Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750822AbWITBJr (ORCPT ); Tue, 19 Sep 2006 21:09:47 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750825AbWITBJr (ORCPT ); Tue, 19 Sep 2006 21:09:47 -0400 Received: from tomts25-srv.bellnexxia.net ([209.226.175.188]:51892 "EHLO tomts25-srv.bellnexxia.net") by vger.kernel.org with ESMTP id S1750822AbWITBJq (ORCPT ); Tue, 19 Sep 2006 21:09:46 -0400 Date: Tue, 19 Sep 2006 21:09:43 -0400 From: Mathieu Desnoyers To: Alan Cox Cc: Martin Bligh , prasanna@in.ibm.com, Andrew Morton , "Frank Ch. Eigler" , Ingo Molnar , Paul Mundt , linux-kernel , Jes Sorensen , Tom Zanussi , Richard J Moore , Michel Dagenais , Christoph Hellwig , Greg Kroah-Hartman , Thomas Gleixner , William Cohen , ltt-dev@shafik.org, systemtap@sources.redhat.com Subject: Re: [PATCH] Linux Kernel Markers Message-ID: <20060920010943.GA28097@Krystal> References: <20060919081124.GA30394@elte.hu> <451008AC.6030006@google.com> <20060919154612.GU3951@redhat.com> <4510151B.5070304@google.com> <20060919093935.4ddcefc3.akpm@osdl.org> <45101DBA.7000901@google.com> <20060919063821.GB23836@in.ibm.com> <45102641.7000101@google.com> <20060919175405.GC26339@Krystal> <1158710925.32598.120.camel@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline In-Reply-To: <1158710925.32598.120.camel@localhost.localdomain> X-Editor: vi X-Info: http://krystal.dyndns.org:8080 X-Operating-System: Linux/2.4.32-grsec (i686) X-Uptime: 21:03:17 up 27 days, 22:11, 2 users, load average: 0.13, 0.17, 0.09 User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2120 Lines: 44 * Alan Cox (alan@lxorguk.ukuu.org.uk) wrote: > Ar Maw, 2006-09-19 am 13:54 -0400, ysgrifennodd Mathieu Desnoyers: > > Very good idea.. However, overwriting the second instruction with a jump could > > be dangerous on preemptible and SMP kernels, because we never know if a thread > > has an IP in any of its contexts that would return exactly at the middle of the > > jump. > > No: on x86 it is the *same* case for all of these even writing an int3. > One byte or a megabyte, > > You MUST ensure that every CPU executes a serializing instruction before > it hits code that was modified by another processor. Otherwise you get > CPU errata and the CPU produces results which vendors like to describe > as "undefined". > > Thus you have to serialize, and if you are serializing it really doesn't > matter if you write a byte, a paragraph or a page. > Hi Alan, What I am trying to address is not "code patching with INT3", but "code patching with a 5 bytes JMP". The errata you point to applies to both and kprobes mechanism already takes care of this with the serialization method you describe. However, there is a supplemental problem with the fact that a JMP is 5 bytes, not 1. You are right about saying that overwriting code with any amount of *int3* does not matter, but what happens when you put one or more 5 bytes long jumps instead ? Think about it : if you are replacing 1-2-3 or 4 bytes long instruction and, unluckily, on any stack of any thread preempted from any CPU, you have a current instruction pointer pointing at the middle of the region where you want to put the 5 bytes JMP, the processor will likely trigger an illegal instruction fault when this particular thread is scheduled back. Mathieu OpenPGP public key: http://krystal.dyndns.org:8080/key/compudj.gpg Key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - 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/