netdev - Re: Questions on sk_no

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Date:	Wed, 07 May 2014 16:15:11 -0400 (EDT)
From:	David Miller <davem@...hat.com>
To:	therbert@...gle.com
Cc:	netdev@...r.kernel.org
Subject: Re: Questions on sk_no_check implemenation

From: Tom Herbert <therbert@...gle.com>
Date: Mon, 5 May 2014 19:34:40 -0700

> Hi, I'm looking at sk_no_check implementation and am hoping to get
> clarification on a couple of use cases.
> 
> 1) In l2tp_core.c, if sk_no_check is set it looks like this will
> completely bypass UDP checksum verification on RX even for packets
> with nonzero checksums.
> 
> d2cf33616 (Benjamin LaHaise 2012-04-27 08:24:18 +0000 505) if
> (sk->sk_no_check || skb_csum_unnecessary(skb))
> 
> I don't believe this is standards compliant and seems pretty risky
> otherwise. What is the requirement here? Is this still needed?

I think this is simply a thinko and the code should be checking non-zero
csums.

> 2) The constant UDP_CSUM_NORCV was introduced for sunrpc and in fact
> this looks like the only code that sets it. AFAICT the only effect
> this would have would be to disable receive checksum verification in
> L2TP as described above. What was the intent of UDP_CSUM_NORCV and why
> was using this specific to sunrpc?

I have no idea about this case.
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