netdev - Re: [PATCH net] net: Allow flow dissector to handle non 4-byte aligned headers

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Date:	Tue, 2 Feb 2016 14:41:23 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Alexander Duyck <alexander.duyck@...il.com>,
	Sowmini Varadhan <sowmini.varadhan@...cle.com>
Cc:	Tom Herbert <tom@...bertland.com>,
	David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>, Kernel Team <kernel-team@...com>
Subject: Re: [PATCH net] net: Allow flow dissector to handle non 4-byte
 aligned headers

On 02.02.2016 04:56, Alexander Duyck wrote:
>> @@ -394,7 +407,7 @@ ip_proto_again:
>>                                                     data, hlen, &_eth);
>>                          if (!eth)
>>                                  goto out_bad;
>> -                       proto = eth->h_proto;
>> +                       proto = get_unaligned_be16(&eth->h_proto);
>>                          nhoff += sizeof(*eth);
>>                  }
>
> This piece doesn't make any sense to me.  It is already only 2 bytes
> wide.  I'm not sure why we should be seeing this trigger an unaligned
> access.  Are you sure it wasn't something like the keyid causing the
> issue?  I'd be interested in seeing what the compiler did here that it
> is triggering the problem.
>

Correct, and the __packed attribute of struct ethhdr already causes all 
members to have an assumed-alignment of '1', so gcc doesn't create any 
16 bit width accesses anyway.

Bye,
Hannes