netdev - Re: [PATCH net-next v2 4/7] net: packet: use skb->dev as storage for skb orig len instead of skb->cb[]

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Date:	Sat, 28 Feb 2015 22:38:04 +0200
From:	Eyal Birger <eyal.birger@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	Willem de Bruijn <willemb@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Shmulik Ladkani <shmulik.ladkani@...il.com>,
	linux-bluetooth@...r.kernel.org,
	Marcel Holtmann <marcel@...tmann.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v2 4/7] net: packet: use skb->dev as storage for
 skb orig len instead of skb->cb[]

On Sat, Feb 28, 2015 at 10:08 PM, David Miller <davem@...emloft.net> wrote:
> From: Eyal Birger <eyal.birger@...il.com>
> Date: Sat, 28 Feb 2015 21:39:34 +0200
>
>> On Sat, Feb 28, 2015 at 9:21 PM, David Miller <davem@...emloft.net> wrote:
>>> From: Eyal Birger <eyal.birger@...il.com>
>>> Date: Thu, 26 Feb 2015 21:07:01 +0200
>>>
>>>> As part of an effort to move skb->dropcount to skb->cb[], 4 bytes
>>>> of additional room are needed in skb->cb[] in packet sockets.
>>>>
>>>> Store the skb original length in skb->dev instead of skb->cb[] for
>>>> this purpose.
>>>>
>>>> Signed-off-by: Eyal Birger <eyal.birger@...il.com>
>>>
>>> I'm a little confused, why is this even needed?
>>>
>>> packet_skb_cb is 24 bytes by my calculations, which is much
>>> smaller than the cb[] size which is 48 bytes.
>>
>> Note the BUILD_BUG_ON in packet_rcv().
>>
>> packet_skb_cb may contain an address as large as MAX_ADDR_LEN (32)
>> Therefore the required space is sizeof(packet_skb_cb) + MAX_ADDR_LEN - 8
>> which is 48 bytes before this change.
>
> So let's take a step back.
>
> What link layer we support has 32-byte hardware addresses?

The largest one I've been hinted of is INFINIBAND_ALEN which is 20 bytes.

> If the answer is lower than 32, we should use that value instead.

Won't that value become the 'real' MAX_ADDR_LEN in that case?

My concern is that any value I pick based on the existing implementations
would need to be adjusted come a protocol with a larger address length.

Also, I would have to assert the available amount of space at run-time as
the address length is provided by a call to dev_parse_header() instead of
using a build-time assert.

Regards,
Eyal.
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