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 80B81C56201 for ; Wed, 18 Nov 2020 20:45:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3263C246C2 for ; Wed, 18 Nov 2020 20:45:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727234AbgKRUoi (ORCPT ); Wed, 18 Nov 2020 15:44:38 -0500 Received: from mail.kernel.org ([198.145.29.99]:52212 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726416AbgKRUoi (ORCPT ); Wed, 18 Nov 2020 15:44:38 -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 AB5CD24686; Wed, 18 Nov 2020 20:44:34 +0000 (UTC) Date: Wed, 18 Nov 2020 15:44:32 -0500 From: Steven Rostedt To: Segher Boessenkool Cc: Florian Weimer , Nick Desaulniers , Peter Zijlstra , Sami Tolvanen , Mathieu Desnoyers , linux-kernel , Matt Mullins , Ingo Molnar , Alexei Starovoitov , Daniel Borkmann , Dmitry Vyukov , Martin KaFai Lau , Song Liu , Yonghong Song , Andrii Nakryiko , John Fastabend , KP Singh , netdev , bpf , Kees Cook , Josh Poimboeuf , linux-toolchains@vger.kernel.org Subject: Re: violating function pointer signature Message-ID: <20201118154432.3e6e9c80@gandalf.local.home> In-Reply-To: <20201118194837.GO2672@gate.crashing.org> References: <375636043.48251.1605642440621.JavaMail.zimbra@efficios.com> <20201117153451.3015c5c9@gandalf.local.home> <20201118132136.GJ3121378@hirez.programming.kicks-ass.net> <20201118121730.12ee645b@gandalf.local.home> <20201118181226.GK2672@gate.crashing.org> <87o8jutt2h.fsf@mid.deneb.enyo.de> <20201118135823.3f0d24b7@gandalf.local.home> <20201118191127.GM2672@gate.crashing.org> <20201118143343.4e86e79f@gandalf.local.home> <20201118194837.GO2672@gate.crashing.org> X-Mailer: Claws Mail 3.17.3 (GTK+ 2.24.32; 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: netdev@vger.kernel.org On Wed, 18 Nov 2020 13:48:37 -0600 Segher Boessenkool wrote: > > With it_func being the func from the struct tracepoint_func, which is a > > void pointer, it is typecast to the function that is defined by the > > tracepoint. args is defined as the arguments that match the proto. > > If you have at most four or so args, what you wnat to do will work on > all systems the kernel currently supports, as far as I can tell. It > is not valid C, and none of the compilers have an extension for this > either. But it will likely work. Well, unfortunately, there's tracepoints with many more than 4 arguments. I think there's one with up to 13! > > > The problem we are solving is on the removal case, if the memory is tight, > > it is possible that the new array can not be allocated. But we must still > > remove the called function. The idea in this case is to replace the > > function saved with a stub. The above loop will call the stub and not the > > removed function until another update happens. > > > > This thread is about how safe is it to call: > > > > void tp_stub_func(void) { return ; } > > > > instead of the function that was removed? > > Exactly as safe as calling a stub defined in asm. The undefined > behaviour happens if your program has such a call, it doesn't matter > how the called function is defined, it doesn't have to be C. > > > Thus, we are indeed calling that stub function from a call site that is not > > using the same parameters. > > > > The question is, will this break? > > It is unlikely to break if you use just a few arguments, all of simple > scalar types. Just hope you will never encounter a crazy ABI :-) But in most cases, all the arguments are of scaler types, as anything else is not recommended, because copying is always slower than just passing a pointer, especially since it would need to be copied for every instance of that loop. I could do an audit to see if there's any that exist, and perhaps even add some static checker to make sure they don't. -- Steve