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

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Date:   Sat, 4 Dec 2021 14:00:48 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Eric Dumazet' <edumazet@...gle.com>,
        kernel test robot <lkp@...el.com>
CC:     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

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.

	David

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