netdev - Re: [Patch net] igb: pass the correct maxlen for eth_get

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Date:	Wed, 22 Apr 2015 13:14:46 -0700
From:	Cong Wang <cwang@...pensource.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	Cong Wang <xiyou.wangcong@...il.com>,
	netdev <netdev@...r.kernel.org>,
	intel-wired-lan@...ts.osuosl.org,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>
Subject: Re: [Patch net] igb: pass the correct maxlen for eth_get_headlen()

On Wed, Apr 22, 2015 at 12:43 PM, Alexander Duyck
<alexander.duyck@...il.com> wrote:
> On 04/22/2015 10:45 AM, Cong Wang wrote:
>> The second parameter of eth_get_headlen() is the length of
>> the frame buffer, not the header length of skb.
>>
>> Cc: Jeff Kirsher <jeffrey.t.kirsher@...el.com>
>> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
>> ---
>>  drivers/net/ethernet/intel/igb/igb_main.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
>> index a0a9b1f..7b3a370 100644
>> --- a/drivers/net/ethernet/intel/igb/igb_main.c
>> +++ b/drivers/net/ethernet/intel/igb/igb_main.c
>> @@ -6852,7 +6852,9 @@ static void igb_pull_tail(struct igb_ring *rx_ring,
>>       /* we need the header to contain the greater of either ETH_HLEN or
>>        * 60 bytes if the skb->len is less than 60 for skb_pad.
>>        */
>> -     pull_len = eth_get_headlen(va, IGB_RX_HDR_LEN);
>> +     pull_len = eth_get_headlen(va, skb_frag_size(frag));
>> +     if (unlikely(pull_len > IGB_RX_HDR_LEN))
>> +             pull_len = IGB_RX_HDR_LEN;
>>
>>       /* align pull length to size of long to optimize memcpy performance */
>>       skb_copy_to_linear_data(skb, va, ALIGN(pull_len, sizeof(long)));
>
> You have this part right.  The length represents the maximum length we
> are willing to traverse in the buffer.  So if we 100% want to get the
> entire header regardless of what we can copy into then we could follow
> your approach.  However, since the allocated space in the skb is only
> IGB_RX_HDR_LEN we only really want to traverse up to that length.  Then
> that is all we copy out of the header.

But the frag size could be smaller than IGB_RX_HDR_LEN which is
the case this patch tries to fix.

>
> As a result we don't need the extra code for putting the upper limit on
> pull_len since that is factored in by passing IGB_RX_HDR_LEN as the
> maximum traversal length.

That is why I put unlikely() there, even with tunnel headers we unlikely
pull header longer than IGB_RX_HDR_LEN, the code is just for
completeness.
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