netdev - CHECKSUM_UNNECESSARY semantics with driver modified.created packets

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Date:   Fri, 17 Feb 2017 13:04:34 -0800
From:   Tom Herbert <tom@...bertland.com>
To:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: CHECKSUM_UNNECESSARY semantics with driver modified.created packets

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.

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

Tom