netdev - RE: How exactly does CHECKSUM

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Message-ID: <1412105832.30721.95.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Tue, 30 Sep 2014 12:37:12 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Yuval Mintz <Yuval.Mintz@...gic.com>
Cc:	"Tom Herbert (Partner - google)" <therbert@...gle.com>,
	netdev <netdev@...r.kernel.org>
Subject: RE: How exactly does CHECKSUM_COMPLETE works?

On Tue, 2014-09-30 at 12:22 -0700, Eric Dumazet wrote:
> On Tue, 2014-09-30 at 19:05 +0000, Yuval Mintz wrote:
> 
> > Can you pinpoint me to what exactly in the code corrects NAPI_GRO_CB(skb)->csum
> > by negating the ipv4 part in the ones' complete checksum?
> > 
> > E.g., for ipv6 [in ipv6_gro_receive()] I can see that skb_gro_postpull_rcsum() is
> > called after pulling the ipv6 header [and before proceeding to the next protocol],
> > but I can't seem to find anything similar to this for ipv4 [in inet_gro_receive()].
> > 
> 
> 
> IPv4 header is supposed to have a 0 checksum ;)
> 

Or if you prefer :

diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index 28e589c5f32d..92db7a69f2b9 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -1369,6 +1369,9 @@ static struct sk_buff **inet_gro_receive(struct sk_buff **head,
 	 * immediately following this IP hdr.
 	 */
 
+	/* Note : No need to call skb_gro_postpull_rcsum() here,
+	 * as we already checked checksum over ipv4 header was 0
+	 */
 	skb_gro_pull(skb, sizeof(*iph));
 	skb_set_transport_header(skb, skb_gro_offset(skb));
 


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