From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932364Ab2ICPYH (ORCPT ); Mon, 3 Sep 2012 11:24:07 -0400 Received: from mx1.redhat.com ([209.132.183.28]:25069 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932350Ab2ICPYF (ORCPT ); Mon, 3 Sep 2012 11:24:05 -0400 Date: Mon, 3 Sep 2012 17:26:09 +0200 From: Oleg Nesterov To: Ingo Molnar , Peter Zijlstra , Srikar Dronamraju Cc: Ananth N Mavinakayanahalli , Anton Arapov , "H. Peter Anvin" , Linus Torvalds , Roland McGrath , Sebastian Andrzej Siewior , linux-kernel@vger.kernel.org Subject: [PATCH 4/7] ptrace: Partly fix set_task_blockstep()->update_debugctlmsr() logic Message-ID: <20120903152609.GA9071@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120903152525.GA9028@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() and TIF_BLOCKSTEP 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. 4. It is not safe to play with TIF_BLOCKSTEP if task != current. DEBUGCTLMSR_BTF and TIF_BLOCKSTEP should always match each other if the task is running. The tracee is stopped but it can be SIGKILL'ed right before set/clear_tsk_thread_flag(). 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. Unfortunately this patch doesn't solve the last problem, we need another fix. Probably we should teach ptrace_stop() to set/clear single/block stepping after resume. And afaics there is yet another problem: perf can play with MSR_IA32_DEBUGCTLMSR from nmi, this obviously means that even __switch_to_xtra() has problems. Signed-off-by: Oleg Nesterov --- arch/x86/kernel/step.c | 14 +++++++++++++- 1 files changed, 13 insertions(+), 1 deletions(-) diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c index 7a51498..f89cdc6 100644 --- a/arch/x86/kernel/step.c +++ b/arch/x86/kernel/step.c @@ -161,6 +161,16 @@ static void set_task_blockstep(struct task_struct *task, bool on) { unsigned long debugctl; + /* + * Ensure irq/preemption can't change debugctl in between. + * Note also that both TIF_BLOCKSTEP and debugctl should + * be changed atomically wrt preemption. + * FIXME: this means that set/clear TIF_BLOCKSTEP is simply + * wrong if task != current, SIGKILL can wakeup the stopped + * tracee and set/clear can play with the running task, this + * can confuse the next __switch_to_xtra(). + */ + local_irq_disable(); debugctl = get_debugctlmsr(); if (on) { debugctl |= DEBUGCTLMSR_BTF; @@ -169,7 +179,9 @@ static void set_task_blockstep(struct task_struct *task, bool on) debugctl &= ~DEBUGCTLMSR_BTF; clear_tsk_thread_flag(task, TIF_BLOCKSTEP); } - update_debugctlmsr(debugctl); + if (task == current) + update_debugctlmsr(debugctl); + local_irq_enable(); } /* -- 1.5.5.1