From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH v3 04/22] x86/doublefault: Make memmove() notrace/NOKPROBE Date: Wed, 19 Feb 2020 17:34:09 +0100 Message-ID: <20200219163409.GI18400@hirez.programming.kicks-ass.net> References: <20200219144724.800607165@infradead.org> <20200219150744.604459293@infradead.org> <20200219103614.2299ff61@gandalf.local.home> <20200219154031.GE18400@hirez.programming.kicks-ass.net> <20200219155715.GD14946@hirez.programming.kicks-ass.net> <20200219160442.GE14946@hirez.programming.kicks-ass.net> <20200219111228.44c2999b@gandalf.local.home> <20200219162747.GX2935@paulmck-ThinkPad-P72> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20200219162747.GX2935@paulmck-ThinkPad-P72> Sender: linux-kernel-owner@vger.kernel.org To: "Paul E. McKenney" Cc: Steven Rostedt , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, mingo@kernel.org, joel@joelfernandes.org, gregkh@linuxfoundation.org, gustavo@embeddedor.com, tglx@linutronix.de, josh@joshtriplett.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, luto@kernel.org, tony.luck@intel.com, frederic@kernel.org, dan.carpenter@oracle.com, mhiramat@kernel.org List-Id: linux-arch.vger.kernel.org On Wed, Feb 19, 2020 at 08:27:47AM -0800, Paul E. McKenney wrote: > On Wed, Feb 19, 2020 at 11:12:28AM -0500, Steven Rostedt wrote: > > On Wed, 19 Feb 2020 17:04:42 +0100 > > Peter Zijlstra wrote: > > > > > > - memmove(&gpregs->ip, (void *)regs->sp, 5*8); > > > > + for (i = 0; i < count; i++) { > > > > + int idx = (dst <= src) ? i : count - i; > > > > > > That's an off-by-one for going backward; 'count - 1 - i' should work > > > better, or I should just stop typing for today ;-) > > > > Or, we could just cut and paste the current memmove and make a notrace > > version too. Then we don't need to worry bout bugs like this. > > OK, I will bite... > > Can we just make the core be an inline function and make a notrace and > a trace caller? Possibly going one step further and having one call > the other? (Presumably the traceable version invoking the notrace > version, but it has been one good long time since I have looked at > function preambles.) One complication is that GCC (and others) are prone to stick their own implementation of memmove() (and other string functions) in at 'random'. That is, it is up to the compiler's discretion wether or not to put a call to memmove() in or just emit some random giberish they feel has the same effect. So if we go play silly games like that, we need be careful (or just call __memmove I suppose, which is supposed to avoid that IIRC). From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from merlin.infradead.org ([205.233.59.134]:33020 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726712AbgBSQex (ORCPT ); Wed, 19 Feb 2020 11:34:53 -0500 Date: Wed, 19 Feb 2020 17:34:09 +0100 From: Peter Zijlstra Subject: Re: [PATCH v3 04/22] x86/doublefault: Make memmove() notrace/NOKPROBE Message-ID: <20200219163409.GI18400@hirez.programming.kicks-ass.net> References: <20200219144724.800607165@infradead.org> <20200219150744.604459293@infradead.org> <20200219103614.2299ff61@gandalf.local.home> <20200219154031.GE18400@hirez.programming.kicks-ass.net> <20200219155715.GD14946@hirez.programming.kicks-ass.net> <20200219160442.GE14946@hirez.programming.kicks-ass.net> <20200219111228.44c2999b@gandalf.local.home> <20200219162747.GX2935@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200219162747.GX2935@paulmck-ThinkPad-P72> Sender: linux-arch-owner@vger.kernel.org List-ID: To: "Paul E. McKenney" Cc: Steven Rostedt , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, mingo@kernel.org, joel@joelfernandes.org, gregkh@linuxfoundation.org, gustavo@embeddedor.com, tglx@linutronix.de, josh@joshtriplett.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, luto@kernel.org, tony.luck@intel.com, frederic@kernel.org, dan.carpenter@oracle.com, mhiramat@kernel.org Message-ID: <20200219163409.V0OShbxcAWIWGic4w7m-6ZWO76myRY5_fkFL9WON5hA@z> On Wed, Feb 19, 2020 at 08:27:47AM -0800, Paul E. McKenney wrote: > On Wed, Feb 19, 2020 at 11:12:28AM -0500, Steven Rostedt wrote: > > On Wed, 19 Feb 2020 17:04:42 +0100 > > Peter Zijlstra wrote: > > > > > > - memmove(&gpregs->ip, (void *)regs->sp, 5*8); > > > > + for (i = 0; i < count; i++) { > > > > + int idx = (dst <= src) ? i : count - i; > > > > > > That's an off-by-one for going backward; 'count - 1 - i' should work > > > better, or I should just stop typing for today ;-) > > > > Or, we could just cut and paste the current memmove and make a notrace > > version too. Then we don't need to worry bout bugs like this. > > OK, I will bite... > > Can we just make the core be an inline function and make a notrace and > a trace caller? Possibly going one step further and having one call > the other? (Presumably the traceable version invoking the notrace > version, but it has been one good long time since I have looked at > function preambles.) One complication is that GCC (and others) are prone to stick their own implementation of memmove() (and other string functions) in at 'random'. That is, it is up to the compiler's discretion wether or not to put a call to memmove() in or just emit some random giberish they feel has the same effect. So if we go play silly games like that, we need be careful (or just call __memmove I suppose, which is supposed to avoid that IIRC).