskb;
>> + page = virt_to_page(skb->data);
>> + offset = offset_in_page(skb->data);
>> + }
>> +
>> spin_lock_irqsave(&queue->tx_lock, flags);
>
> I think you also have to recalculate 'len' in this case too, as
> skb_headlen() will definitely be different for nskb.
>
> In fact, I can't see how this code can work properly without that fix.
Thank you for your feedback David,
in my testing (even when I tried doing skb_copy() for all skbs
unconditionally) skb_headlen(nskb) always equals 'len' so I was under an
impression that both 'skb->len' and 'skb->data_len' remain the same when
we do skb_copy(). However, in case you think there are cases when
headlen changes, I see no problem with re-calculating 'len' as it won't
bring any significant performace penalty compared to the already added
skb_copy().
I'll send 'v2'.
--
Vitaly
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