netdev - Re: [PATCH 2/2] net: Discard and warn about LRO'd skbs received for forwarding

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Message-ID: <20080619224246.GA29360@solarflare.com>
Date:	Thu, 19 Jun 2008 23:42:51 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	linux-net-drivers@...arflare.com,
	Kieran Mansley <kmansley@...arflare.com>,
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH 2/2] net: Discard and warn about LRO'd skbs received for forwarding

Stephen Hemminger wrote:
> On Thu, 19 Jun 2008 19:08:34 +0100
> Ben Hutchings <bhutchings@...arflare.com> wrote:
> 
> > Add skb_warn_if_lro() to test whether an skb was received with LRO and
> > warn if so.
> > 
> > Change br_forward(), ip_forward() and ip6_forward() to call it) and
> > discard the skb if it returns true.
> 
> Actually shouldn't ip forwarding work because of refragmentation?

At the moment, LRO'd skbs have gso_size != 0 and ip_summed ==
CHECKSUM_UNNECESSARY (typically), and this results in taking the
GSO path on output and hitting a BUG() or WARN() in skb_segment_gso().
If you want to avoid this without dropping packets, you can avoid the
condition by either:

- setting gso_size = 0 - should result in fragmentation
- setting ip_summed = CHECKSUM_NONE - should result in GSO
  (maybe skb_forward_csum() could do that)

But I didn't try either of these because I think it's fundamentally
dodgy to apply LRO to forwarded packets.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
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