netdev - Re: [Xen-devel] [PATCH net-next] xen-netfront: avoid packet loss when ethernet header crosses page boundary

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Message-ID: <87wpizamxv.fsf@vitty.brq.redhat.com>
Date:   Mon, 29 Aug 2016 12:28:44 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     David Vrabel <david.vrabel@...rix.com>
Cc:     <netdev@...r.kernel.org>, Juergen Gross <jgross@...e.com>,
        Patrick Talbert <ptalbert@...hat.com>,
        <linux-kernel@...r.kernel.org>, <xen-devel@...ts.xenproject.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [Xen-devel] [PATCH net-next] xen-netfront: avoid packet loss when ethernet header crosses page boundary

Vitaly Kuznetsov <vkuznets@...hat.com> writes:

> David Vrabel <david.vrabel@...rix.com> writes:
>
>> On 22/08/16 16:42, Vitaly Kuznetsov wrote:
>>> 
>>> I see two ways to fix the issue:
>>> - Change the 'wire' protocol between netfront and netback to start keeping
>>>   the original SKB structure. We'll have to add a flag indicating the fact
>>>   that the particular request is a part of the original linear part and not
>>>   a frag. We'll need to know the length of the linear part to pre-allocate
>>>   memory.
>>
>> I don't think there needs to be a protocol change.  I think the check in
>> netback is bogus -- it's the total packet length that must be >
>> HLEN_ETH.  The upper layers will pull any headers from the frags as
>> needed
>
> I'm afraid this is not always true, just removing the check leads us to
> the following:
>
> [  495.442186] kernel BUG at ./include/linux/skbuff.h:1927! 
> [  495.468789] invalid opcode: 0000 [#1] SMP 

What I wanted to say here is that this test makes me think the
description of the patch I suggested is correct: an SKB can't have its
linear part shorter than ETH_HLEN as the header is being pointed directly,
upper network layers don't assemble it from frags, the check in netback
is valid.

So, how can we proceed here?

-- 
  Vitaly