From: "H. Peter Anvin" Subject: Re: [PATCH 1/1] x86: fix text_poke Date: Fri, 25 Apr 2008 12:19:43 -0700 Message-ID: <48122ECF.405@zytor.com> References: <20080425151931.GA25510@elte.hu> <20080425152650.GA894@elte.hu> <20080425154854.GC3265@one.firstfloor.org> <20080425161916.GD3265@one.firstfloor.org> <20080425163035.GE9503@Krystal> <481209F2.4050908@zytor.com> <20080425170929.GA16180@Krystal> <20080425183748.GB16180@Krystal> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Andi Kleen , Linus Torvalds , Ingo Molnar , Jiri Slaby , David Miller , zdenek.kabelac@gmail.com, rjw@sisk.pl, paulmck@linux.vnet.ibm.com, akpm@linux-foundation.org, linux-ext4@vger.kernel.org, herbert@gondor.apana.org.au, penberg@cs.helsinki.fi, clameter@sgi.com, linux-kernel@vger.kernel.org, pageexec@freemail.hu, Jeremy Fitzhardinge To: Mathieu Desnoyers Return-path: Received: from terminus.zytor.com ([198.137.202.10]:46137 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754096AbYDYT1c (ORCPT ); Fri, 25 Apr 2008 15:27:32 -0400 In-Reply-To: <20080425183748.GB16180@Krystal> Sender: linux-ext4-owner@vger.kernel.org List-ID: Mathieu Desnoyers wrote: > >>> b) there might be a jump into the middle of this instruction sequence? >>> >> If we change that, as discussed above, so the liveliness of ZF and of >> the %al register is still insured by leaving the mov and test >> instructions in place, we end up only modifying a single instruction and >> the problem fades away. We would end up changing a jne for a jmp. > > So, if we do is I propose here, we have to take into account this > question too. Any jump that jumps in the middle of this instruction > sequence would have to insure correct liveliness of %al and ZF. However, > since we just limited the scope of their liveliness, there are no other > code paths which can jump in the middle of our instruction sequence and > insure correct ZF and %al liveliness. > I wanted to point out that this, in particular, is utter nonsense. Consider a sequence that looks something like this: if (foo ? bar : imv_cond(var)) { blah(); } An entirely sane transformation of this (as far as gcc is concerned), is something like: cmpl $0,foo je 1f cmpl $0,bar jmp 2f 1: #APP movb var,%al /* This is your imv */ #NO_APP testb %al,%al 2: je 3f call blah 3: Your code would take the movb-testb-je sequence and combine them, then we jump into the middle of the new instruction when jumping at 2! There are only two ways to deal with this - extensive analysis of the entire flow of control, or telling the compiler exactly what is *actually* going on. The latter is the preferred way, obviously. -hpa