netdev - Re: [Xen-devel] [PATCH 2/4] xen-netfront: drop skb when skb->len

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Date:	Tue, 9 Apr 2013 15:59:51 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Christoph Egger <chegger@...zon.de>
CC:	Ian Campbell <Ian.Campbell@...rix.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"annie.li@...cle.com" <annie.li@...cle.com>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	Wei Liu <wei.liu2@...rix.com>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>
Subject: Re: [Xen-devel] [PATCH 2/4] xen-netfront: drop skb when skb->len >
 65535

On Tue, 2013-04-09 at 16:53 +0200, Christoph Egger wrote:
> On 09.04.13 16:45, Ben Hutchings wrote:
> > On Tue, 2013-04-09 at 15:30 +0100, Ian Campbell wrote:
> >> On Tue, 2013-03-19 at 21:28 +0000, Ben Hutchings wrote:
> >>> On Tue, 2013-03-19 at 21:24 +0000, Ben Hutchings wrote:
> >>>> On Mon, 2013-03-18 at 15:07 +0000, Ian Campbell wrote:
> >>>>> On Mon, 2013-03-18 at 15:04 +0000, Wei Liu wrote:
> >>>>>> On Mon, 2013-03-18 at 14:54 +0000, Ian Campbell wrote:
> >>>>>>> On Mon, 2013-03-18 at 14:40 +0000, Wei Liu wrote:
> >>>>>>>> On Mon, 2013-03-18 at 11:42 +0000, Ian Campbell wrote:
> >>>>>>>>> On Mon, 2013-03-18 at 10:35 +0000, Wei Liu wrote:
> >>>>>>>>>> The `size' field of Xen network wire format is uint16_t, anything bigger than
> >>>>>>>>>> 65535 will cause overflow.
> >>>>>>>>>>
> >>>>>>>>>> Signed-off-by: Wei Liu <wei.liu2@...rix.com>
> >>>>>>>>>> ---
> >>>>>>>>>>   drivers/net/xen-netfront.c |   12 ++++++++++++
> >>>>>>>>>>   1 file changed, 12 insertions(+)
> >>>>>>>>>>
> >>>>>>>>>> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> >>>>>>>>>> index 5527663..8c3d065 100644
> >>>>>>>>>> --- a/drivers/net/xen-netfront.c
> >>>>>>>>>> +++ b/drivers/net/xen-netfront.c
> >>>>>>>>>> @@ -547,6 +547,18 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
> >>>>>>>>>>   	unsigned int len = skb_headlen(skb);
> >>>>>>>>>>   	unsigned long flags;
> >>>>>>>>>>
> >>>>>>>>>> +	/*
> >>>>>>>>>> +	 * wire format of xen_netif_tx_request only supports skb->len
> >>>>>>>>>> +	 * < 64K, because size field in xen_netif_tx_request is
> >>>>>>>>>> +	 * uint16_t.
> >>>>>>>>>
> >>>>>>>>> Is there some field we can set e.g. in struct ethernet_device which
> >>>>>>>>> would stop this from happening?
> >>>>>>>>>
> >>>>>>>>
> >>>>>>>> struct ethernet_device? I could not find it.
> >>>>>>>>
> >>>>>>>> And for struct net_device,
> >>>>>>>
> >>>>>>> I meant struct net_device.
> >>>>>>>
> >>>>>>>>   there is no field for this AFAICT.
> >>>>>>>
> >>>>>>> Interesting. Are hardware devices expected to cope with arbitrary sized
> >>>>>>> GSO skbs then I wonder.
> >>>>>>>
> >>>>>>
> >>>>>> No idea. But there is a macro called GSO_MAX_SIZE (65536) in struct
> >>>>>> net_device. :-)
> >>>>>
> >>>>> But aren't we seeing skb's bigger than that?
> >>>>>
> >>>>> Maybe this is just a historical bug in some older guests?
> >>>>
> >>>> GSO_MAX_SIZE is the maximum payload length, not the maximum total length
> >>>> of an skb.
> >>>
> >>> ...and it's actually just the default value assigned to
> >>> dev->gso_max_size.  You'll want to change it to your actual maximum
> >>> (65535 - maximum length of headers) before registering your net devices.
> >>
> >> Thanks.
> >>
> >> "maximum length of headers" might be a bit tricky to determine
> >> generically :-(.
> >
> > Well you don't need to be generic, you need to know the maximum length
> > of headers that might appear in a TSO skb.
> >
> > Ethernet + VLAN tag + IPv6 + TCP + timestamp option = 90 bytes, but I'm
> > not sure whether there can be other IP or TCP options in a TSO skb.  I'd
> > really like to get the TSO requirements clearly documented somewhere.
> 
> What about encapsulated IPSEC, IP-in-IP-tunnels, etc. ?

xen-netfront doesn't offload GSO for those, unless I'm much mistaken.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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