netdev - Re: [PATCH net v2 3/4] udp: do not transition UDP fraglist to unnecessary checksum

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Date: Thu, 21 Mar 2024 14:13:35 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Antoine Tenart <atenart@...nel.org>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 davem@...emloft.net, 
 edumazet@...gle.com, 
 kuba@...nel.org, 
 pabeni@...hat.com
Cc: steffen.klassert@...unet.com, 
 willemdebruijn.kernel@...il.com, 
 netdev@...r.kernel.org
Subject: Re: [PATCH net v2 3/4] udp: do not transition UDP fraglist to
 unnecessary checksum

Antoine Tenart wrote:
> Quoting Willem de Bruijn (2024-03-21 15:58:17)
> > Antoine Tenart wrote:
> > > 
> > > If I sum up our discussion CHECKSUM_NONE conversion is wanted,
> > > CHECKSUM_UNNECESSARY conversion is a no-op and CHECKSUM_PARTIAL
> > > conversion breaks things. What about we just convert CHECKSUM_NONE to
> > > CHECKSUM_UNNECESSARY?
> > 
> > CHECKSUM_NONE cannot be converted to CHECKSUM_UNNECESSARY in the
> > receive path. Unless it is known to have been locally generated,
> > this means that the packet has not been verified yet.
> 
> I'm not sure to follow, non-partial checksums are being verified by
> skb_gro_checksum_validate_zero_check in udp4/6_gro_receive before ending
> up in udp4/6_gro_complete. That's also probably what the original commit
> msg refers to: "After validating the csum, we mark ip_summed as
> CHECKSUM_UNNECESSARY for fraglist GRO packets".
> 
> With fraglist, the csum can then be converted to CHECKSUM_UNNECESSARY.

Oh yes, of course.

> Except for CHECKSUM_PARTIAL, as we discussed.

Because that is treated as equivalent to CHECKSUM_UNNECESSARY in the
ingress path, and if forwarded to an egress path, csum_start and
csum_off are set correctly. Also for all segs after segmentation.
Okay, that sounds fine then.

There are two cases here: csum_start points to the outer header or it
points to the inner header. I suppose that does not matter for
correctness post segmentation.

> Does that make sense? Anything we can do to help moving this forward?
> 
> Thanks!
> Antoine