netdev - Re: [patch 5/7] CAN: Add virtual CAN netdevice driver

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Date:	Sat, 02 Jun 2007 11:51:58 +0200
From:	Oliver Hartkopp <oliver@...tkopp.net>
To:	Patrick McHardy <kaber@...sh.net>,
	Urs Thuermann <urs@...ogud.escape.de>
CC:	David Miller <davem@...emloft.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Oliver Hartkopp <oliver.hartkopp@...kswagen.de>,
	Urs Thuermann <urs.thuermann@...kswagen.de>,
	netdev@...r.kernel.org
Subject: Re: [patch 5/7] CAN: Add virtual CAN netdevice driver

Patrick McHardy wrote:
> Oliver Hartkopp wrote:
>   
>> Patrick McHardy wrote:
>>     
>
> Yes, its working, but only in certain combinations and you're breaking
> the rules for skb->cb, making it impossible for other layers to use.
> skb->sk is "stable" at the output path, the regular loopback device
> orphans the skb in hard_start_xmit. So you can at least use it there.
>
>   
>> Would therefore skb->cb left unchanged in my skb's? Or is there any flag
>> that can be set in the skb to keep the packet scheduler's hands off?
>>     
>
>
> No, and I don't think we want a flag to signal that something is
> violating the rules for skb->cb, there are other users of this
> besides qdiscs.
>   

Hm - regarding Patricks and Urs' last mails i just had the idea to put
the sk-reference that's needed for this special
CAN-only-loopback-functionality into the data section of the skb, e.g.
by introducing a new struct can_skb_data:

struct can_skb_data {
    struct can_frame cf;
    sock *txsk;
};

So instead of allocating the space of struct can_frame the alloc_skb()
would allocate the size of struct can_skb_data.
The needed txsk would be stable in any case and could be used like the
currently missused skb->cb. This would also lead to a type proof(!)
implementation.

In raw_rcv() in raw.c there could be a check for the size of struct
can_skb_data first before checking the txsk - this would also guarantee
the backward compatibility for current CAN drivers that allocate only
the size of struct can_frame. For me this looks like a safe and
compatible (Kernel & CAN) solution.

Any objections/comments for this approach?

Best regards,
Oliver

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