Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752999AbdGMUWM (ORCPT ); Thu, 13 Jul 2017 16:22:12 -0400 Received: from mail-lf0-f67.google.com ([209.85.215.67]:35230 "EHLO mail-lf0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752745AbdGMUWK (ORCPT ); Thu, 13 Jul 2017 16:22:10 -0400 Subject: Re: [PATCH] Revert "x86/uaccess: Add stack frame output operand in get_user() inline asm" To: Matthias Kaehlcke , Josh Poimboeuf Cc: Chris J Arges , Borislav Petkov , Thomas Gleixner , Ingo Molnar , "H . Peter Anvin" , x86@kernel.org, linux-kernel@vger.kernel.org, Douglas Anderson , Michael Davidson , Greg Hackmann , Nick Desaulniers , Stephen Hines , Kees Cook , Arnd Bergmann , Bernhard.Rosenkranzer@linaro.org References: <20170712212744.23660-1-mka@chromium.org> <20170712221242.puv5illztsla4nph@treble> <20170712222040.GD95735@google.com> <20170712223547.fyra43dizqooosbs@treble> <20170712223630.gb237t4vhrqeu5zd@treble> <20170712232213.GE95735@google.com> <20170713180001.mvwzdmudht56hdk5@treble> <20170713184748.GF95735@google.com> From: Andrey Rybainin Message-ID: <75850bb7-a60e-057d-d88b-afa0c79e94a0@gmail.com> Date: Thu, 13 Jul 2017 23:20:04 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0 MIME-Version: 1.0 In-Reply-To: <20170713184748.GF95735@google.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2212 Lines: 61 On 07/13/2017 09:47 PM, Matthias Kaehlcke wrote: > Thanks for your analysis! > >> What happens if you try the below patch instead of the revert? Any >> chance the offending instruction goes away? >> >> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h >> index 11433f9..beac907 100644 >> --- a/arch/x86/include/asm/uaccess.h >> +++ b/arch/x86/include/asm/uaccess.h >> @@ -171,7 +171,7 @@ __typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL), 0ULL, 0UL)) >> might_fault(); \ >> asm volatile("call __get_user_%P4" \ >> : "=a" (__ret_gu), "=r" (__val_gu), "+r" (__sp) \ >> - : "0" (ptr), "i" (sizeof(*(ptr)))); \ >> + : "0" (ptr), "i" (sizeof(*(ptr))), "r" (__sp)); \ >> (x) = (__force __typeof__(*(ptr))) __val_gu; \ >> __builtin_expect(__ret_gu, 0); \ >> }) > > The generated code is basically the same, only that now the value from > the stack is stored in a register and written twice to RSP: > AFAIR clang works much better with global named registers. Could you check if the patch bellow helps? --- arch/x86/include/asm/uaccess.h | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h index a059aac9e937..121204387978 100644 --- a/arch/x86/include/asm/uaccess.h +++ b/arch/x86/include/asm/uaccess.h @@ -157,15 +157,18 @@ __typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL), 0ULL, 0UL)) * Clang/LLVM cares about the size of the register, but still wants * the base register for something that ends up being a pair. */ + +register unsigned long __current_sp asm(_ASM_SP); + #define get_user(x, ptr) \ ({ \ int __ret_gu; \ register __inttype(*(ptr)) __val_gu asm("%"_ASM_DX); \ - register void *__sp asm(_ASM_SP); \ __chk_user_ptr(ptr); \ might_fault(); \ asm volatile("call __get_user_%P4" \ - : "=a" (__ret_gu), "=r" (__val_gu), "+r" (__sp) \ + : "=a" (__ret_gu), "=r" (__val_gu), \ + "+r" (__current_sp) \ : "0" (ptr), "i" (sizeof(*(ptr)))); \ (x) = (__force __typeof__(*(ptr))) __val_gu; \ __builtin_expect(__ret_gu, 0); \ -- 2.13.0