netdev - Re: [PATCH] IPv4: skip loopback checksums in ip

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Date:	Mon, 19 Oct 2009 23:34:29 +0200
From:	Torsten Schmidt <torsten.schmidt@...06.tu-chemnitz.de>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [PATCH] IPv4: skip loopback checksums in ip_rcv()

Eric Dumazet:
> Torsten Schmidt a écrit :
>> Eric Dumazet wrotes:
>>> This is bogus IMHO.
>>>
>>> One bit could be corrupted in iph, and ntohl(iph->daddr) becomes 0x7fxxyyzz,
>>> we then accept a bogus frame. This is a RFC violation.
>>>
>>> This also slows down non loopback devices, adding an extra test.
>>>
>>> ip_fast_csum() is really fast (about 16 instructions).
>> 
>> Yes, you are right. So it would be better to only skip csum if *dev is 
>> our loopback interface ? Right ?
> 
> An application could send a bogus IP packet using RAW interface, and we still should
> check IP checksum before delivering this packet, even on loopback device.

Yes, then we should fix this comment in ip_rcv():

	RFC1122: 3.2.1.2 MUST silently discard any IP frame that fails the checksum.
	Is the datagram acceptable?
	1.	Length at least the size of an ip header
	2.	Version of 4
->	3.	Checksums correctly. [Speed optimisation for later, skip loopback checksums]
	4.	Doesn't have a bogus length

Right ?
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