From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S936413AbZDIUur (ORCPT ); Thu, 9 Apr 2009 16:50:47 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S936772AbZDIUue (ORCPT ); Thu, 9 Apr 2009 16:50:34 -0400 Received: from iolanthe.rowland.org ([192.131.102.54]:55985 "HELO iolanthe.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S936684AbZDIUud (ORCPT ); Thu, 9 Apr 2009 16:50:33 -0400 Date: Thu, 9 Apr 2009 16:50:31 -0400 (EDT) From: Alan Stern X-X-Sender: stern@iolanthe.rowland.org To: "K.Prasad" cc: Ingo Molnar , Linux Kernel Mailing List , Andrew Morton , Benjamin Herrenschmidt , Frederic Weisbecker , , Roland McGrath , Steven Rostedt Subject: Re: [Patch 00/11] Hardware Breakpoint interfaces In-Reply-To: <20090407082224.GA22500@in.ibm.com> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 7 Apr 2009, K.Prasad wrote: > A slight change though...writes to DR0-DR3 may fail if the address is > invalid. This behaviour is true even in existing implementation of > ptrace_set_debugreg(). That's okay. > > > > > > /* Lazy debug register switching */ > > > if (per_cpu(last_debugged_task, get_cpu()) != current) { > > > switch_to_none_hw_breakpoint(); > > > put_cpu_no_resched(); > > > } > > > > I just noticed that the lines saving DR7 and setting it to 0 need to > > come here. Otherwise switch_to_none_hw_breakpoint() might set DR7 back > > to a nonzero value, and it might not match the value stored in dr7. > > > > arch_uninstall_thread_hw_breakpoint()<--switch_to_none_hw_breakpoint() > will store 'kdr7' (which contains all kernel-space breakpoints in > encoded format) to DR7 physical register. Given that the current() > process should not have TIF_DEBUG() set (if it were set, > switch_to_thread_hw_breakpoint() would have been invoked to set > last_debugged_task), we will wipe out all user-space breakpoints and > store only kdr7. No, you don't understand. The code looks like this: > + get_debugreg(dr7, 7); > + > + /* Disable breakpoints during exception handling */ > + set_debugreg(0UL, 7); ... > + /* Lazy debug register switching */ > + if (per_cpu(last_debugged_task, get_cpu()) != current) { > + switch_to_none_hw_breakpoint(); > + put_cpu_no_resched(); > + } ... > + set_debugreg(dr7, 7); > + return rc; The first few lines will set dr7 to a value which includes the user breakpoints and will set DR7 to 0. The next few lines will set DR7 to kdr7, which might be non-zero. This is wrong; we need DR7 to be 0. Then the second-to-last line will set DR7 back to dr7, which is also wrong -- it should be set to kdr7. > > For each breakpoint where we decide it's a case of lazy DR switching or > > we invoke a "triggered" callback, the corresponding bit in dr6 should > > be cleared. This is a way of indicating to do_debug() that the handler > > has taken care of these causes of the exception. > > > > Similarly, the kprobe routine should clear the single-step bit in dr6 > > when it handles a single-step exception. When the notifier chain > > completes, the only bits remaining in dr6 should be for events that > > still need to be handled. > > > > Alan Stern > > > > This does sound like good design, but unfortunately there are pieces in > do_debug() which rely upon bits in dr6 being set even after the actual > breakpoint is handled (the get_si_code() is one such example). If necessary, do_debug() can keep two copies of dr6: the original version read from DR6 and the version modified by the notification handlers. > Do we go about changing them to use thread->debugreg6 instead of dr6? If > yes, wouldn't that be better done outside the HW Breakpoint patches as a > part of some cleanup initiative? Should they use thread->debugreg6? If they should, then change them. And no, the change should not be in a separate patch; it should be part of your series. Otherwise there would be intermediate kernels that behaved incorrectly. Alan Stern