netdev - RE: [Xen-devel] xennet_start

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Date:   Thu, 19 Jan 2017 11:31:14 +0000
From:   Paul Durrant <Paul.Durrant@...rix.com>
To:     Sowmini Varadhan <sowmini.varadhan@...cle.com>
CC:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Wei Liu <wei.liu2@...rix.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
Subject: RE: [Xen-devel] xennet_start_xmit assumptions

> -----Original Message-----
> From: Sowmini Varadhan [mailto:sowmini.varadhan@...cle.com]
> Sent: 19 January 2017 11:14
> To: Paul Durrant <Paul.Durrant@...rix.com>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>; Wei Liu
> <wei.liu2@...rix.com>; netdev@...r.kernel.org; xen-
> devel@...ts.xenproject.org
> Subject: Re: [Xen-devel] xennet_start_xmit assumptions
> 
> On (01/19/17 09:36), Paul Durrant wrote:
> >
> > Hi Sowmini,
> >
> >   Sounds like a straightforward bug to me... netfront should be able
> > to handle an empty skb and clearly, if it's relying on skb_headlen()
> > being non-zero, that's not the case.
> >
> >   Paul
> 
> I see. Seems like there are 2 things broken here: recovering
> from skb->len = 0, and recovering from  the more complex
> case of (skb->len > 0 && skb_headlen(skb) == 0)
> 
> Do you folks want to take a shot at fixing this,
> since you know the code better? If you are interested,
> I can share my test program to help you reproduce the
> simpler skb->len == 0 case, but it's the fully non-linear
> skbs that may be more interesting to reproduce/fix.

Sowmini,

Yeah, it would be useful to verify any change fixes the particular issue you're seeing so please share the program. For the non-empty non-linear case I'd hope that catching this and doing a pull of some sensible amount of header (which might coincide with the least amount that netback expects to see in the first frag) would be enough.
I can take a shot at a patch for this in the next few days; I'll add your 'Reported-by' so you should get cc-ed.

Cheers,

  Paul

> 
> I'll probably work on fixing packet_snd to return -EINVAL
> or similar when the len is zero this week.
> 
> --Sowmini