netdev - Re: [Patch net] net: invert the check of detecting hardware RX checksum fault

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Date:   Mon, 19 Nov 2018 11:25:47 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Tom Herbert <tom@...bertland.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [Patch net] net: invert the check of detecting hardware RX
 checksum fault

On Sun, Nov 18, 2018 at 8:02 PM Herbert Xu <herbert@...dor.apana.org.au> wrote:
>
> On Fri, Nov 16, 2018 at 01:32:50PM -0800, Cong Wang wrote:
> >
> > This is true only when there is a skb_checksum_init*() or
> > skb_checksum_validate*() prior to it, it seems not true for
> > nf_ip_checksum() where skb->csum is correctly set to pesudo header
> > checksum but there is no validation of the original skb->csum.
> > So this check should be still inverted there??
> >
> > Or am I still missing anything here?
>
> What do you mean? My copy of nf_ip_checksum seems to be doing the
> right thing as far as verifying CHECKSUM_COMPLETED goes.

Hmm, it calls csum_tcpudp_magic() directly instead of
__skb_checksum_validate_complete(), but it also sets ip_summed
to CHECKSUM_UNNECESSARY:

 20                 if ((protocol == 0 && !csum_fold(skb->csum)) ||
 21                     !csum_tcpudp_magic(iph->saddr, iph->daddr,
 22                                        skb->len - dataoff, protocol,
 23                                        skb->csum)) {
 24                         skb->ip_summed = CHECKSUM_UNNECESSARY;
 25                         break;
 26                 }

which means the rx checksum fault won't be triggered.