netdev - Re: [PATCH net 1/1] openvswitch: Fix setting ipv6 fields causing hw csum failure

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Message-ID: <5ee3304d-162a-e5b2-f8e9-5a4d52c71216@nvidia.com>
Date:   Thu, 10 Feb 2022 10:53:24 +0200
From:   Paul Blakey <paulb@...dia.com>
To:     Jakub Kicinski <kuba@...nel.org>
CC:     <dev@...nvswitch.org>, <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Pravin B Shelar <pshelar@....org>, <davem@...emloft.net>,
        Eelco Chaudron <echaudro@...hat.com>,
        Oz Shlomo <ozsh@...dia.com>, "Vlad Buslov" <vladbu@...dia.com>,
        Roi Dayan <roid@...dia.com>,
        Ariel Levkovich <lariel@...dia.com>
Subject: Re: [PATCH net 1/1] openvswitch: Fix setting ipv6 fields causing hw
 csum failure



On Wed, 9 Feb 2022, Jakub Kicinski wrote:

> On Mon, 7 Feb 2022 16:41:01 +0200 Paul Blakey wrote:
> > Ipv6 ttl, label and tos fields are modified without first
> > pulling/pushing the ipv6 header, which would have updated
> > the hw csum (if available). This might cause csum validation
> > when sending the packet to the stack, as can be seen in
> > the trace below.
> > 
> > Fix this by calling postpush/postpull checksum calculation
> > which will update the hw csum if needed.
> 
> > -static void set_ipv6_fl(struct ipv6hdr *nh, u32 fl, u32 mask)
> > +static void set_ipv6_dsfield(struct sk_buff *skb, struct ipv6hdr *nh, __u8 ipv6_tclass, __u8 mask)
> >  {
> > +	skb_postpull_rcsum(skb, nh, 4);
> > +
> > +	ipv6_change_dsfield(nh, ~mask, ipv6_tclass);
> > +
> > +	skb_postpush_rcsum(skb, nh, 4);
> > +}
> 
> The calls seem a little heavy for single byte replacements.
> Can you instead add a helper based on csum_replace4() maybe?
> 
> BTW doesn't pedit have the same problem?
> 


I don't think they are heavier then csum_replace4, but they are more
bulletproof in my opinion, since they handle both the COMPLETE and PARTIAL
csum cases (in __skb_postpull_rcsum()) and resemble what editing of the 
packet should have done - pull the header, edit, and then push it back.

RE pedit, not really the issue here, but i guess pedit should be followed 
by act_csum with the relevant checksum fields (as we do when offloading 
ovs set() action to tc pedit + csum actions).