netdev - Re: [PATCH v3] filter: Optimize instruction revalidation code.

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Date:	Wed, 17 Nov 2010 17:06:10 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	eric.dumazet@...il.com
Cc:	mjt@....msk.ru, davem@...emloft.net, drosenberg@...curity.com,
	hagen@...u.net, xiaosuo@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH v3] filter: Optimize instruction revalidation code.

Eric Dumazet wrote:
> > > But how about this:
> > > 
> > > enum {
> > >         BPF_S_RET_K = 1,
> > 
> > If BPF_S_* are only for kernel internal use, I think we don't need to translate
> > from the beginning because only net/core/filter.c uses BPF_S_*.
> > 
> > BPF_S_* are exposed to userspace via /usr/include/linux/filter.h since 2.6.36.
> > Is it no problem to change?
> 
> No problem, and Changli posted patch to move them to net/core/filter.c
> anyway.

> Please repost it when Changli patch is accepted by David 
> (if accepted :)), to get rid of the "+ 1"

But if Changli's patch is accepted, we can remove BPF_S_* like below.

 	switch (fentry->code) {
-	case BPF_S_ALU_ADD_X:
+	case BPF_ALU|BPF_ADD|BPF_X: /* BPF_S_ALU_ADD_X */
 		A += X;
 		continue;
-	case BPF_S_ALU_ADD_K:
+	case BPF_ALU|BPF_ADD|BPF_K: /* BPF_S_ALU_ADD_K */
 		A += f_k;
 		continue;
-	case BPF_S_ALU_SUB_X:
+	case BPF_ALU|BPF_SUB|BPF_X: /* BPF_S_ALU_SUB_X */
 		A -= X;
 		continue;
-	case BPF_S_ALU_SUB_K:
+	case BPF_ALU|BPF_SUB|BPF_K: /* BPF_S_ALU_SUB_K */
 		A -= f_k;
 		continue;

If compilers can generate better code for continuous case values
than discontinuous case values, then keeping BPF_S_* makes sense.
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