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Date:	Wed, 9 Apr 2014 18:08:47 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	David Miller <davem@...emloft.net>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"olaf@...fle.de" <olaf@...fle.de>,
	"apw@...onical.com" <apw@...onical.com>,
	"jasowang@...hat.com" <jasowang@...hat.com>
Subject: RE: [PATCH net 3/3] Drivers: net: hyperv: Address UDP checksum
 issues



> -----Original Message-----
> From: David Miller [mailto:davem@...emloft.net]
> Sent: Wednesday, April 9, 2014 10:01 AM
> To: KY Srinivasan
> Cc: netdev@...r.kernel.org; linux-kernel@...r.kernel.org;
> devel@...uxdriverproject.org; olaf@...fle.de; apw@...onical.com;
> jasowang@...hat.com
> Subject: Re: [PATCH net 3/3] Drivers: net: hyperv: Address UDP checksum
> issues
> 
> From: "K. Y. Srinivasan" <kys@...rosoft.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.

Thanks Dave. I will COW the SKB before changing the checksum value. I do have a question though.
I looked at a bunch of hardware drivers and they modify the header information, specially checksum
field and I could not see where they had COWed the skb. I was grepping for skb_cow in these drivers.


Regards,

K. Y
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