netdev - how to use skb_postpush

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Message-ID: <a8b99311d732d2627d57beb3970fab9cdcd0e4d2.camel@sipsolutions.net>
Date:   Tue, 21 Apr 2020 17:20:18 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     netdev@...r.kernel.org
Cc:     linux-wireless@...r.kernel.org
Subject: how to use skb_postpush_rcsum()?

Hi,

This is probably a stupid question but I'm hitting my head against the
wall ...

I have an skb. I have this:

        if (skb->ip_summed == CHECKSUM_COMPLETE) {
                printk(KERN_DEBUG "csum before\n");
                printk(KERN_DEBUG "  hw = 0x%.4x\n", skb->csum);
                printk(KERN_DEBUG "  sw = 0x%.4x\n", csum_fold(skb_checksum(skb, 0, skb->len, 0)));
        }

	ehdr = skb_push(skb, ETH_HLEN);
	memcpy(ehdr, &tmp, ETH_HLEN);
	skb_postpush_rcsum(skb, &tmp, ETH_HLEN);

        if (skb->ip_summed == CHECKSUM_COMPLETE) {
                printk(KERN_DEBUG "csum after\n");
                printk(KERN_DEBUG "  hw = 0x%.4x\n", csum_fold(skb->csum));
                printk(KERN_DEBUG "  sw = 0x%.4x\n", csum_fold(skb_checksum(skb, 0, skb->len, 0)));
        }

Why does this print, for example:

csum before
  hw = 0xce81
  sw = 0xce81
csum after
  hw = 0x5f36
  sw = 0xfc39

I'm clearly doing something wrong, but most of the examples seem to do
things this way, so what I'm I doing wrong?

johannes