netdev - Re: [PATCH v2] skbuff: align sk

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Message-ID: <1264835220.2919.10.camel@edumazet-laptop>
Date:	Sat, 30 Jan 2010 08:07:00 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Felix Fietkau <nbd@...nwrt.org>
Cc:	netdev@...r.kernel.org, Lennert Buytenhek <buytenh@...tstofly.org>,
	David Daney <ddaney@...iumnetworks.com>
Subject: Re: [PATCH v2] skbuff: align sk_buff::cb to 64 bit

Le samedi 30 janvier 2010 à 01:38 +0100, Felix Fietkau a écrit :
> 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] __aligned(8);
>   	unsigned int		len,
>  				data_len;
> 
> --

Without a detailed analysis of holes added on x86_32 and/or x86_64, I
guess this patch is not acceptable as is.

You certainly can find a better way to do this, without adding holes in
sk_buff structure. Size matters a lot :)

Thanks


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