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

From: Hannes Frederic Sowa <[email-masked]>
Date: Tue, 2 Feb 2016 14:41:23 +0100

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