linux-kernel - Re: Fw: asm-x86/byteorder.h, CONFIG_X86

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Date:	Sat, 26 Jul 2008 10:00:37 -0700
From:	Harvey Harrison <harvey.harrison@...il.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Olaf Hering <olaf@...fle.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Fw: asm-x86/byteorder.h, CONFIG_X86_BSWAP leaks to userland

On Sat, 2008-07-26 at 12:48 -0400, H. Peter Anvin wrote:
> Harvey Harrison wrote:
> >>
> >> An undefined CONFIG_foo defaults to 0 (I think), so bswap is never used.
> >> Is this done on purpose, or can the CONFIG_ foo be moved inside
> >> __KERNEL__ somehow?
> > 
> > I believe it's there to prevent the bswap instruction from being used on
> > early x86_32 models (i386/i486).  As this will be 0 in userspace it is
> > effectively never using the bswap instruction for these routines.
> > 
> 
> i386, specifically.
> 
> However, you shouldn't leak these symbols to userspace; there is a 
> warning option in gcc for undefined macros, and it's a *good thing* to 
> use it.  Causing warnings in user space is not nice.

True, but the existing header in Linus' tree does exactly this, so unless
you beat me to it, I'll have a look to see what can be done here.

> 
> > I'm not sure if it's time yet to make the bswap ones be exported, as they
> > would no longer be usable for those early machines.  X86 guys CC:d.
> 
> On i386 we still default to i386-compatible binaries; I *think* gcc has 
> macros telling you if the user has used -march=i486 etc.
> 

Hmm, I wasn't aware of that, hopefully google will oblige.

Cheers,

Harvey

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