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 C73BFC433EF for ; Wed, 24 Nov 2021 17:42:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1349838AbhKXRpi (ORCPT ); Wed, 24 Nov 2021 12:45:38 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:56084 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S243479AbhKXRp3 (ORCPT ); Wed, 24 Nov 2021 12:45:29 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1637775739; 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=a8lhy+VtLw+2GnZQ7mw/ecSfRhwGxktu9nwcEFIo9tZ3VR+W3NRmZsvqgRRLzRelvumMmq 2MRE8j90PTr7ziGEUMFFY1MVZYAuY6sqfnkTTfN2nwDgaInyFm+F4vUf9cBmapnAaqeEzg Y90VjBgsn1w/Vo5X902/SQewsoH0Fb0= 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-344-yro7IeCYOLCIOwihWFcPIQ-1; Wed, 24 Nov 2021 12:42:18 -0500 X-MC-Unique: yro7IeCYOLCIOwihWFcPIQ-1 Received: by mail-qt1-f199.google.com with SMTP id v19-20020ac85793000000b002b19184b2bfso2677419qta.14 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=E9pmWZxz2fpFcZ7WbVk/FJ33BE0dOCNHYghURjd6wCwhQwnS7QsS6VNfDt91FxFBj+ 58YQkQSavT8oOFyUXFVWkmZOPOUywcFceemcYFfZszDH2cbyh7nAAYzfxuE4exwR2lAf aetzVc/KZ4/Fq22UcIHjMCXClrCmwGBSKpBt7PUDriy/SqU0Un2+6hthUMNNlXhzmvsh tnvJ3QcSOy0TauFpATT9tArPLaKJljpcZaTFeA/GFxVHBgg0hIGE3WC6KYPejR5YT4br tI9PENfR0c1dS+Z34/6gT6Wh7jidhSORXN+UpFdqyH10dOzUB59mn2l5IPRoKUCWoJNX 53xA== X-Gm-Message-State: AOAM53397u2zzpcGIp87TSUkp3a+8JEITj45QM8qi0WDh1E2nMNNI+kY uLIxNy5UYcEGSoSmIWf2Uo50/TAnZvwuxrg5enfRnAZOUk7+3mRLNu2zpjJ/BT4r8vQHoUezi7V Xf4D9XHLFETYAHWNKaVEAkV9x X-Received: by 2002:ac8:5acb:: with SMTP id d11mr9480979qtd.109.1637775737716; 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 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@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