netdev - Re: [PATCH] flow-dissector: Fix alignment issue in __skb_flow_get

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Date:	Fri, 10 Oct 2014 15:32:38 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	alexander.h.duyck@...hat.com
Cc:	alexander.duyck@...il.com, eric.dumazet@...il.com,
	David.Laight@...LAB.COM, netdev@...r.kernel.org
Subject: Re: [PATCH] flow-dissector: Fix alignment issue in
 __skb_flow_get_ports

From: Alexander Duyck <alexander.h.duyck@...hat.com>
Date: Fri, 10 Oct 2014 11:53:35 -0700

> On 10/10/2014 11:22 AM, David Miller wrote:
>> From: David Miller <davem@...emloft.net>
>> Date: Fri, 10 Oct 2014 14:15:59 -0400 (EDT)
>>
>>> Your original code works because you do things like "byte[12] & 0xf0"
>>> to
>>> extract these fields.
>> Changing that th->doff sequence to instead be:
>>
>> 		const u8 *bp;
>> 		u8 buf[13];
>>
>> 		bp = __skb_header_pointer(skb, poff, sizeof(buf),
>> 					  data, hlen, &buf);
>> 		if (!bp)
>> 			return poff;
>>
>> 		poff += max_t(u32, sizeof(struct tcphdr), (bp[12] & 0xf0) >> 2);
>> 		break;
>>
>> on top of your v3 patch works for me.
>>
>> Please double-check my calculations.
> 
> Any reason why you are grabbing all 13 bytes instead of just the 1 we
> care about?  Seems like we could just use a u8 buf instead of the
> array since we are only grabbing doff.

No reason, just a thinko, my brain implemented this as if skb_header_pointer
worked like skb_pull :-/
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