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

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Message-ID: <474F01F6.2030509@zytor.com>
Date:	Thu, 29 Nov 2007 10:16:22 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Chuck Ebbert <cebbert@...hat.com>,
	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

Linus Torvalds wrote:
> 
> But this one is correct:
> 
>>>  	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.
> 

Brief summary/reminder:

The kernel uses %fs in 32-bit mode and %gs in 64-bit mode.
User space TLS uses %gs in 32-bit mode and %fs in 64-bit mode.

The 64-bit kernel has to use %gs in order for SWAPGS to be available to 
it (by which time the 32-bit ABI was already fixed.)  It is advantageous 
for user space to use the register the kernel typically won't, in order 
to speed up system call entry/exit.

	-hpa
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