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 0B3E2C433EF for ; Wed, 24 Nov 2021 17:42:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1349713AbhKXRpc (ORCPT ); Wed, 24 Nov 2021 12:45:32 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:38773 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1349710AbhKXRpa (ORCPT ); Wed, 24 Nov 2021 12:45:30 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1637775740; h=from:from:reply-to:subject:subject: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=izYL/FQ2z7YLasw10KZFgXFc7Wmsk5GHBcxx7ULv/IU=; b=c4CwZJP/u04NP7Q9AF0ILeBuXBgTZgrN9ebaZxyKGnzSb5HsXSk1I1YVq9auoI1vl+7PWS vrBMMAcLEoYRcGYNsZDfCwQmtzmXtFkUpNDIudxLPU/aeohBjU67uYgyOusHfQ68sj2Gda SJ4wAMBeTviQnD8aqDbkG0GuewAEzh8= Received: from mail-qt1-f199.google.com (mail-qt1-f199.google.com [209.85.160.199]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-210-bOonBuCgPmyGp3qfKlxHCQ-1; Wed, 24 Nov 2021 12:42:18 -0500 X-MC-Unique: bOonBuCgPmyGp3qfKlxHCQ-1 Received: by mail-qt1-f199.google.com with SMTP id p7-20020a05622a00c700b002b2f6944e7dso2684115qtw.10 for ; Wed, 24 Nov 2021 09:42:18 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=izYL/FQ2z7YLasw10KZFgXFc7Wmsk5GHBcxx7ULv/IU=; b=FGH+MsUHmV2OUqfPpOz21nCWNPirKIyxaAS+nnzAZ0yZY8TwXuTQiXy0PkZHxSFWKb xXRoleNhBvDTpd6xyQvkdkxMF5KA/p3goprK7/jqwAqt/vzI5L7eoK/j8JybcVRXFxOu OmbFvaCr6+USuoOcj04kLqbZ33uHGU22HLdFrO2CyFUX1KiPUjnR88X9fii2aKAyouxk ypMNS8IXnXa5DVYPz4qHM3ioPbAwNxiNflR91UeQoj3FO7OpX+XzvVo9fKShc7nHVbRi UZHVy8dXbyiPdcDb3CQvOk698xklyiDJGFa96YQfowkHeyLulzMdOD78vpR0beKb43hz 8njg== X-Gm-Message-State: AOAM532exVbW5YmIzeE0ne+4ebj94S3l2E0Bg/9wfZ3yflF8uebnFnXT BzbaWTvMUq6E3RbuwoXAx20SOwKV5e9J89/s304AjqI23mnHRJ38n5KJRdJDUS9Q+7hYVvqdwL6 7hcoYuConSuyFZ4d+O8ZCjFovK7Fy3w== X-Received: by 2002:ac8:5acb:: with SMTP id d11mr9480984qtd.109.1637775737719; Wed, 24 Nov 2021 09:42:17 -0800 (PST) X-Google-Smtp-Source: ABdhPJwEk1ANv+XdmOXXh7xqRV6RKm7Oa+OWbJGtmCj1kiN5VP0aoRVqfKaRZ2d6egiSWtnPRlOdSw== X-Received: by 2002:ac8:5acb:: with SMTP id d11mr9480924qtd.109.1637775737447; Wed, 24 Nov 2021 09:42:17 -0800 (PST) Received: from treble ([2600:1700:6e32:6c00::15]) by smtp.gmail.com with ESMTPSA id u18sm198553qki.69.2021.11.24.09.42.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 24 Nov 2021 09:42:16 -0800 (PST) Date: Wed, 24 Nov 2021 09:42:13 -0800 From: Josh Poimboeuf To: Petr Mladek 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: <20211124174213.mspehbgomdqarxea@treble> References: <20211109210736.GV174703@worktop.programming.kicks-ass.net> <2734a37ebed2432291345aaa8d9fd47e@AcuMS.aculab.com> <20211112015003.pefl656m3zmir6ov@treble> MIME-Version: 1.0 In-Reply-To: Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=jpoimboe@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Precedence: bulk List-ID: X-Mailing-List: linux-toolchains@vger.kernel.org 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. -- Josh