Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933002AbXK2SRd (ORCPT ); Thu, 29 Nov 2007 13:17:33 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755295AbXK2SRI (ORCPT ); Thu, 29 Nov 2007 13:17:08 -0500 Received: from terminus.zytor.com ([198.137.202.10]:36021 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932953AbXK2SRG (ORCPT ); Thu, 29 Nov 2007 13:17:06 -0500 Message-ID: <474F01F6.2030509@zytor.com> Date: Thu, 29 Nov 2007 10:16:22 -0800 From: "H. Peter Anvin" User-Agent: Thunderbird 2.0.0.9 (X11/20071115) MIME-Version: 1.0 To: Linus Torvalds CC: Chuck Ebbert , Roland McGrath , Andrew Morton , linux-kernel@vger.kernel.org, Thomas Gleixner , Ingo Molnar Subject: Re: [PATCH x86/mm 6/6] x86-64 ia32 ptrace get/putreg32 current task References: <20071129003849.428E026F8E7@magilla.localdomain> <20071129004222.E49AD26F8E7@magilla.localdomain> <474EF824.3020806@redhat.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1062 Lines: 30 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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/