Invalid GSO - from 4.x (TBA) to 5.5-rc1. Was: Re: 64 bit time regression in recvmmsg()

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

On 06/12/2019 20:06, Arnd Bergmann wrote:
> On Fri, Dec 6, 2019 at 6:50 PM Anton Ivanov
> <anton.ivanov@cambridgegreys.com> wrote:
>> On 29/11/2019 16:34, Anton Ivanov wrote:
>>
>> I apologize, problem elsewhere. I have narrowed it down, it is a host
>> regression at the end, not a recvmmsg/time one.
>>
>> The EINVAL is occasionally returned from the guts of packet_rcv_vnet
>>
>> https://elixir.bootlin.com/linux/latest/ident/packet_rcv_vnet
>>
>> in af_packet. I am going to try to figure out exactly when it happens
>> and why.
>>
>> My sincere apologies,
> 
> No, worries, I'm glad it wasn't me this time ;-)

At some point in late 4.x (I am going to try narrowing down the 
version), gso code introduced a condition which should not occur:

We have

sinfo(skb)->gso_type = 0

while

sinfo(skb)->gso_size = 752.
skb->len = 818
skb->data_len = 0

As a result we get a skb which is GSO, but it has a no GSO type.

This shows up in virtio_net_hdr_from_skb() which detects the condition 
as invalid and returns an -EINVAL

A few interesting things.

1. Size is always 752.

2. I have found it while tracing an -EINVAL when using recvmmsg() on raw 
sockets. No idea if it shows up elsewhere.

3. The environment under test is reading/writing to a raw socket opened 
on a vEth pair configured as follows:


ip link add l-veth0 type veth peer name p-veth0 && ifconfig p-veth0 up
ifconfig l-veth0 192.168.97.1 netmask 255.255.255.252

the UML linux instance used as a reader/writer relies on 
recvmmsg/sendmmesg raw socket with vnet headers enabled.

virtio_net_hdr_from_skb() is called from the af_packet packet_recv_vnet 
function.

This condition simply should not occur. A skb with no data in the frags, 
gso on, gso size less than MTU and no type looks bogus.


> 
>       Arnd
> 
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-- 
Anton R. Ivanov
Cambridgegreys Limited. Registered in England. Company Number 10273661
https://www.cambridgegreys.com/