netdev - Re: [PATCH net] virtio-net: fix for skb_over

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Date:   Fri, 4 Jun 2021 11:00:25 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        virtualization@...ts.linux-foundation.org,
        Corentin Noël <corentin.noel@...labora.com>,
        netdev@...r.kernel.org
Subject: Re: [PATCH net] virtio-net: fix for skb_over_panic inside big mode


在 2021/6/4 上午10:30, Xuan Zhuo 写道:
> On Fri, 4 Jun 2021 10:28:41 +0800, Jason Wang <jasowang@...hat.com> wrote:
>> 在 2021/6/4 上午1:09, Xuan Zhuo 写道:
>>> In virtio-net's large packet mode, there is a hole in the space behind
>>> buf.
>>
>> before the buf actually or behind the vnet header?
>>
>>
>>>       hdr_padded_len - hdr_len
>>>
>>> We must take this into account when calculating tailroom.
>>>
>>> [   44.544385] skb_put.cold (net/core/skbuff.c:5254 (discriminator 1) net/core/skbuff.c:5252 (discriminator 1))
>>> [   44.544864] page_to_skb (drivers/net/virtio_net.c:485) [   44.545361] receive_buf (drivers/net/virtio_net.c:849 drivers/net/virtio_net.c:1131)
>>> [   44.545870] ? netif_receive_skb_list_internal (net/core/dev.c:5714)
>>> [   44.546628] ? dev_gro_receive (net/core/dev.c:6103)
>>> [   44.547135] ? napi_complete_done (./include/linux/list.h:35 net/core/dev.c:5867 net/core/dev.c:5862 net/core/dev.c:6565)
>>> [   44.547672] virtnet_poll (drivers/net/virtio_net.c:1427 drivers/net/virtio_net.c:1525)
>>> [   44.548251] __napi_poll (net/core/dev.c:6985)
>>> [   44.548744] net_rx_action (net/core/dev.c:7054 net/core/dev.c:7139)
>>> [   44.549264] __do_softirq (./arch/x86/include/asm/jump_label.h:19 ./include/linux/jump_label.h:200 ./include/trace/events/irq.h:142 kernel/softirq.c:560)
>>> [   44.549762] irq_exit_rcu (kernel/softirq.c:433 kernel/softirq.c:637 kernel/softirq.c:649)
>>> [   44.551384] common_interrupt (arch/x86/kernel/irq.c:240 (discriminator 13))
>>> [   44.551991] ? asm_common_interrupt (./arch/x86/include/asm/idtentry.h:638)
>>> [   44.552654] asm_common_interrupt (./arch/x86/include/asm/idtentry.h:638)
>>>
>>> Fixes: fb32856b16ad ("virtio-net: page_to_skb() use build_skb when there's sufficient tailroom")
>>> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
>>> Reported-by: Corentin Noël <corentin.noel@...labora.com>
>>> Tested-by: Corentin Noël <corentin.noel@...labora.com>
>>> ---
>>>    drivers/net/virtio_net.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>> index fa407eb8b457..78a01c71a17c 100644
>>> --- a/drivers/net/virtio_net.c
>>> +++ b/drivers/net/virtio_net.c
>>> @@ -406,7 +406,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>>>    	 * add_recvbuf_mergeable() + get_mergeable_buf_len()
>>>    	 */
>>>    	truesize = headroom ? PAGE_SIZE : truesize;
>>> -	tailroom = truesize - len - headroom;
>>> +	tailroom = truesize - len - headroom - (hdr_padded_len - hdr_len);
>>
>> The patch looks correct and I saw it has been merged.
>>
>> But I prefer to do that in receive_big() instead of here.
>>
>> Thanks
> How?
>
> change truesize or headroom?
>
> I didn't find a good way. Do you have a good way?


Something like the following? The API is designed to let the caller to 
pass a correct headroom instead of figure it out by itself.

         struct sk_buff *skb =
                 page_to_skb(vi, rq, page, 0, len, PAGE_SIZE, true, 0, 
hdr_padded_len - hdr_len);

Thanks


>
> Thanks.
>
>>
>>
>>>    	buf = p - headroom;
>>>
>>>    	len -= hdr_len;