From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 070ECC433F5 for ; Thu, 25 Nov 2021 08:20:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1348646AbhKYIXX (ORCPT ); Thu, 25 Nov 2021 03:23:23 -0500 Received: from smtp-out1.suse.de ([195.135.220.28]:37786 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1348819AbhKYIVW (ORCPT ); Thu, 25 Nov 2021 03:21:22 -0500 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id C084921B36; Thu, 25 Nov 2021 08:18:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1637828290; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=e3lTvL7X/pLPkMChteUmxKRux3aIsQksbSl01FKz2GE=; b=WDt3JM+g5EKhl7/+0IB6E9eEn4PIn/sOOufrX1Vr22rNIfKaxC7dyYvVUuCvMw4I+eAyf6 o6hesNFh9GKyC2w3eFFQ5uekDJfLeFgjZHokRr/yMjbhKrLLcl+AYzHxU8QesiqUqyYt04 t+LlRjmWOhh0CIybJAX82uTBBC4RsiU= Received: from suse.cz (unknown [10.100.216.66]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id 8CF79A3B85; Thu, 25 Nov 2021 08:18:10 +0000 (UTC) Date: Thu, 25 Nov 2021 09:18:09 +0100 From: Petr Mladek To: Josh Poimboeuf Cc: David Laight , 'Peter Zijlstra' , Nick Desaulniers , Bill Wendling , "x86@kernel.org" , "linux-kernel@vger.kernel.org" , "mark.rutland@arm.com" , "dvyukov@google.com" , "seanjc@google.com" , "pbonzini@redhat.com" , "mbenes@suse.cz" , "llvm@lists.linux.dev" , "linux-toolchains@vger.kernel.org" , live-patching@vger.kernel.org Subject: Re: [PATCH 20/22] x86,word-at-a-time: Remove .fixup usage Message-ID: References: <20211109210736.GV174703@worktop.programming.kicks-ass.net> <2734a37ebed2432291345aaa8d9fd47e@AcuMS.aculab.com> <20211112015003.pefl656m3zmir6ov@treble> <20211124174213.mspehbgomdqarxea@treble> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20211124174213.mspehbgomdqarxea@treble> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 2021-11-24 09:42:13, Josh Poimboeuf wrote: > On Mon, Nov 22, 2021 at 06:46:44PM +0100, Petr Mladek wrote: > > On Thu 2021-11-11 17:50:03, Josh Poimboeuf wrote: > > > On Wed, Nov 10, 2021 at 12:20:47PM +0000, David Laight wrote: > > > > > > Wouldn't moving part of a function to .text.cold (or .text.unlikely) > > > > > > generate the same problems with the stack backtrace code as the > > > > > > .text.fixup section you are removing had?? > > > > > > > > > > GCC can already split a function into func and func.cold today (or > > > > > worse: func, func.isra.N, func.cold, func.isra.N.cold etc..). > > > > > > > > > > I'm assuming reliable unwind and livepatch know how to deal with this. > > > > > > > > They'll have 'proper' function labels at the top - so backtrace > > > > stands a chance. > > > > Indeed you (probably) want it to output "func.irsa.n.cold" rather > > > > than just "func" to help show which copy it is in. > > > > > I guess that livepatch will need separate patches for each > > > > version of the function - which might be 'interesting' if > > > > all the copies actually need patching at the same time. > > > > You'd certainly want a warning if there seemed to be multiple > > > > copies of the function. > > > > > > Hm, I think there is actually a livepatch problem here. > > > > > > If the .cold (aka "child") function actually had a fentry hook then we'd > > > be fine. Then we could just patch both "parent" and "child" functions > > > at the same time. We already have the ability to patch multiple > > > functions having dependent interface changes. > > > > > > But there's no fentry hook in the child, so we can only patch the > > > parent. > > > > > > If the child schedules out, and then the parent gets patched, things can > > > go off-script if the child later jumps back to the unpatched version of > > > the parent, and then for example the old parent tries to call another > > > patched function with a since-changed ABI. > > > > This thread seems to be motivation for the patchset > > https://lore.kernel.org/all/20211119090327.12811-1-mbenes@suse.cz/ > > I am trying to understand the problem here, first. And I am > > a bit lost. > > > > How exactly is child called in the above scenario, please? > > How could parent get livepatched when child is sleeping? > > > > I imagine it the following way: > > > > parent_func() > > fentry > > > > /* some parent code */ > > jmp child > > /* child code */ > > jmp back_to_parent > > /* more parent code */ > > ret > > Right. > > > In the above example, parent_func() would be on stack and could not > > get livepatched even when the process is sleeping in the child code. > > > > The livepatching is done via ftrace. Only code with fentry could be > > livepatched. And code called via fentry must be visible on stack. > > How would parent_func() be on the stack? If it jumps to the child then > it leaves no trace on the stack. Grr, sure. It was off-by-one error on my side. /o\ Thanks for explanation. Best Regards, Petr