netdev - Re: [PATCH] skbuff: align sk

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Message-ID: <4B63643B.3070400@caviumnetworks.com>
Date:	Fri, 29 Jan 2010 14:42:03 -0800
From:	David Daney <ddaney@...iumnetworks.com>
To:	Felix Fietkau <nbd@...nwrt.org>
CC:	netdev@...r.kernel.org, Lennert Buytenhek <buytenh@...tstofly.org>
Subject: Re: [PATCH] skbuff: align sk_buff::cb to 64 bit

Felix Fietkau wrote:
> The alignment requirement for 64-bit load/store instructions on ARM is
> implementation defined. Some CPUs (such as Marvell Feroceon) do not
> generate an exception, if such an instruction is executed with an
> address that is not 64 bit aligned. 
> In such a case, the Feroceon corrupts adjacent memory, which showed up
> in my tests as a crash in the rx path of ath9k that only occured with
> CONFIG_XFRM set. This crash happened, because the first field of the
> mac80211 rx status info in the cb is an u64, and changing it corrupted
> the skb->sp field.
> 
> Signed-off-by: Felix Fietkau <nbd@...nwrt.org>
> Cc: stable@...nel.org
> ---
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -329,7 +329,7 @@ struct sk_buff {
>  	 * want to keep them across layers you have to do a skb_clone()
>  	 * first. This is owned by whoever has the skb queued ATM.
>  	 */
> -	char			cb[48];
> +	char			cb[48] __attribute__((aligned(8)));
>  


s/__attribute__((aligned(8)))/__aligned(8)/

Could the same thing be achieved by swapping the order of the dev and 
tstamp fields instead of adding the alignment attribute?

What is the sizeof(void *) on this thing?

David Daney
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