netdev - Re: [PATCH] net: multicast: calculate csum of looped-back and forwarded packets

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Date:   Sun, 24 Oct 2021 15:38:40 +0200
From:   Cyril Strejc <cyril.strejc@...da.cz>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH] net: multicast: calculate csum of looped-back and
 forwarded packets

On 2021-10-23T22:41:06-0400, Willem de Bruijn wrote:
> >
> > Alternatively, we could solve the CHECKSUM_NONE case by a simple,
> > practical and historical compatible "TX->RX translation" of ip_summed
> > in dev_loopback_xmit(), which keeps CHECKSUM_PARTIAL and leaves
> > __skb_checksum_validate_needed() as is:
> >
> >         if (skb->ip_summed == CHECKSUM_NONE)
> >                 skb->ip_summed = CHECKSUM_UNNECESSARY;
> >
> > or:
> >         if (skb->ip_summed != CHECKSUM_PARTIAL)
> >                 skb->ip_summed = CHECKSUM_UNNECESSARY;
> 
> Based on the idea that these packets are fully checksummed, so even if
> they loop to the tx path again with ip_summed CHECKSUM_UNNECESSARY,
> they will not cause the bug that you originally reported?
> 

It won't cause the bug. The original bug is caused solely by
CHECKSUM_PARTIAL being unconditionally translated to
CHECKSUM_UNNECESSARY in dev_loopback_xmit(). Adding the condition to
keep CHECKSUM_PARTIAL solves the issue.

> Yes, that looks like a nice solution.

I will double-check and send PATCH v2 to this e-mail thread.