From: David Daney <ddaney@caviumnetworks•com>
To: Eric Dumazet <eric.dumazet@gmail•com>
Cc: Felix Fietkau <nbd@openwrt•org>,
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 10:26:51 -0800 [thread overview]
Message-ID: <4B671CEB.5080505@caviumnetworks.com> (raw)
In-Reply-To: <1264835220.2919.10.camel@edumazet-laptop>
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.
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?
David Daney
next prev parent reply other threads:[~2010-02-01 18:27 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 [this message]
2010-02-01 18:37 ` Felix Fietkau
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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