Subject: Re: [PATCH v2] skbuff: align sk_buff::cb to 64 bit

From: Felix Fietkau <[email-masked]>
Date: Mon, 01 Feb 2010 19:37:45 +0100

On 2010-02-01 7:26 PM, David Daney wrote:
> Eric Dumazet wrote:
>> 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 <[email-masked]>
>>> Cc: [email-masked]
>>> ---
>>> --- 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 :)
>> 
> 
> Can't we just move cb[] up so that it comes after an even number of 
> pointers under all configs?
> 
> Then perhaps add __aligned(8) to the entire structure instead of just 
> this field.
Makes sense, I'll send a patch for that.

> Alternatively, could you fix the driver so that it adds the necessary 
> alignment to its use of the cb[] array?
> 
> How common it it to have sizeof(void *) == 4 *and* require 8-byte 
> alignment on other things?  cb[] is fairly large, can you afford to burn 
> 4 bytes for alignment purposes in your driver?
No, I can't afford to burn a single byte on this, in some places
mac80211 uses all of the cb[] area up to the last byte.

- Felix