netdev - Re: [PATCH net v3 3/4] udp: do not transition UDP GRO fraglist partial checksums to unnecessary

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Message-ID: <171136303579.5526.5377651702776757800@kwain>
Date: Mon, 25 Mar 2024 11:37:15 +0100
From: Antoine Tenart <atenart@...nel.org>
To: 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 v3 3/4] udp: do not transition UDP GRO fraglist partial checksums to unnecessary

Quoting Willem de Bruijn (2024-03-22 18:29:40)
> Antoine Tenart wrote:
> > UDP GRO validates checksums and in udp4/6_gro_complete fraglist packets
> > are converted to CHECKSUM_UNNECESSARY to avoid later checks. However
> > this is an issue for CHECKSUM_PARTIAL packets as they can be looped in
> > an egress path and then their partial checksums are not fixed.
> > 
> > Different issues can be observed, from invalid checksum on packets to
> > traces like:
> > 
> >   gen01: hw csum failure
> >   skb len=3008 headroom=160 headlen=1376 tailroom=0
> >   mac=(106,14) net=(120,40) trans=160
> >   shinfo(txflags=0 nr_frags=0 gso(size=0 type=0 segs=0))
> >   csum(0xffff232e ip_summed=2 complete_sw=0 valid=0 level=0)
> >   hash(0x77e3d716 sw=1 l4=1) proto=0x86dd pkttype=0 iif=12
> >   ...
> > 
> > Fix this by only converting CHECKSUM_NONE packets to
> > CHECKSUM_UNNECESSARY by reusing __skb_incr_checksum_unnecessary.
> > 
> > Fixes: 9fd1ff5d2ac7 ("udp: Support UDP fraglist GRO/GSO.")
> > Signed-off-by: Antoine Tenart <atenart@...nel.org>
> 
> Should fraglist UDP GRO and non-fraglist (udp_gro_complete_segment)
> have the same checksumming behavior?

They can't as non-fraglist GRO packets can be aggregated, csum can't
just be converted there. It seems non-fraglist handles csum as it should
already, except for tunneled packets but that's why patch 4 prevents
those packets from being GROed.

> Second, this leaves CHECKSUM_COMPLETE as is. Is that intentional? I
> don't immediately see where GSO skb->csum would be updated.

That is intentional, fraglist GSO packets aren't modified and csums
don't need to be updated. The issues are with converting the checksum
type: partial checksum information can be lost.

Thanks,
Antoine