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From:   Eric Dumazet <edumazet@google.com>
Date:   Wed, 24 Nov 2021 18:56:31 -0800
Message-ID: <CANn89iJdQ1VTLYUKu1hYNm4wF__ZzrwNYr28v_vGM0MCybJpxg@mail.gmail.com>
Subject: Re: [tip:x86/core 1/1] arch/x86/um/../lib/csum-partial_64.c:98:12:
 error: implicit declaration of function 'load_unaligned_zeropad'
To:     Noah Goldstein <goldstein.w.n@gmail.com>
Cc:     Johannes Berg <johannes@sipsolutions.net>, alexanderduyck@fb.com,
        kbuild-all@lists.01.org, linux-kernel@vger.kernel.org,
        linux-um@lists.infradead.org, lkp@intel.com, peterz@infradead.org,
        x86@kernel.org
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On Wed, Nov 24, 2021 at 5:59 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>

>
> Hi, I'm not sure if this is intentional or not, but I noticed that the output
> of 'csum_partial' is different after this patch. I figured that the checksum
> algorithm is fixed so just wanted mention it incase its a bug. If not sorry
> for the spam.
>
> Example on x86_64:
>
> Buff: [ 87, b3, 92, b7, 8b, 53, 96, db, cd, 0f, 7e, 7e ]
> len : 11
> sum : 0
>
> csum_partial new : 2480936615
> csum_partial HEAD: 2472089390

No worries.

skb->csum is 32bit, but really what matters is the 16bit folded value.

So make sure to apply csum_fold() before comparing the results.

A minimal C and generic version of csum_fold() would be something like

static unsigned short csum_fold(u32 csum)
{
  u32 sum = csum;
  sum = (sum & 0xffff) + (sum >> 16);
  sum = (sum & 0xffff) + (sum >> 16);
  return ~sum;
}

I bet that csum_fold(2480936615) == csum_fold(2472089390)

It would be nice if we had a csum test suite, hint, hint ;)

Thanks !