Re: [PATCH] virtio: Work around frames incorrectly marked as gso

[Anton Ivanov <anton.ivanov@cambridgegreys.com>]

On 13/02/2020 15:53, Michael S. Tsirkin wrote:
> On Thu, Feb 13, 2020 at 07:44:06AM -0800, Eric Dumazet wrote:
>>
>> On 2/13/20 2:00 AM, Michael S. Tsirkin wrote:
>>> On Wed, Feb 12, 2020 at 05:38:09PM +0000, Anton Ivanov wrote:
>>>>
>>>> On 11/02/2020 10:37, Michael S. Tsirkin wrote:
>>>>> On Tue, Feb 11, 2020 at 07:42:37AM +0000, Anton Ivanov wrote:
>>>>>> On 11/02/2020 02:51, Jason Wang wrote:
>>>>>>> On 2020/2/11 上午12:55, Anton Ivanov wrote:
>>>>>>>>
>>>>>>>> On 09/12/2019 10:48, anton.ivanov@cambridgegreys.com wrote:
>>>>>>>>> From: Anton Ivanov <anton.ivanov@cambridgegreys.com>
>>>>>>>>>
>>>>>>>>> Some of the frames marked as GSO which arrive at
>>>>>>>>> virtio_net_hdr_from_skb() have no GSO_TYPE, no
>>>>>>>>> fragments (data_len = 0) and length significantly shorter
>>>>>>>>> than the MTU (752 in my experiments).
>>>>>>>>>
>>>>>>>>> This is observed on raw sockets reading off vEth interfaces
>>>>>>>>> in all 4.x and 5.x kernels I tested.
>>>>>>>>>
>>>>>>>>> These frames are reported as invalid while they are in fact
>>>>>>>>> gso-less frames.
>>>>>>>>>
>>>>>>>>> This patch marks the vnet header as no-GSO for them instead
>>>>>>>>> of reporting it as invalid.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
>>>>>>>>> ---
>>>>>>>>>     include/linux/virtio_net.h | 8 ++++++--
>>>>>>>>>     1 file changed, 6 insertions(+), 2 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
>>>>>>>>> index 0d1fe9297ac6..d90d5cff1b9a 100644
>>>>>>>>> --- a/include/linux/virtio_net.h
>>>>>>>>> +++ b/include/linux/virtio_net.h
>>>>>>>>> @@ -112,8 +112,12 @@ static inline int
>>>>>>>>> virtio_net_hdr_from_skb(const struct sk_buff *skb,
>>>>>>>>>                 hdr->gso_type = VIRTIO_NET_HDR_GSO_TCPV4;
>>>>>>>>>             else if (sinfo->gso_type & SKB_GSO_TCPV6)
>>>>>>>>>                 hdr->gso_type = VIRTIO_NET_HDR_GSO_TCPV6;
>>>>>>>>> -        else
>>>>>>>>> -            return -EINVAL;
>>>>>>>>> +        else {
>>>>>>>>> +            if (skb->data_len == 0)
>>>>>>>>> +                hdr->gso_type = VIRTIO_NET_HDR_GSO_NONE;
>>>>>>>>> +            else
>>>>>>>>> +                return -EINVAL;
>>>>>>>>> +        }
>>>>>>>>>             if (sinfo->gso_type & SKB_GSO_TCP_ECN)
>>>>>>>>>                 hdr->gso_type |= VIRTIO_NET_HDR_GSO_ECN;
>>>>>>>>>         } else
>>>>>>>>>
>>>>>>>> ping.
>>>>>>>>
>>>>>>> Do you mean gso_size is set but gso_type is not? Looks like a bug
>>>>>>> elsewhere.
>>>>>>>
>>>>>>> Thanks
>>>>>>>
>>>>>>>
>>>>>> Yes.
>>>>>>
>>>>>> I could not trace it where it is coming from.
>>>>>>
>>>>>> I see it when doing recvmmsg on raw sockets in the UML vector network
>>>>>> drivers.
>>>>>>
>>>>> I think we need to find the culprit and fix it there, lots of other things
>>>>> can break otherwise.
>>>>> Just printing out skb->dev->name should do the trick, no?
>>>> The printk in virtio_net_hdr_from_skb says NULL.
>>>>
>>>> That is probably normal for a locally originated frame.
>>>>
>>>> I cannot reproduce this with network traffic by the way - it happens only if the traffic is locally originated on the host.
>>>>
>>>> A,
>>> OK so is it code in __tcp_transmit_skb that sets gso_size to non-null
>>> when gso_type is 0?
>>>
>> Correct way to determine if a packet is a gso one is by looking at gso_size.
>> Then only it is legal looking at gso_type
>>
>>
>> static inline bool skb_is_gso(const struct sk_buff *skb)
>> {
>>      return skb_shinfo(skb)->gso_size;
>> }
>>
>> /* Note: Should be called only if skb_is_gso(skb) is true */
>> static inline bool skb_is_gso_v6(const struct sk_buff *skb)
>> ...
>>
>>
>> There is absolutely no relation between GSO and skb->data_len, skb can be linearized
>> for various orthogonal reasons.
> The reported problem is that virtio gets a packet where gso_size
> is !0 but gso_type is 0.
>
> It currently drops these on the assumption that it's some type
> of a gso packet it does not know how to handle.
>
>
> So you are saying if skb_is_gso we can still have gso_type set to 0,
> and that's an expected configuration?
>
> So the patch should just be:
>
>
> -        if (skb_is_gso(skb)) {
> +        if (skb_is_gso(skb) && sinfo->gso_type) {
>
Yes, provided that skb_is_gso(skb) and sinfo->gso_type == 0 is a valid state.

I agree with Jason, there may be something wrong going on here and we need to find the source which creates these packets.

A.

>
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-- 
Anton R. Ivanov
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https://www.cambridgegreys.com/