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, USER_AGENT_SANE_1 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 951D0C64E69 for ; Wed, 18 Nov 2020 19:20:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5FD1822227 for ; Wed, 18 Nov 2020 19:20:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727434AbgKRTU1 (ORCPT ); Wed, 18 Nov 2020 14:20:27 -0500 Received: from gate.crashing.org ([63.228.1.57]:43809 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725948AbgKRTU0 (ORCPT ); Wed, 18 Nov 2020 14:20:26 -0500 Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id 0AIJBT7B020479; Wed, 18 Nov 2020 13:11:29 -0600 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 0AIJBR35020478; Wed, 18 Nov 2020 13:11:27 -0600 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Wed, 18 Nov 2020 13:11:27 -0600 From: Segher Boessenkool To: Steven Rostedt 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: <20201118191127.GM2672@gate.crashing.org> References: <47463878.48157.1605640510560.JavaMail.zimbra@efficios.com> <20201117142145.43194f1a@gandalf.local.home> <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> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201118135823.3f0d24b7@gandalf.local.home> User-Agent: Mutt/1.4.2.3i Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Wed, Nov 18, 2020 at 01:58:23PM -0500, Steven Rostedt wrote: > I wonder if we should define on all architectures a void void_stub(void), > in assembly, that just does a return, an not worry about gcc messing up the > creation of the stub function. > > On x86_64: > > GLOBAL(void_stub) > retq > > > And so on. I don't see how you imagine a compiler to mess this up? void void_stub(void) { } will do fine? .globl void_stub .type void_stub, @function void_stub: .LFB0: .cfi_startproc ret .cfi_endproc .LFE0: .size void_stub, .-void_stub Calling this via a different declared function type is undefined behaviour, but that is independent of how the function is *defined*. Your program can make ducks appear from your nose even if that function is never called, if you do that. Just don't do UB, not even once! Segher