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

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Date:   Fri, 3 Dec 2021 20:40:51 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     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,
        netdev <netdev@...r.kernel.org>,
        Vladimir Oltean <vladimir.oltean@....com>,
        David Laight <David.Laight@...lab.com>,
        David Lebrun <dlebrun@...gle.com>
Subject: Re: [PATCH net-next] net: fix recent csum changes

On Fri, Dec 3, 2021 at 7:34 PM kernel test robot <lkp@...el.com> wrote:
>
> Hi Eric,
>
> I love your patch! Perhaps something to improve:
>
> [auto build test WARNING on net-next/master]
>
> url:    https://github.com/0day-ci/linux/commits/Eric-Dumazet/net-fix-recent-csum-changes/20211204-025401
> base:   https://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git 43332cf97425a3e5508c827c82201ecc5ddd54e0
> config: parisc-randconfig-s031-20211203 (https://download.01.org/0day-ci/archive/20211204/202112041104.gPgP3Z6U-lkp@intel.com/config)
> compiler: hppa64-linux-gcc (GCC) 11.2.0
> reproduce:
>         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
>         chmod +x ~/bin/make.cross
>         # apt-get install sparse
>         # sparse version: v0.6.4-dirty
>         # https://github.com/0day-ci/linux/commit/c13fbd113358fb59f76f76d25a1fdb57379c4b9c
>         git remote add linux-review https://github.com/0day-ci/linux
>         git fetch --no-tags linux-review Eric-Dumazet/net-fix-recent-csum-changes/20211204-025401
>         git checkout c13fbd113358fb59f76f76d25a1fdb57379c4b9c
>         # save the config file to linux build tree
>         mkdir build_dir
>         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-11.2.0 make.cross C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__' O=build_dir ARCH=parisc SHELL=/bin/bash net/core/ net/ipv4/
>
> If you fix the issue, kindly add following tag as appropriate
> Reported-by: kernel test robot <lkp@...el.com>
>

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