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 X-Spam-Level: X-Spam-Status: No, score=-11.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 91164C4363A for ; Thu, 29 Oct 2020 15:03:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 41CD120759 for ; Thu, 29 Oct 2020 15:03:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727839AbgJ2PDN (ORCPT ); Thu, 29 Oct 2020 11:03:13 -0400 Received: from mx2.suse.de ([195.135.220.15]:51376 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727836AbgJ2PDN (ORCPT ); Thu, 29 Oct 2020 11:03:13 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id C9D1CAF0C; Thu, 29 Oct 2020 15:03:11 +0000 (UTC) Date: Thu, 29 Oct 2020 16:03:09 +0100 (CET) From: Miroslav Benes To: Petr Mladek cc: Steven Rostedt , linux-kernel@vger.kernel.org, Masami Hiramatsu , Andrew Morton , Josh Poimboeuf , Jiri Kosina , Joe Lawrence , live-patching@vger.kernel.org Subject: Re: [PATCH 6/9] livepatch/ftrace: Add recursion protection to the ftrace callback In-Reply-To: <20201029145709.GD16774@alley> Message-ID: References: <20201028115244.995788961@goodmis.org> <20201028115613.291169246@goodmis.org> <20201029145709.GD16774@alley> User-Agent: Alpine 2.21 (LSU 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Precedence: bulk List-ID: X-Mailing-List: live-patching@vger.kernel.org On Thu, 29 Oct 2020, Petr Mladek wrote: > On Thu 2020-10-29 14:51:06, Miroslav Benes wrote: > > On Wed, 28 Oct 2020, Steven Rostedt wrote: > > > Hm, I've always thought that we did not need any kind of recursion > > protection for our callback. It is marked as notrace and it does not call > > anything traceable. In fact, it does not call anything. I even have a note > > in my todo list to mark the callback as RECURSION_SAFE :) > > Well, it calls WARN_ON_ONCE() ;-) Oh my, I learned to ignore these. Of course there is printk hidden everywhere. > > At the same time, it probably does not hurt and the patch is still better > > than what we have now without RECURSION_SAFE if I understand the patch set > > correctly. > > And better be on the safe side. Agreed. > > > Cc: Josh Poimboeuf > > > Cc: Jiri Kosina > > > Cc: Miroslav Benes > > > Cc: Petr Mladek > > > Cc: Joe Lawrence > > > Cc: live-patching@vger.kernel.org > > > Signed-off-by: Steven Rostedt (VMware) > > > --- > > > kernel/livepatch/patch.c | 5 +++++ > > > 1 file changed, 5 insertions(+) > > > > > > diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c > > > index b552cf2d85f8..6c0164d24bbd 100644 > > > --- a/kernel/livepatch/patch.c > > > +++ b/kernel/livepatch/patch.c > > > @@ -45,9 +45,13 @@ static void notrace klp_ftrace_handler(unsigned long ip, > > > struct klp_ops *ops; > > > struct klp_func *func; > > > int patch_state; > > > + int bit; > > > > > > ops = container_of(fops, struct klp_ops, fops); > > > > > > + bit = ftrace_test_recursion_trylock(); > > > + if (bit < 0) > > > + return; > > > > This means that the original function will be called in case of recursion. > > That's probably fair, but I'm wondering if we should at least WARN about > > it. > > Yeah, the early return might break the consistency model and > unexpected things might happen. We should be aware of it. > Please use: > > if (WARN_ON_ONCE(bit < 0)) > return; > > WARN_ON_ONCE() might be part of the recursion. But it should happen > only once. IMHO, it is worth the risk. Agreed. Miroslav