Subject: Re: [PATCH] lro: IP fragment checking

From: Ben Hutchings <[email-masked]>
Date: Tue, 02 Dec 2008 15:18:48 +0000

On Tue, 2008-12-02 at 09:42 -0500, Andrew Gallatin wrote:
> Ben Hutchings wrote:
> > On Mon, 2008-12-01 at 19:02 -0500, Andrew Gallatin wrote:
> >> Ben Hutchings wrote:
[...]
> >>> 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.
> 
> Indeed, and that is the patch I intend to submit if the fragment
> check in inet_lro is rejected.  I still think the check belongs
> in the inet lro code though, and I'm worried it is being rejected
> for the wrong reasons..

There's a wide variety of capabilities of different hardware:

1. No checksum offload. Probably not worth using LRO.
2. Full-checksum generation. Driver passes packets to inet_lro;
get_frag_header() or get_skb_header() parses packets to check that they
are TCP/IPv4 and to validate the checksum. inet_lro does further checks.
3. L4 packet parsing and checksum validation. Driver passes TCP/IPv4
packets to inet_lro. inet_lro does further checks.
4. Hardware/firmware LRO. inet_lro not needed.

You seem to be proposing that a check that is only needed in case (2)
should also be applied in case (3).  Maybe it would make more sense to
define a generic implementation of get_frag_header() for full-checksum
devices, if that's possible?

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.