From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755131AbbCXTWr (ORCPT ); Tue, 24 Mar 2015 15:22:47 -0400 Received: from mail.skyhub.de ([78.46.96.112]:37979 "EHLO mail.skyhub.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754937AbbCXTWn (ORCPT ); Tue, 24 Mar 2015 15:22:43 -0400 Date: Tue, 24 Mar 2015 20:20:57 +0100 From: Borislav Petkov To: Ingo Molnar Cc: Denys Vlasenko , Andy Lutomirski , Linus Torvalds , Steven Rostedt , "H. Peter Anvin" , Oleg Nesterov , Frederic Weisbecker , Alexei Starovoitov , Will Drewry , Kees Cook , x86@kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] x86/asm/entry/64: Improve the THREAD_INFO() macro explanation Message-ID: <20150324192057.GD11525@pd.tnic> References: <1426785469-15125-1-git-send-email-dvlasenk@redhat.com> <20150324184311.GA14760@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20150324184311.GA14760@gmail.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 On Tue, Mar 24, 2015 at 07:43:11PM +0100, Ingo Molnar wrote: > 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) Vehemently-with-both-thumbs-up-acked-by: Borislav Petkov -- Regards/Gruss, Boris. ECO tip #101: Trim your mails when you reply. --