On Wed, Nov 24, 2021 at 5:59 PM Noah Goldstein [off-list ref] 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 !

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