From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932372AbbCXSnS (ORCPT ); Tue, 24 Mar 2015 14:43:18 -0400 Received: from mail-wi0-f176.google.com ([209.85.212.176]:37835 "EHLO mail-wi0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932350AbbCXSnQ (ORCPT ); Tue, 24 Mar 2015 14:43:16 -0400 Date: Tue, 24 Mar 2015 19:43:11 +0100 From: Ingo Molnar To: Denys Vlasenko Cc: Andy Lutomirski , Linus Torvalds , Steven Rostedt , Borislav Petkov , "H. Peter Anvin" , Oleg Nesterov , Frederic Weisbecker , Alexei Starovoitov , Will Drewry , Kees Cook , x86@kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH] x86/asm/entry/64: Improve the THREAD_INFO() macro explanation Message-ID: <20150324184311.GA14760@gmail.com> References: <1426785469-15125-1-git-send-email-dvlasenk@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1426785469-15125-1-git-send-email-dvlasenk@redhat.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org >>From 0229a184997a7d4ad4398ee3ac2f5ae78c1c1a03 Mon Sep 17 00:00:00 2001 From: Ingo Molnar Date: Tue, 24 Mar 2015 18:57:13 +0100 Subject: [PATCH] x86/asm/entry/64: Improve the THREAD_INFO() macro explanation Explain the background, and add a real example. Cc: Alexei Starovoitov Cc: Andy Lutomirski Cc: Andy Lutomirski Cc: Borislav Petkov Cc: Borislav Petkov Cc: Denys Vlasenko Cc: Frederic Weisbecker Cc: H. Peter Anvin Cc: Kees Cook Cc: Linus Torvalds Cc: Oleg Nesterov Cc: Steven Rostedt Cc: Will Drewry Signed-off-by: Ingo Molnar --- arch/x86/include/asm/thread_info.h | 27 +++++++++++++++++++++++---- 1 file changed, 23 insertions(+), 4 deletions(-) diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h index ad0ee3423da5..813dfbb867a7 100644 --- a/arch/x86/include/asm/thread_info.h +++ b/arch/x86/include/asm/thread_info.h @@ -206,10 +206,29 @@ static inline unsigned long current_stack_pointer(void) _ASM_SUB $(THREAD_SIZE),reg ; /* - * ASM operand which evaluates to thread_info address - * if it is known that "reg" is exactly "off" bytes below stack top. - * Example (fetch thread_info->fieldname): - * mov TI_fieldname+THREAD_INFO(reg, off),%eax + * ASM operand which evaluates to a 'thread_info' address of + * the current task, if it is known that "reg" is exactly "off" + * bytes below the top of the stack currently. + * + * ( The kernel stack's size is known at build time, it is usually + * 2 or 4 pages, and the bottom of the kernel stack contains + * the thread_info structure. So to access the thread_info very + * quickly from assembly code we can calculate down from the + * top of the kernel stack to the bottom, using constant, + * build-time calculations only. ) + * + * For example, to fetch the current thread_info->flags value into %eax + * on x86-64 defconfig kernels, in syscall entry code where RSP is + * currently at exactly SIZEOF_PTREGS bytes away from the top of the + * stack: + * + * mov TI_flags+THREAD_INFO(%rsp, SIZEOF_PTREGS), %eax + * + * will translate to: + * + * 8b 84 24 b8 c0 ff ff mov -0x3f48(%rsp), %eax + * + * which is below the current RSP by almost 16K. */ #define THREAD_INFO(reg, off) ((off)-THREAD_SIZE)(reg)