Subject: Re: [PATCH] gainfar.c : skb_over_panic

From: Eran Liberty <[email-masked]>
Date: Mon, 21 Jun 2010 12:13:29 +0300

David Miller wrote:
> From: Eran Liberty <[email-masked]>
> Date: Thu, 17 Jun 2010 19:32:54 +0300
>
>   
>> I have demonstrated skb_over_panic with linux 2.6.32.15 on a mpc8548
>> based product.
>>     
>
> A fix for a similar bug was necessary for the ucc_geth driver,
> see below.
>
> The real problem is that skb->data assignment, the rest of the
> SKB state has to be reset, and not doing that is what results in
> the skb_over_panic calls.
>
> >From db176edc89abbf22e6db6853f8581f9475fe8ec1 Mon Sep 17 00:00:00 2001
> From: Sergey Matyukevich <[email-masked]>
> Date: Mon, 14 Jun 2010 06:35:20 +0000
> Subject: [PATCH] ucc_geth: fix for RX skb buffers recycling
>
> This patch implements a proper modification of RX skb buffers before
> recycling. Adjusting only skb->data is not enough because after that
> skb->tail and skb->len become incorrect.
>
> Signed-off-by: Sergey Matyukevich <[email-masked]>
> Signed-off-by: David S. Miller <[email-masked]>
> ---
>  drivers/net/ucc_geth.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
>
> diff --git a/drivers/net/ucc_geth.c b/drivers/net/ucc_geth.c
> index 4a34833..807470e 100644
> --- a/drivers/net/ucc_geth.c
> +++ b/drivers/net/ucc_geth.c
> @@ -3215,6 +3215,8 @@ static int ucc_geth_rx(struct ucc_geth_private *ugeth, u8 rxQ, int rx_work_limit
>  					   __func__, __LINE__, (u32) skb);
>  			if (skb) {
>  				skb->data = skb->head + NET_SKB_PAD;
> +				skb->len = 0;
> +				skb_reset_tail_pointer(skb);
>  				__skb_queue_head(&ugeth->rx_recycle, skb);
>  			}
>  
David,

I have compared the suggested patch with what the function skb_recycle_check() does. Both patch and skb_recycle_check()
 have skb_reset_tail_pointer(). While the patch zero only skb->len, skb_recycle_check()
 clears the whole skb (up to tail). On top of that skb_recycle_check() preforms a whole set of other checks and cleanups. 
The question is, which action is MORE correct: the pin-point action of the patch suggested or the broader checks of skb_recycle_check() function?

-- Liberty