From: Felix Fietkau <nbd@openwrt•org>
To: David Daney <ddaney@caviumnetworks•com>
Cc: Eric Dumazet <eric.dumazet@gmail•com>,
netdev@vger•kernel.org,
Lennert Buytenhek <buytenh@wantstofly•org>
Subject: Re: [PATCH v2] skbuff: align sk_buff::cb to 64 bit
Date: Mon, 01 Feb 2010 19:37:45 +0100 [thread overview]
Message-ID: <4B671F79.8090808@openwrt.org> (raw)
In-Reply-To: <4B671CEB.5080505@caviumnetworks.com>
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 <nbd@openwrt•org>
>>> Cc: stable@kernel•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 :)
>>
>
> 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
next prev parent reply other threads:[~2010-02-01 18:37 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-01-30 0:38 [PATCH v2] skbuff: align sk_buff::cb to 64 bit Felix Fietkau
2010-01-30 7:07 ` Eric Dumazet
2010-02-01 18:26 ` David Daney
2010-02-01 18:37 ` Felix Fietkau [this message]
2010-02-12 20:13 ` David Miller
2010-02-23 21:45 ` [PATCH v3] skbuff: align sk_buff::cb to 64 bit and close some potential holes Felix Fietkau
2010-02-27 11:17 ` David Miller
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