netdev - Aw: Re: question about checksumming and tcp

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Message-ID: <trinity-afda3924-c5a6-4998-98b5-ee047f5cbe74-1446837255645@3capp-1and1-bs04>
Date:	Fri, 6 Nov 2015 20:14:15 +0100
From:	ronny.hegewald@...ine.de
To:	"Tom Herbert" <tom@...bertland.com>
Cc:	netdev@...r.kernel.org
Subject: Aw: Re: question about checksumming and tcp_sendpage

> Well, looking at the code I don't readily see how this works either,
> but note that tcp_sendmsg has the same check also. If your analysis is
> correct then checksum for IPv6 would commonly be broken (i.e. several
> devices support IPv4 checksum but not IPv6)-- so I find that hard to
> believe so I'm probably missing something too! (at least this
> ambiguity is one more reason why we need to get rid of NETIF_F_IP_CSUM
> and NETIF_F_V6_CSUM!).

I think the checksum-building is not broken because of this on itself. The
problem i see is for example in case pages are sent from a block-device 
over the net. 

Depending on the block-device this pages might be changed at any time. 
Because of the described issue the page-data wouldn't be copied as a call 
to sock_no_sendpage would do it. So the page-data might change at any time
during or after the checksum-building. From my understanding this would just
cause a crc-error anytime after sending and would only lead to a resent 
of the data, probably with a new checksum which will then be correct So
there wouldn't be a obvious impact except some bad tcp packages.

> Can you try to verify this a bug with some testing?

Will do.
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