netdev - Re: BUG:af_packet fails to TX TSO frames

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Message-ID: <e301fbfd-a283-caa1-5915-8be15677ed74@cambridgegreys.com>
Date:   Thu, 12 Oct 2017 16:44:26 +0100
From:   Anton Ivanov <anton.ivanov@...bridgegreys.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>
Subject: Re: BUG:af_packet fails to TX TSO frames

Found it.

Two bugs canceling each other.
The bind sequence in:  psock_txring_vnet.c is wrong.

It does the following addr.sll_protocol =    htons(ETH_P_IP);
before calling bind.

If you set addr.sll_protocol to ETH_P_ALL where it should have been in 
the first place the test program blows up with -ENOBUFS

I think what is happening is that this value is taken into account when 
looking at "what should I use to segment it with" in skb_mac_gso_segment 
which is invoked at the end of the verification chain which starts in 
packet_direct_xmit in af_packet.c

I have not tried the other test cases like setting it to ETH_P_IP and 
giving it IPv6 traffic or the opposite, but my guess is that these will 
fail too if they need GSO to be applied.

A.

On 10/12/17 15:12, Anton Ivanov wrote:
>
>
> On 10/12/17 14:39, Willem de Bruijn wrote:
>>> If I produce a real vnet frame out of a live kernel frame using
>>> virtio_net_hdr_from_skb() and try to send it it fails on the check in
>>> af_packet, while succeeding for tap. If I remove the af_packet check 
>>> the
>>> frame is accepted by the hardware too.
>>>
>>> If I produce it a synthetic frame + vnet header using the test 
>>> program - it
>>> works. Go figure.
>> Besides looking at the raw frame bytes, also compare the setup
>> of virtio_net_header, as well as the tcp checksum field. The stack
>> expects the pseudo header to have already been calculated.
>
> I am feeding it a skb which is coming up in the tx routine of a User 
> Mode Linux device which is marked as NETIF_F_HW_CSUM and SG - that 
> results in a skb with csum-ed headers, body set up for CSUM_PARTIAL 
> and multiple fragments (always at least 1 more frag besides the TCP 
> head).
>
> That has everything in order as expected by virtio_net_hdr_from_skb 
> and this is what I use to generate the vnet header. It works correctly 
> for csum and GRO with af_packet and it works correctly for everything 
> using a tap device. It fails only on GSO + af_packet TX.
>
> What I am doing is the same thing virtio_net does - it just takes the 
> output of virtio_net_hdr_from_skb and does nothing more. There should 
> be no need to do anything more :(
>
> It should just work.
>
> Unless there is a gremlin somewhere in the machinery and that gremlin 
> needs some light to be flushed out.
>>
>>> I am going to continue digging into it.
>>>
>>> At the very least I now have a positive test case which uses the same
>>> semantics as my code so I have something to compare to.
>> Glad to hear that the test is helpful. I wrote it because I
>> have run into these exact same issues in the past.
>
> It is. I have changes ready for it so it also supports vector IO, need 
> to finish fighting with it.
>
> A.
>
>>
>

-- 
Anton R. Ivanov

Cambridge Greys Limited, England and Wales company No 10273661
http://www.cambridgegreys.com/