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

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Date:	Mon, 19 Oct 2009 22:42:02 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Torsten Schmidt <torsten.schmidt@...06.tu-chemnitz.de>
CC:	"David S. Miller" <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [PATCH] IPv4: skip loopback checksums in ip_rcv()

Torsten Schmidt a écrit :
> Skip loopback checksum in ip_rcv() for speed optimisation.
> 
> Signed-off-by: Torsten Schmidt <schmto@....tu-chemnitz.de>
> ---
>  net/ipv4/ip_input.c |    7 ++++---
>  1 files changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/net/ipv4/ip_input.c b/net/ipv4/ip_input.c
> index 6c98b43..dc72286 100644
> --- a/net/ipv4/ip_input.c
> +++ b/net/ipv4/ip_input.c
> @@ -406,7 +406,7 @@ int ip_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt,
>  	 *
>  	 *	1.	Length at least the size of an ip header
>  	 *	2.	Version of 4
> -	 *	3.	Checksums correctly. [Speed optimisation for later, skip loopback checksums]
> +	 *	3.	Checksums correctly. [Speed optimisation: skip loopback checksums]
>  	 *	4.	Doesn't have a bogus length
>  	 */
>  
> @@ -418,8 +418,9 @@ int ip_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt,
>  
>  	iph = ip_hdr(skb);
>  
> -	if (unlikely(ip_fast_csum((u8 *)iph, iph->ihl)))
> -		goto inhdr_error;
> +	if (!ipv4_is_loopback(iph->daddr))
> +		if (unlikely(ip_fast_csum((u8 *)iph, iph->ihl)))
> +			goto inhdr_error;
>  
>  	len = ntohs(iph->tot_len);
>  	if (skb->len < len) {

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).

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