netdev - Re: [PATCH 2/2 v2] bonding: COW before overwriting the destination MAC address

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Date:	Thu, 03 Mar 2011 08:55:01 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Changli Gao <xiaosuo@...il.com>
Cc:	Jay Vosburgh <fubar@...ibm.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH 2/2 v2] bonding: COW before overwriting the destination
 MAC address

Le jeudi 03 mars 2011 à 15:07 +0800, Changli Gao a écrit :
> When there is a ptype handler holding a clone of this skb, whose
> destination MAC addresse is overwritten, the owner of this handler may
> get a corrupted packet.
> 
> Signed-off-by: Changli Gao <xiaosuo@...il.com>
> ---
> v2: fix the bug in the previous one. Thank him.
>  drivers/net/bonding/bond_main.c |    8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
> index 912b416..7b7ca97 100644
> --- a/drivers/net/bonding/bond_main.c
> +++ b/drivers/net/bonding/bond_main.c
> @@ -1511,9 +1511,13 @@ static struct sk_buff *bond_handle_frame(struct sk_buff *skb)
>  	if (bond_dev->priv_flags & IFF_MASTER_ALB &&
>  	    bond_dev->priv_flags & IFF_BRIDGE_PORT &&
>  	    skb->pkt_type == PACKET_HOST) {
> -		u16 *dest = (u16 *) eth_hdr(skb)->h_dest;
>  
> -		memcpy(dest, bond_dev->dev_addr, ETH_ALEN);
> +		if (unlikely(skb_cow_head(skb,
> +					  skb->data - skb_mac_header(skb)))) {
> +			kfree_skb(skb);
> +			return NULL;
> +		}
> +		memcpy(eth_hdr(skb)->h_dest, bond_dev->dev_addr, ETH_ALEN);
>  	}
>  
>  	return skb;



Thats minor, but using :

u16 *dest = eth_hdr(skb)->h_dest;

memcpy(dest, ptr, ETH_ALEN);

Is better because compiler knows both destination and source are at
least aligned on shorts.

On some arches, it helps to not using 6 bytes copy, but 3 shorts.



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