From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753576Ab2HCQdU (ORCPT ); Fri, 3 Aug 2012 12:33:20 -0400 Received: from mx1.redhat.com ([209.132.183.28]:64899 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753475Ab2HCQdR (ORCPT ); Fri, 3 Aug 2012 12:33:17 -0400 Date: Fri, 3 Aug 2012 18:29:54 +0200 From: Oleg Nesterov To: Ingo Molnar Cc: Ananth N Mavinakayanahalli , Anton Arapov , "H. Peter Anvin" , Peter Zijlstra , Roland McGrath , Sebastian Andrzej Siewior , Srikar Dronamraju , linux-kernel@vger.kernel.org Subject: [PATCH 2/2] ptrace: fix set_task_blockstep()->update_debugctlmsr() logic Message-ID: <20120803162954.GA19806@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120803162912.GA19767@redhat.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Afaics the usage of update_debugctlmsr() in step.c was always very wrong. 1. update_debugctlmsr() was simply unneeded. The child sleeps TASK_TRACED, __switch_to_xtra(next_p => child) should notice TIF_BLOCKSTEP and set/clear DEBUGCTLMSR_BTF after resume if needed. 2. It is wrong. The state of DEBUGCTLMSR_BTF bit in CPU register should always match the state of current's TIF_BLOCKSTEP bit. 3. Even get_debugctlmsr() + update_debugctlmsr() itself does not look right. Irq can change other bits in MSR_IA32_DEBUGCTLMSR register or the caller can be preempted in between. However, now that uprobes uses user_enable_single_step(current) we can't simply remove update_debugctlmsr(). So this patch adds the additional "task == current" check and disables irqs to avoid the race with interrupts/preemption. Signed-off-by: Oleg Nesterov --- arch/x86/kernel/step.c | 6 ++++++ 1 files changed, 6 insertions(+), 0 deletions(-) diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c index afa60db..636402e 100644 --- a/arch/x86/kernel/step.c +++ b/arch/x86/kernel/step.c @@ -166,12 +166,18 @@ static void set_task_blockstep(struct task_struct *task, bool on) else clear_tsk_thread_flag(task, TIF_BLOCKSTEP); + if (task != current) + return; + + /* ensure irq/preemption can't change debugctl in between */ + local_irq_disable(); debugctl = get_debugctlmsr(); if (on) debugctl |= DEBUGCTLMSR_BTF; else debugctl &= ~DEBUGCTLMSR_BTF; update_debugctlmsr(debugctl); + local_irq_enable(); } /* -- 1.5.5.1