netdev - Re: CHECKSUM_UNNECESSARY semantics with driver modified.created packets

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Date:   Fri, 17 Feb 2017 13:29:39 -0800
From:   Tom Herbert <tom@...bertland.com>
To:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: CHECKSUM_UNNECESSARY semantics with driver modified.created packets

On Fri, Feb 17, 2017 at 1:04 PM, Tom Herbert <tom@...bertland.com> wrote:
> Alexei and I were looking at the mlx5 LRO code this morning. One
> discrepancy I noticed with GRO is that CHECKSUM_UNNECESSARY is set for
> the assembled packet whereas in GRO we use CHECKSUM_PARTIAL. The
> effect of using checksum-unnecessary is that the actual TCP checksum
> in the packet is not correct for the packet. Since gso_type set this
> probably is okay when packet is forwarding (checksums recomputed), but
> this seems to make some assumptions about the meaning of
> checksum-unnecessary. Looking if the rules of checksum in skbuff.h
> we're no explicit in saying whether the checksum actually in the
> packet must be correct.
>
> I think there's some possible ways to address this:
>
> 1) Allow actual checksum to be incorrect in packet when the packet is
> gso. On TX checksum must always be computed then.
> 2) Change the instances where driving is modifying a packet or
> creating a new one as in LRO case to use CHECKSUM_PARTIAL.
> 3) Modify the checksum in the packet so that it is correct. We do this
> in nearly all other cases where we modify the packet (e.g. NAT). Would
> be hard to do in LRO though.
>
On second thought, I don't think #1 is correct. For instance, if we
ever applied this to UDP that would break checksum-unnecessary
conversion. Also, the stack should always have the prerogative to not
trust checksum-unnecessary mark and do the calculation itself.
checksum-unnecessary should mean the checksum has been verified to be
correct and it is correct in the packet.

For LRO, drivers should use checksum-partial I think.

Tom

> In any case, I think we need to update sk_buff.h to clarify what the
> semantics are.
>
> Tom