From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965129AbWGKEVI (ORCPT ); Tue, 11 Jul 2006 00:21:08 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S965135AbWGKEVH (ORCPT ); Tue, 11 Jul 2006 00:21:07 -0400 Received: from ns.oss.ntt.co.jp ([222.151.198.98]:27524 "EHLO serv1.oss.ntt.co.jp") by vger.kernel.org with ESMTP id S965129AbWGKEVG (ORCPT ); Tue, 11 Jul 2006 00:21:06 -0400 Subject: Re: [Fastboot] [PATCH 1/3] stack overflow safe kdump (2.6.18-rc1-i386) - safe_smp_processor_id From: Fernando Luis =?ISO-8859-1?Q?V=E1zquez?= Cao To: "Eric W. Biederman" Cc: Keith Owens , akpm@osdl.org, James.Bottomley@steeleye.com, fastboot@lists.osdl.org, linux-kernel@vger.kernel.org, ak@suse.de In-Reply-To: References: <5742.1152520068@ocs3.ocs.com.au> <1152526550.3003.24.camel@localhost.localdomain> Content-Type: text/plain; charset=utf-8 Organization: =?UTF-8?Q?NTT=E3=82=AA=E3=83=BC=E3=83=97=E3=83=B3=E3=82=BD=E3=83=BC?= =?UTF-8?Q?=E3=82=B9=E3=82=BD=E3=83=95=E3=83=88=E3=82=A6=E3=82=A7?= =?UTF-8?Q?=E3=82=A2=E3=82=BB=E3=83=B3=E3=82=BF?= Date: Tue, 11 Jul 2006 13:21:01 +0900 Message-Id: <1152591661.2414.11.camel@localhost.localdomain> Mime-Version: 1.0 X-Mailer: Evolution 2.6.2 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Hi Eric! On Mon, 2006-07-10 at 05:37 -0600, Eric W. Biederman wrote: > Fernando Luis Vázquez Cao writes: > > > Hi Keith, > > > > Thank you for the comments. > > > > On Mon, 2006-07-10 at 18:27 +1000, Keith Owens wrote: > >> Fernando Luis Vazquez Cao (on Mon, 10 Jul 2006 16:50:52 +0900) wrote: > >> >On the event of a stack overflow critical data that usually resides at > >> >the bottom of the stack is likely to be stomped and, consequently, its > >> >use should be avoided. > >> > > >> >In particular, in the i386 and IA64 architectures the macro > >> >smp_processor_id ultimately makes use of the "cpu" member of struct > >> >thread_info which resides at the bottom of the stack. x86_64, on the > >> >other hand, is not affected by this problem because it benefits from > >> >the use of the PDA infrastructure. > >> > > >> >To circumvent this problem I suggest implementing > >> >"safe_smp_processor_id()" (it already exists in x86_64) for i386 and > >> >IA64 and use it as a replacement for smp_processor_id in the reboot path > >> >to the dump capture kernel. This is a possible implementation for i386. > >> > >> I agree with avoiding the use of thread_info when the stack might be > >> corrupt. However your patch results in reading apic data and scanning > >> NR_CPU sized tables for each IPI that is sent, which will slow down the > >> sending of all IPIs, not just dump. > > This patch only affects IPIs sent using send_IPI_allbutself which is > > rarely called, so the impact in performance should be negligible. > > Well smp_call_function uses it so I don't know if rarely called applies. > > However when called with the NMI vector every instance of send_IPI_allbutself > transforms this into send_IPI_mask. Which is why we need to know our current > cpu in the first place. > > Therefore why don't we just do that explicitly in crash.c > i.e. > > static void smp_send_nmi_allbutself(void) > { > cpumask_t mask = cpu_online_map; > cpu_clear(safe_smp_processor_id(), mask); > send_IPI_mask(mask, NMI_VECTOR); > } > > That will guarantee that any effects this code paranoia may have > are only seen in the crash dump path. That is a good idea, but I have on concern. In mach-default by default we use __send_IPI_shortcut (no_broadcast==0) instead of send_IPI_mask. Is it always safe to ignore the no_broadcast setting? In other words, can __send_IPI_shortcut be replaced by send_IPI_mask safely? The implementation of send_IPI_allbutself in the different architectures follows: smp_send_nmi_allbutself send_IPI_allbutself * mach-bigsmp send_IPI_allbutself cpu_clear(smp_processor_id(), mask) send_IPI_mask send_IPI_mask_sequence apic_wait_icr_idle * mach-default send_IPI_allbutself __local_send_IPI_allbutself if (no_broadcast) { cpu_clear(smp_processor_id(), mask) send_IPI_mask(mask, vector) send_IPI_mask_bitmask apic_wait_icr_idle } else { __send_IPI_shortcut(APIC_DEST_ALLBUT, vector) apic_wait_icr_idle } * mach-es7000 send_IPI_allbutself cpu_clear(smp_processor_id(), mask); send_IPI_mask send_IPI_mask_sequence apic_wait_icr_idle * mach-numaq send_IPI_allbutself cpu_clear(smp_processor_id(), mask) send_IPI_mask send_IPI_mask_sequence apic_wait_icr_idle * mach-summit send_IPI_allbutself cpu_clear(smp_processor_id(), mask) send_IPI_mask send_IPI_mask_sequence apic_wait_icr_idle Regards, Fernando > > > >> It would be far cheaper to define > >> a per-cpu variable containing the logical cpu number, set that variable > >> once as each cpu is brought up and just read it in cases where you > >> might not trust the integrity of struct thread_info. safe_smp_processor_id() > >> resolves to just a read of the per cpu variable. > > But to read a per-cpu variable you need to index the corresponding array > > with processor id of the current CPU (see code below), but that is > > precisely what we are trying to figure out. Anyway as > > send_IPI_allbutself is not a fast path (correct if this assumption is > > wrong) the current implementation of safe_smp_processor_id should be > > fine. > > > > #define get_cpu_var(var) (*({ preempt_disable(); > > &__get_cpu_var(var); })) > > #define __get_cpu_var(var) per_cpu(var, smp_processor_id()) > > > > Am I missing something obvious? > > No. Except that other architectures have cheaper per pointers so they > don't have that problem. > > Eric