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:

quoted

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));