netdev - BUG:af_packet fails to TX TSO frames

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Message-ID: <c3002d4f-62ff-1d0b-65fb-b366be45a6d8@cambridgegreys.com>
Date:   Wed, 11 Oct 2017 09:39:47 +0100
From:   Anton Ivanov <anton.ivanov@...bridgegreys.com>
To:     netdev@...r.kernel.org
Cc:     davem@...emloft.net
Subject: BUG:af_packet fails to TX TSO frames

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.

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);

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