linux-kernel - Re: [PATCH net] tuntap: properly align skb->head before building skb

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Date:   Fri, 27 Oct 2017 11:03:57 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Wei Wei <dotweiba@...il.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH net] tuntap: properly align skb->head before building skb



On 2017年10月26日 22:11, Eric Dumazet wrote:
> On Thu, Oct 26, 2017 at 5:15 AM, Jason Wang <jasowang@...hat.com> wrote:
>> An unaligned alloc_frag->offset caused by previous allocation will
>> result an unaligned skb->head. This will lead unaligned
>> skb_shared_info and then unaligned dataref which requires to be
>> aligned for accessing on some architecture. Fix this by aligning
>> alloc_frag->offset before the frag refilling.
>>
>> Fixes: 0bbd7dad34f8 ("tun: make tun_build_skb() thread safe")
>> Cc: Eric Dumazet <edumazet@...gle.com>
>> Cc: Willem de Bruijn <willemdebruijn.kernel@...il.com>
>> Cc: Wei Wei <dotweiba@...il.com>
>> Cc: Dmitry Vyukov <dvyukov@...gle.com>
>> Cc: Mark Rutland <mark.rutland@....com>
>> Reported-by: Wei Wei <dotweiba@...il.com>
>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>> ---
>> - The patch is needed for -stable.
>> - Wei, can you try this patch to see if it solves your issue?
>> ---
>>   drivers/net/tun.c | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index b9973fb..60e44f2 100644
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -1286,6 +1286,7 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
>>          buflen += SKB_DATA_ALIGN(len + pad);
>>          rcu_read_unlock();
>>
>> +       alloc_frag->offset = ALIGN((u64)alloc_frag->offset, TUN_RX_PAD);
> You have to align to one cache line (SMP_CACHE_BYTES), or SKB_DATA_ALIGN(1)

Oh right.

> Then eventually use skb_reserve() for NET_IP_ALIGN, but I guess it is
> already done.

Yes.

Thanks