linux-kernel - Re: [GIT PULL] Additional x86 fixes for 2.6.31-rc5

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Message-ID: <alpine.LFD.2.01.0908021811130.3352@localhost.localdomain>
Date:	Sun, 2 Aug 2009 18:14:40 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Tejun Heo <tj@...nel.org>
cc:	"H. Peter Anvin" <hpa@...or.com>,
	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] Additional x86 fixes for 2.6.31-rc5



On Mon, 3 Aug 2009, Tejun Heo wrote:
> 
> Yeap, this part is strange.  I can't understand why it was done that
> way.  Interestingly, the segment override doesn't trigger any
> exception, it just gets ignored.  Is there a way to retrieve %gs value
> without accessing memory?  ie. other than %gs:identity_gs?

No. There's a "load segment limit" instruction, but not a base one. It's 
intentional - segments are supposed to be "anonymous" in protected mode 
(in real more of vm86 mode, the base is normally the segment register 
shifted left by four of course, but even that isn't _strictly_ always 
true).

You're very much not supposed to look at the base.

			Linus
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