Re: [PATCH] lro: IP fragment checking

[Andrew Gallatin]

Ben Hutchings wrote:
> 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?

Or maybe a generic lro_check_header() that can be called from
everybody's get_frag_header()/get_skb_header().  I guess what
bothers me is the division of checks between the get_*_header()
routine and lro_tcp_ip_checks() and the inevitable code
duplication in the get_*_header routines.

I still don't understand why an unneeded check for fragmentation
in case (3) is any more objectionable than the existing tcp
flags checks in lro_tcp_ip_check(), many of which are surely
not needed in case (3) either.

Drew
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