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 7E0A2C5519F for ; Wed, 18 Nov 2020 19:05:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2140E21D1A for ; Wed, 18 Nov 2020 19:05:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726297AbgKRTFF (ORCPT ); Wed, 18 Nov 2020 14:05:05 -0500 Received: from gate.crashing.org ([63.228.1.57]:54376 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726163AbgKRTFF (ORCPT ); Wed, 18 Nov 2020 14:05:05 -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 0AIIthGu019175; Wed, 18 Nov 2020 12:55:43 -0600 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 0AIIteOV019174; Wed, 18 Nov 2020 12:55:40 -0600 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Wed, 18 Nov 2020 12:55:40 -0600 From: Segher Boessenkool To: Florian Weimer Cc: Steven Rostedt , 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: <20201118185540.GL2672@gate.crashing.org> References: <20201116175107.02db396d@gandalf.local.home> <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> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87o8jutt2h.fsf@mid.deneb.enyo.de> User-Agent: Mutt/1.4.2.3i Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Wed, Nov 18, 2020 at 07:31:50PM +0100, Florian Weimer wrote: > * Segher Boessenkool: > > > On Wed, Nov 18, 2020 at 12:17:30PM -0500, Steven Rostedt wrote: > >> I could change the stub from (void) to () if that would be better. > > > > Don't? In a function definition they mean exactly the same thing (and > > the kernel uses (void) everywhere else, which many people find clearer). > > And I think () functions expected a caller-provided parameter save > area on powerpc64le, while (void) functions do not. Like I said (but you cut off, didn't realise it matters I guess): > > In a function declaration that is not part of a definition it means no > > information about the arguments is specified, a quite different thing. Since the caller does not know if the callee will need a save area, it has to assume it does. Similar is true for many ABIs. > It does not > matter for an empty function, but GCC prefers to use the parameter > save area instead of setting up a stack frame if it is present. So > you get stack corruption if you call a () function as a (void) > function. (The other way round is fine.) If you have no prototype for a function, you have to assume worst case, yes. Calling things "a () function" can mean two things (a declaration that is or isn't a definition, two very different things), so it helps to be explicit about it. Just use (void) and do not worry :-) Segher