netdev - Re: GRE: Refactor GRE tunneling code.

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Message-ID: <20130402074441.GA28378@unicorn.suse.cz>
Date:	Tue, 2 Apr 2013 09:44:42 +0200
From:	Michal Kubecek <mkubecek@...e.cz>
To:	Dan Carpenter <dan.carpenter@...cle.com>
Cc:	pshelar@...ira.com, netdev@...r.kernel.org
Subject: Re: GRE: Refactor GRE tunneling code.

On Tue, Apr 02, 2013 at 09:34:21AM +0300, Dan Carpenter wrote:
> 
> I had a question about c54419321455: "GRE: Refactor GRE tunneling code."
> from Mar 25, 2013:
> 
> net/ipv4/ip_gre.c
>    392                  if (tpi->flags&TUNNEL_CSUM &&
>    393                      !(skb_shinfo(skb)->gso_type & SKB_GSO_GRE)) {
>    394                          *(__sum16 *)ptr = 0;
>    395                          *(__sum16 *)ptr = csum_fold(skb_checksum(skb, 0,
>    396                                                                   skb->len, 0));
> 
> What are we doing here when we set *ptr = 0 and then reset it on the
> next line?  Is there a ptr++ or ptr-- missing?

AFAICS, skb_checksum() calculates the checksum over a block containing
the checksum field the result is to be written into (which is where ptr
points). This fields needs to be reset first.

                                                        Michal Kubecek

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