netdev - Re: [PATCH nf-next-2.6] netfilter: add xt

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Date:	Thu, 22 Jul 2010 18:24:14 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jan Engelhardt <jengelh@...ozas.de>
Cc:	Patrick McHardy <kaber@...sh.net>,
	Netfilter Development Mailinglist 
	<netfilter-devel@...r.kernel.org>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH nf-next-2.6] netfilter: add xt_cpu match

Le jeudi 22 juillet 2010 à 17:39 +0200, Jan Engelhardt a écrit :

> Never say never. "long" already bit people in the past, and now we
> have that CONFIG_COMPAT stuff.
> 

I know pretty well the "long" problem, I received one of the first alpha
machine ever built in the world (DEC 3000 AXP, with a fast 133 MHz
cpu ;) ), before I began to use Linux :)


> If invert is the only flag, perhaps it makes sense to use __u8 
> for it. 
> 

Quite frankly it brings more problems than plain u32

- Possible security problems (padding bytes). Not applicable to
iptables.

- Some arches have slow byte/short accesses (21064 for example :) )

"int" is the natural type, fast on all arches.

- Given alignment requirements of iptables rules, using less than 32bits
here saves no ram.

But I dont care that much.

I even see compiler doesnt want to use a XOR instruction :

00000018 <cpu_mt>:
  18:	55                   	push   %ebp
  19:	8b 42 04             	mov    0x4(%edx),%eax
  1c:	64 8b 15 00 00 00 00 	mov    %fs:0x0,%edx
  23:	89 e5                	mov    %esp,%ebp
  25:	5d                   	pop    %ebp
  26:	39 10                	cmp    %edx,(%eax)
  28:	0f 94 c2             	sete   %dl
  2b:	0f b6 d2             	movzbl %dl,%edx
  2e:	3b 50 04             	cmp    0x4(%eax),%edx
  31:	0f 95 c0             	setne  %al
  34:	c3                   	ret    




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