linux-kernel - Re: [PATCH x86/mm 6/6] x86-64 ia32 ptrace get/putreg32 current task

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Message-ID: <474F449B.70603@redhat.com>
Date:	Thu, 29 Nov 2007 18:00:43 -0500
From:	Chuck Ebbert <cebbert@...hat.com>
To:	Roland McGrath <roland@...hat.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH x86/mm 6/6] x86-64 ia32 ptrace get/putreg32 current task

On 11/29/2007 05:21 PM, Roland McGrath wrote:
>>>  	case offsetof(struct user32, regs.gs):
>>>  		*val = child->thread.gsindex;
>>> +		if (child == current)
>>> +			asm("movl %%gs,%0" : "=r" (*val));
>> Won't this return the kernel's GS instead of the user's?
> [...]
>> But this is x86_64, where swapgs is done on kernel entry.
> 
> As I understand it, and from what the documentation I have says, swapgs has
> nothing to do with the %gs selector.  It affects the "GS base register",
> i.e. the MSR.
> 

Yep, I confused the GS selector with the base address in the descriptor.
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