From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1161428AbXBZXwj (ORCPT ); Mon, 26 Feb 2007 18:52:39 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1161421AbXBZXwi (ORCPT ); Mon, 26 Feb 2007 18:52:38 -0500 Received: from gundega.hpl.hp.com ([192.6.19.190]:64923 "EHLO gundega.hpl.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1161428AbXBZXwh (ORCPT ); Mon, 26 Feb 2007 18:52:37 -0500 Date: Mon, 26 Feb 2007 15:51:54 -0800 From: Stephane Eranian To: linux-kernel@vger.kernel.org Cc: linux-ia64@vger.kernel.org, Stephane Eranian , ak@suse.de, akpm@linux-foundation.org, tony.luck@intel.com Subject: debug registers and fork Message-ID: <20070226235154.GB19630@frankl.hpl.hp.com> Reply-To: eranian@hpl.hp.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.4.1i Organisation: HP Labs Palo Alto Address: HP Labs, 1U-17, 1501 Page Mill road, Palo Alto, CA 94304, USA. E-mail: eranian@hpl.hp.com X-HPL-MailScanner: Found to be clean X-HPL-MailScanner-From: eranian@hpl.hp.com Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Hello, I have come across an issue with a monitoring using the hardware debug registers on ia64/i386/x86-64. It seems that the way debug registers are inherited across fork differs between ia-64 and i386/x86-64. On ia-64, the debug registers are NEVER inherited in the child. The copy_thread() routine clears the necessary thread flags to avoid reloading the debug registers in the child. Now, on x86-64, it appears that the TIF_DEBUG flag is inherited via setup_thread_stack(). By virtue of dup_task_struct() the debug registers get copied into the child task on fork. So the child has active breakpoints, unless I am mistaken somewhere. Given the way the ptrace() interface works, I would tend to think that the ia-64 way is the correct one. Any comment? Furthermore, on i386/x86-64, when switching out from a task with TIF_DEBUG enabled to another which does not, it seems we do not clear the debug registers (at least dr7) so they become inactive. -- -Stephane