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

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Message-ID: <474F0389.5030003@zytor.com>
Date:	Thu, 29 Nov 2007 10:23:05 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Chuck Ebbert <cebbert@...hat.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Roland McGrath <roland@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH x86/mm 6/6] x86-64 ia32 ptrace get/putreg32 current task

Chuck Ebbert wrote:
> On 11/29/2007 01:09 PM, Linus Torvalds 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?
>> No, %gs is untouched by the kernel, so it contains user space version, and 
>> getting the value directly from %gs looks correct.
>>
> 
> But this is x86_64, where swapgs is done on kernel entry.
> 

For i386-x86_64 sharing, getting to the user segments probably should be 
macroized.  I'm thinking something like 
get_user_[cs|ds|es|fs|gs|ss](thread) in <asm/processor.h> doing the 
appropriate thing for different configurations.

	-hpa
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