Subject: Re: [PATCH] lro: IP fragment checking
From: Ben Hutchings <bhutchings@solarflare.com>
To: Andrew Gallatin <gallatin@myri.com>
Cc: David Miller <davem@davemloft.net>, ossthema@de.ibm.com,
       netdev@vger.kernel.org, linux-kernel@vger.kernel.org, tklein@de.ibm.com,
       raisch@de.ibm.com, jb.billaud@gmail.com, hering2@de.ibm.com
In-Reply-To: <49347B0B.8030705@myri.com>
References: <4933A74F.3050809@de.ibm.com>	<493423D7.5030203@myri.com>
	 <20081201.131810.158631503.davem@davemloft.net> <49345CCA.1030209@myri.com>
	 <1228169379.3073.13.camel@achroite> <49347B0B.8030705@myri.com>
Content-Type: text/plain
Organization: Solarflare Communications
Date: Tue, 02 Dec 2008 00:18:49 +0000
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On Mon, 2008-12-01 at 19:02 -0500, Andrew Gallatin wrote:
> Ben Hutchings wrote:
> > On Mon, 2008-12-01 at 16:53 -0500, Andrew Gallatin wrote:
> >> David Miller wrote:
> >>> From: Andrew Gallatin <gallatin@myri.com>
> >>> Date: Mon, 01 Dec 2008 12:50:15 -0500
> >>>
> >>>> As to whether or not to do it in the drivers/hardware or in the
> >>>> LRO code, I favor doing it in the LRO code just so that it is not
> >>>> missed in some driver.
> >>> Then there is no point in the hardware doing the check, if
> >>> we're going to check it anyways.
> >>>
> >>> That's part of my point about why this check doesn't belong
> >>> here.
> >> What hardware does an explicit check for fragmentation?
> > 
> > Any that implements TCP/UDP checksumming properly.
> 
> How many do?

Good question. ;-)

> >> In most cases, aren't we just relying on the hardware checksum
> >> to be wrong on fragmented packets? That works 99.999% of the time,
> >> but the TCP checksum is pretty weak, and it is possible to
> >> have a fragmented packet where the first fragment has the same
> >> checksum as the entire packet.
> > [...]
> > 
> > If your hardware/firmware wrongly claims to be able to verify the
> > TCP/UDP checksum for an IP fragment, it seems to me you should deal with
> > that in your driver or fix the firmware.
> 
> We do partial checksums.

So you should check for IP fragmentation in your get_frag_header() along
with all the other checks you've got to do.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
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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