linux-kernel - Re: [PATCH net 2/2] macvtap: correctly linearize skb when zerocopy is used

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Date:	Wed, 10 Jul 2013 13:17:51 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 2/2] macvtap: correctly linearize skb when zerocopy
 is used

On 07/09/2013 06:35 PM, Michael S. Tsirkin wrote:
> On Tue, Jul 09, 2013 at 06:10:51PM +0800, Jason Wang wrote:
>> Userspace may produce vectors greater than MAX_SKB_FRAGS. When we try to
>> linearize parts of the skb to let the rest of iov to be fit in
>> the frags, we need count copylen into linear when calling macvtap_alloc_skb()
>> instead of partly counting it into data_len. Since this breaks
>> zerocopy_sg_from_iovec() since its inner counter assumes nr_frags should
>> be zero at beginning. This cause nr_frags to be increased wrongly without
>> setting the correct frags.
>>
>> This bug were introduced from b92946e2919134ebe2a4083e4302236295ea2a73
>> (macvtap: zerocopy: validate vectors before building skb).
>>
>> Cc: Michael S. Tsirkin <mst@...hat.com>
>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>> ---
>> This patch is needed for stable.
>> ---
>>  drivers/net/macvtap.c |    3 ++-
>>  1 files changed, 2 insertions(+), 1 deletions(-)
>>
>> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
>> index f2c4a3b..b213020 100644
>> --- a/drivers/net/macvtap.c
>> +++ b/drivers/net/macvtap.c
>> @@ -770,7 +770,8 @@ static ssize_t macvtap_get_user(struct macvtap_queue *q, struct msghdr *m,
>>  		copylen = len;
>>  
>>  	skb = macvtap_alloc_skb(&q->sk, NET_IP_ALIGN, copylen,
>> -				vnet_hdr.hdr_len, noblock, &err);
>> +				zerocopy ? copylen : vnet_hdr.hdr_len,
>> +				noblock, &err);
>>  	if (!skb)
>>  		goto err;
> Same comment as for tun - let's add code for the if statement above.
> Thanks!
>

Sure, I will post v2.

Thanks
>> -- 
>> 1.7.1
> --
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