Re: BUG:af_packet fails to TX TSO frames

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

On Wed, Oct 11, 2017 at 4:39 AM, Anton Ivanov
<anton.ivanov@cambridgegreys.com> wrote:
> Hi all,
>
> I am having an issue with af_packet.c
>
> It fails to transmit any TSO frame submitted via raw socket + vnet headers.
> An identical frame is considered valid for tap.

This may be due to validation. As of commit 104ba78c9880 ("packet: on
direct_xmit, limit tso and csum to supported devices") the packet socket
code validates TSO packets and HW support in the direct_xmit path.

Do you have a test program or packet (incl. vnet hdr) to reproduce this
with? I usually test this path with

https://github.com/wdebruij/kerneltools/blob/master/tests/psock_txring_vnet.c

> The frames are generated out of legit linux skbufs (in UML) and vnet headers
> work for checksumming on raw, so I should have the raw initialization right.
>
> The header is supposedly parsed correctly and the newly formed skbuf is sent
> to the device transmit routine (or enqueued) . I have debugged it as far as
> it reaching the following line in packet_snd() (line 2592 in 4.13):
>
> err = po->xmit(skb);

That maps either on to packet_direct_xmit or dev_queue_xmit.

>
> This returns NET_XMIT_DROP for any TSO capable device I tested.

You can also try

  perf record -a -g -e skb:kfree_skb sleep 10
  perf report

to see where these packets are dropped.

> They dislike
> the frame. Same frame is accepted by tap. I have went through the header
> parsing and skb allocation code in both af_packet and tap several times and
> I do not see any material difference (except the new zerocopy stuff). So,
> frankly, I am stuck.
>
> Can someone help me to debug this. I do not see an easy way to debug it, but
> this is not a part of the kernel I am familiar with. Is there a suitable
> helper function to try to segment the frame and see exactly what is wrong
> with it?
>
> Cc-ing DaveM as this has no specific maintainer so it falls under his
> umbrella remit.
>
> --
> Anton R. Ivanov
>
> Cambridge Greys Limited, England and Wales company No 10273661
> http://www.cambridgegreys.com/
>