From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1032750AbdEWUmr (ORCPT ); Tue, 23 May 2017 16:42:47 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:51656 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1161456AbdEWUkE (ORCPT ); Tue, 23 May 2017 16:40:04 -0400 From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Will Deacon , Mark Rutland , Catalin Marinas Subject: [PATCH 4.4 084/103] arm64: xchg: hazard against entire exchange variable Date: Tue, 23 May 2017 22:09:50 +0200 Message-Id: <20170523200903.251772558@linuxfoundation.org> X-Mailer: git-send-email 2.13.0 In-Reply-To: <20170523200856.903752266@linuxfoundation.org> References: <20170523200856.903752266@linuxfoundation.org> User-Agent: quilt/0.65 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 4.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: Mark Rutland commit fee960bed5e857eb126c4e56dd9ff85938356579 upstream. The inline assembly in __XCHG_CASE() uses a +Q constraint to hazard against other accesses to the memory location being exchanged. However, the pointer passed to the constraint is a u8 pointer, and thus the hazard only applies to the first byte of the location. GCC can take advantage of this, assuming that other portions of the location are unchanged, as demonstrated with the following test case: union u { unsigned long l; unsigned int i[2]; }; unsigned long update_char_hazard(union u *u) { unsigned int a, b; a = u->i[1]; asm ("str %1, %0" : "+Q" (*(char *)&u->l) : "r" (0UL)); b = u->i[1]; return a ^ b; } unsigned long update_long_hazard(union u *u) { unsigned int a, b; a = u->i[1]; asm ("str %1, %0" : "+Q" (*(long *)&u->l) : "r" (0UL)); b = u->i[1]; return a ^ b; } The linaro 15.08 GCC 5.1.1 toolchain compiles the above as follows when using -O2 or above: 0000000000000000 : 0: d2800001 mov x1, #0x0 // #0 4: f9000001 str x1, [x0] 8: d2800000 mov x0, #0x0 // #0 c: d65f03c0 ret 0000000000000010 : 10: b9400401 ldr w1, [x0,#4] 14: d2800002 mov x2, #0x0 // #0 18: f9000002 str x2, [x0] 1c: b9400400 ldr w0, [x0,#4] 20: 4a000020 eor w0, w1, w0 24: d65f03c0 ret This patch fixes the issue by passing an unsigned long pointer into the +Q constraint, as we do for our cmpxchg code. This may hazard against more than is necessary, but this is better than missing a necessary hazard. Fixes: 305d454aaa29 ("arm64: atomics: implement native {relaxed, acquire, release} atomics") Acked-by: Will Deacon Signed-off-by: Mark Rutland Signed-off-by: Catalin Marinas Signed-off-by: Greg Kroah-Hartman --- arch/arm64/include/asm/cmpxchg.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- a/arch/arm64/include/asm/cmpxchg.h +++ b/arch/arm64/include/asm/cmpxchg.h @@ -49,7 +49,7 @@ static inline unsigned long __xchg_case_ " swp" #acq_lse #rel #sz "\t%" #w "3, %" #w "0, %2\n" \ " nop\n" \ " " #nop_lse) \ - : "=&r" (ret), "=&r" (tmp), "+Q" (*(u8 *)ptr) \ + : "=&r" (ret), "=&r" (tmp), "+Q" (*(unsigned long *)ptr) \ : "r" (x) \ : cl); \ \