Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754817AbYAAPWn (ORCPT ); Tue, 1 Jan 2008 10:22:43 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752185AbYAAPWg (ORCPT ); Tue, 1 Jan 2008 10:22:36 -0500 Received: from mx3.mail.elte.hu ([157.181.1.138]:36201 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752038AbYAAPWf (ORCPT ); Tue, 1 Jan 2008 10:22:35 -0500 Date: Tue, 1 Jan 2008 16:19:43 +0100 From: Ingo Molnar To: Quentin Barnes Cc: linux-kernel@vger.kernel.org, Masami Hiramatsu Subject: Re: [PATCH] [CFT] Code clarification patch to Kprobes arch code Message-ID: <20080101151943.GE4434@elte.hu> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.17 (2007-11-01) X-ELTE-VirusStatus: clean X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.3 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5340 Lines: 137 * Quentin Barnes wrote: > Since people are discussing some x86 Kprobes code cleanup, I thought I > would contribute a small change as well. When developing the Kprobes > arch code for ARM, I ran across some code found in x86 and s390 > Kprobes arch code which I didn't consider as good as it could be. > > Once I figured out what the code was doing, I changed the code for ARM > Kprobes to work the way I felt was more appropriate. I've tested the > code this way in ARM for about a year and would like to push the same > change to the other affected architectures. thanks Quentin, it looks good to me and i've applied the x86 bit to x86.git. (find the merged patch attached below) small note: > @@ -654,12 +655,12 @@ int __kprobes kprobe_exceptions_notify(struct > notifier_block *self, > ret = NOTIFY_STOP; your email client apparently line-wrapped this portion of the patch - i fixed it up manually (wasnt a big issue). Please see Documentation/email-clients.txt about how to set up your email client. Ingo --------------------> Subject: Code clarification patch to Kprobes arch code From: "Quentin Barnes" When developing the Kprobes arch code for ARM, I ran across some code found in x86 and s390 Kprobes arch code which I didn't consider as good as it could be. Once I figured out what the code was doing, I changed the code for ARM Kprobes to work the way I felt was more appropriate. I've tested the code this way in ARM for about a year and would like to push the same change to the other affected architectures. The code in question is in kprobe_exceptions_notify() which does: ==== /* kprobe_running() needs smp_processor_id() */ preempt_disable(); if (kprobe_running() && kprobe_fault_handler(args->regs, args->trapnr)) ret = NOTIFY_STOP; preempt_enable(); ==== For the moment, ignore the code having the preempt_disable()/ preempt_enable() pair in it. The problem is that kprobe_running() needs to call smp_processor_id() which will assert if preemption is enabled. That sanity check by smp_processor_id() makes perfect sense since calling it with preemption enabled would return an unreliable result. But the function kprobe_exceptions_notify() can be called from a context where preemption could be enabled. If that happens, the assertion in smp_processor_id() happens and we're dead. So what the original author did (speculation on my part!) is put in the preempt_disable()/preempt_enable() pair to simply defeat the check. Once I figured out what was going on, I considered this an inappropriate approach. If kprobe_exceptions_notify() is called from a preemptible context, we can't be in a kprobe processing context at that time anyways since kprobes requires preemption to already be disabled, so just check for preemption enabled, and if so, blow out before ever calling kprobe_running(). I wrote the ARM kprobe code like this: ==== /* To be potentially processing a kprobe fault and to * trust the result from kprobe_running(), we have * be non-preemptible. */ if (!preemptible() && kprobe_running() && kprobe_fault_handler(args->regs, args->trapnr)) ret = NOTIFY_STOP; ==== The above code has been working fine for ARM Kprobes for a year. So I changed the x86 code (2.6.24-rc6) to be the same way and ran the Systemtap tests on that kernel. As on ARM, Systemtap on x86 comes up with the same test results either way, so it's a neutral external functional change (as expected). This issue has been discussed previously on linux-arm-kernel and the Systemtap mailing lists. Pointers to the by base for the two discussions: http://lists.arm.linux.org.uk/lurker/message/20071219.223225.1f5c2a5e.en.html http://sourceware.org/ml/systemtap/2007-q1/msg00251.html Signed-off-by: Quentin Barnes Signed-off-by: Ingo Molnar Signed-off-by: Thomas Gleixner --- arch/x86/kernel/kprobes.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) Index: linux-x86.q/arch/x86/kernel/kprobes.c =================================================================== --- linux-x86.q.orig/arch/x86/kernel/kprobes.c +++ linux-x86.q/arch/x86/kernel/kprobes.c @@ -44,6 +44,7 @@ #include #include #include +#include #include #include #include @@ -951,12 +952,14 @@ int __kprobes kprobe_exceptions_notify(s ret = NOTIFY_STOP; break; case DIE_GPF: - /* kprobe_running() needs smp_processor_id() */ - preempt_disable(); - if (kprobe_running() && + /* + * To be potentially processing a kprobe fault and to + * trust the result from kprobe_running(), we have + * be non-preemptible. + */ + if (!preemptible() && kprobe_running() && kprobe_fault_handler(args->regs, args->trapnr)) ret = NOTIFY_STOP; - preempt_enable(); break; default: break; -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/