linux-kernel - RE: [PATCH v3] net: dccp: Checksum verification enhancement

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Date:   Thu, 13 Jun 2019 06:51:34 +0000
From:   <Markus.Amend@...ekom.de>
To:     <davem@...emloft.net>
CC:     <linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>,
        <dccp@...r.kernel.org>
Subject: RE: [PATCH v3] net: dccp: Checksum verification enhancement

Hi David,

Yes, you are right, I overlooked this. Unfortunately the current receive process in the DCCP layer does from my view not properly support the skb->ip_summed flag verification, because the checksum validation takes place at different places. This would require some dirty hacks...

I see two options.

1. Adding the ip_summed flag verification 

or 2. Learn from the UDP stack

Since UDP/UDP-Lite are very similar to DCCP, at least from a checksum verification point, I ask myself if it would make sense to re-work DCCP's receive process according to the one of UDP/UDP-Lite? 
The relevant process in the udp stack (for IPv4) I identified therefore, can be found in /net/ipv4/udp.c, within the function __udp4_lib_rcv. There it is done, compared to DCCP, the other way round it starts with an udp4_csum_init and most likely a later udp_lib_checksum_complete. Both consider skb->ip_summed. If we would implement similar functions into the DCCP stack and adapt the DCCP rcv checksum validation process to the one in UDP could make probably more sense?!

Personally I prefer the second option, what do you think?

BR

Markus

> -----Original Message-----
> From: David Miller <davem@...emloft.net>
> Sent: Sonntag, 5. Mai 2019 18:53
> To: Amend, Markus <Markus.Amend@...ekom.de>
> Cc: linux-kernel@...r.kernel.org; netdev@...r.kernel.org;
> dccp@...r.kernel.org
> Subject: Re: [PATCH v3] net: dccp: Checksum verification enhancement
> 
> From: <Markus.Amend@...ekom.de>
> Date: Tue, 30 Apr 2019 16:11:07 +0000
> 
> > The current patch modifies the checksum verification of a received
> > DCCP packet, by adding the function __skb_checksum_validate into the
> > dccp_vX_rcv routine. The purpose of the modification is to allow the
> > verification of the skb->ip_summed flags during the checksum
> > validation process (for checksum offload purposes). As
> > __skb_checksum_validate covers the functionalities of skb_checksum and
> > dccp_vX_csum_finish they are needless and therefore removed.
> >
> > Signed-off-by: Nathalie Romo Moreno <natha.ro.moreno@...il.com>
> > Signed-off-by: Markus Amend <markus.amend@...ekom.de>
> 
> I don't see how this can be correct as you're not taking the csum coverage
> value into consideration at all.