netdev - Re: [PATCH net-next] net: fix recent csum changes

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Date:   Sat, 4 Dec 2021 10:33:55 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     kernel test robot <lkp@...el.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        "kbuild-all@...ts.01.org" <kbuild-all@...ts.01.org>,
        netdev <netdev@...r.kernel.org>,
        Vladimir Oltean <vladimir.oltean@....com>,
        David Lebrun <dlebrun@...gle.com>
Subject: Re: [PATCH net-next] net: fix recent csum changes

On Sat, Dec 4, 2021 at 6:00 AM David Laight <David.Laight@...lab.com> wrote:
>
> From: Eric Dumazet
> > Sent: 04 December 2021 04:41
> >
> > On Fri, Dec 3, 2021 at 7:34 PM kernel test robot <lkp@...el.com> wrote:
> > >
> > > I love your patch! Perhaps something to improve:
> ...
> >
> > Yes, keeping sparse happy with these checksum is not easy.
> >
> > I will add and use this helper, unless someone has a better idea.
> >
> > diff --git a/include/net/checksum.h b/include/net/checksum.h
> > index 5b96d5bd6e54532a7a82511ff5d7d4c6f18982d5..5218041e5c8f93cd369a2a3a46add3e6a5e41af7
> > 100644
> > --- a/include/net/checksum.h
> > +++ b/include/net/checksum.h
> > @@ -180,4 +180,8 @@ static inline void remcsum_unadjust(__sum16 *psum,
> > __wsum delta)
> >         *psum = csum_fold(csum_sub(delta, (__force __wsum)*psum));
> >  }
> >
> > +static inline __wsum wsum_negate(__wsum val)
> > +{
> > +       return (__force __wsum)-((__force u32)val);
> > +}
> >  #endif
>
> I was thinking that the expression also requires some comments.
> So maybe put a #define / static inline in checksum.h like:
>
> /* Subract the checksum of a buffer.
>  * The domain is __wsum is [1..~0u] (ie excludes zero)
>  * so ~csum_partial() cannot be used.
>  * The two's compliment gives the right answer provided the old 'csum'
>  * isn't zero - which it shouldn't be. */
> #define csum_partial_sub(buf, len, csum) (-csum_partial(buf, len, -(csum))
>
> and then add the annotations there to keep sparse happy there.
>
> will sparse accept '1 + ~csum' ? The compiler should use negate for it.
> It actually makes it slightly more obvious why the code is right.

Sparse is not happy with  1 + ~csum,

So we unfortunately would need something ugly like

(__force __wsum)(1 + ~(__force u32)csum)

Which most readers of this code will not find obvious.


>
>         David
>
> -
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