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Date: Fri, 11 Apr 2014 17:49:28 -0700 From: Eric Dumazet <eric.dumazet@...il.com> To: Arvid Brodin <arvid.brodin@...en.se> Cc: "netdev@...r.kernel.org" <netdev@...r.kernel.org> Subject: Re: TCP checksum problem On Sat, 2014-04-12 at 02:25 +0200, Arvid Brodin wrote: > Hi, > > I'm trying to insert a 6-byte HSR header into a newly copied skb, > between the ethernet header and the IP header. The skb comes from my > virtual device's ndo_start_xmit() function and is to be sent on to a > physical interface after modification. Essentially, this is what I'm > doing: > > skb = skb_copy_expand(skb, skb_headroom(skb) + HSR_HLEN, 0, > GFP_ATOMIC); > > /* Move ethernet header to make room for the HSR header */ > src = skb_mac_header(skb); > WARN_ON_ONCE(src != skb->data); > dst = skb_push(skb, HSR_HLEN); > memmove(dst, src, ETH_HLEN); > skb_reset_mac_header(skb); > > /* Fill in the 6-byte header */ > hsr_fill_tag(skb, frame, port); > > /* Point the skb to the physical interface */ > skb->dev = port->dev; > > dev_queue_xmit(skb); > > But something goes wrong with the TCP checksum on the way out. Arp, > ping etc works fine, but anything using TCP is discarded by the > receiver due to incorrect TCP checksums. > > Using wireshark and some printks, I've managed to decide that the > updated TCP checksum is written offset by -6 bytes from where it > should be (into what wireshark calls the TCP header "acknowledgement > number" field). > > Any idea why this happens? I've spent a few days on this now and I'm > running out of ideas... > > This is run on the cadence/macb ethernet device. Before the call to > skb_copy_expand(), skb->ip_summed is CHECKSUM_PARTIAL, skb->csum_start > is 120, and skb->csum_offset is 16. The total frame length is 80 bytes > (on the wire as reported by wireshark - I think this is excluding the > FCS). > > You need to set skb->csum_start to the new offset. ( skb_transport_header(skb) - skb->head ) Check how __tcp_v4_send_check() sets csum_start & csum_offset (csum_offset you can leave it as is) -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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