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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_2 autolearn=no 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 B284CC433E0 for ; Fri, 29 Jan 2021 19:03:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 68F0B64E0B for ; Fri, 29 Jan 2021 19:03:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232897AbhA2TD0 (ORCPT ); Fri, 29 Jan 2021 14:03:26 -0500 Received: from mail.kernel.org ([198.145.29.99]:50278 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232896AbhA2TCg (ORCPT ); Fri, 29 Jan 2021 14:02:36 -0500 Received: from gandalf.local.home (cpe-66-24-58-225.stny.res.rr.com [66.24.58.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 49AED64DE3; Fri, 29 Jan 2021 19:01:05 +0000 (UTC) Date: Fri, 29 Jan 2021 14:01:03 -0500 From: Steven Rostedt To: Peter Zijlstra Cc: Alexei Starovoitov , Masami Hiramatsu , Nikolay Borisov , LKML , Alexei Starovoitov , bpf , Josh Poimboeuf Subject: Re: kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter() with nmi_enter()") Message-ID: <20210129140103.3ce971b7@gandalf.local.home> In-Reply-To: <20210129175943.GH8912@worktop.programming.kicks-ass.net> References: <20210128123842.c9e33949e62f504b84bfadf5@gmail.com> <20210129013452.njuh3fomws62m4rc@ast-mbp.dhcp.thefacebook.com> <20210129224011.81bcdb3eba1227c414e69e1f@kernel.org> <20210129105952.74dc8464@gandalf.local.home> <20210129162438.GC8912@worktop.programming.kicks-ass.net> <20210129175943.GH8912@worktop.programming.kicks-ass.net> X-Mailer: Claws Mail 3.17.8 (GTK+ 2.24.33; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 29 Jan 2021 18:59:43 +0100 Peter Zijlstra wrote: > On Fri, Jan 29, 2021 at 09:45:48AM -0800, Alexei Starovoitov wrote: > > Same things apply to bpf side. We can statically prove safety for > > ftrace and kprobe attaching whereas to deal with NMI situation we > > have to use run-time checks for recursion prevention, etc. > > I have no idea what you're saying. You can attach to functions that are > called with random locks held, you can create kprobes in some very > sensitive places. > > What can you staticlly prove about that? I think the main difference is, if you attach a kprobe or ftrace function, you can theoretically analyze the location before you do the attachment. Does, the NMI context mean "in_nmi()" returns true? Because there's cases in ftrace callbacks where that is checked (like the stack tracer). And having ftrace return true for "in_nmi()" will break a lot of existing utilities. -- Steve