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SubjectRe: [PATCH net 3/3] Drivers: net: hyperv: Address UDP checksum issues
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From: "K. Y. Srinivasan" <kys@microsoft.com>
Date: Tue, 8 Apr 2014 12:46:11 -0700

> } else if (net_trans_info & INFO_UDP) {
> - csum_info->transmit.udp_checksum = 1;
> + /* UDP checksum offload is not supported on ws2008r2.
> + * Furthermore, on ws2012 and ws2012r2, there are some
> + * issues with udp checksum offload from Linux guests.
> + * (these are host issues).
> + * For now compute the checksum here.
> + */
> + struct udphdr *uh = udp_hdr(skb);
> + u16 udp_len = ntohs(uh->len);
> +
> + uh->check = 0;
> + uh->check = csum_tcpudp_magic(ip_hdr(skb)->saddr,
> + ip_hdr(skb)->daddr,
> + udp_len, IPPROTO_UDP,
> + csum_partial(uh, udp_len, 0));
> + if (uh->check == 0)
> + uh->check = CSUM_MANGLED_0;
> +
> + csum_info->transmit.udp_checksum = 0;

You absolutely cannot mangle packet header contents without first
COW'ing the SKB to make sure it's writable.

Otherwise network taps like tcpdump, and any other entity with a reference
to the packet, can see inconsistent state.


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