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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 73664ECAAD3 for ; Wed, 14 Sep 2022 12:28:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229853AbiINM2Z (ORCPT ); Wed, 14 Sep 2022 08:28:25 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43756 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229832AbiINM2Y (ORCPT ); Wed, 14 Sep 2022 08:28:24 -0400 Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id BF95C2E6A4; Wed, 14 Sep 2022 05:28:21 -0700 (PDT) Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id 28ECGMjO020934; Wed, 14 Sep 2022 07:16:22 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 28ECGLVD020933; Wed, 14 Sep 2022 07:16:21 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Wed, 14 Sep 2022 07:16:20 -0500 From: Segher Boessenkool To: Josh Poimboeuf Cc: Mark Rutland , Peter Zijlstra , linuxppc-dev@lists.ozlabs.org, Chen Zhongjin , x86@kernel.org, Nick Desaulniers , linux-kernel@vger.kernel.org, Mark Brown , Sathvika Vasireddy , linux-toolchains@vger.kernel.org, Indu Bhagat , live-patching@vger.kernel.org, Miroslav Benes , Will Deacon , Ard Biesheuvel , linux-arm-kernel@lists.infradead.org, "Jose E. Marchesi" , Michael Matz Subject: Re: [RFC] Objtool toolchain proposal: -fannotate-{jump-table,noreturn} Message-ID: <20220914121620.GY25951@gate.crashing.org> References: <20220909180704.jwwed4zhwvin7uyi@treble> <20220912113114.GV25951@gate.crashing.org> <20220914102100.thl5ad35plvazark@treble> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220914102100.thl5ad35plvazark@treble> User-Agent: Mutt/1.4.2.3i Precedence: bulk List-ID: X-Mailing-List: linux-toolchains@vger.kernel.org On Wed, Sep 14, 2022 at 11:21:00AM +0100, Josh Poimboeuf wrote: > On Mon, Sep 12, 2022 at 06:31:14AM -0500, Segher Boessenkool wrote: > > On Fri, Sep 09, 2022 at 11:07:04AM -0700, Josh Poimboeuf wrote: > > > 2) Noreturn functions: > > > > > > There's no reliable way to determine which functions are designated > > > by the compiler to be noreturn (either explictly via function > > > attribute, or implicitly via a static function which is a wrapper > > > around a noreturn function.) > > > > Or just a function that does not return for any other reason. > > > > The compiler makes no difference between functions that have the > > attribute and functions that do not. There are good reasons to not > > have the attribute on functions that do in fact not return. The > > not-returningness of the function may be just an implementation > > accident, something you do not want part of the API, so it *should* not > > have that attribute; or you may want the callers to a function to not be > > optimised according to this knowledge (you cannot *prevent* that, the > > compiler can figure it out it other ways, but still) for any other > > reason. > > Yes, many static functions that are wrappers around noreturn functions > have this "implicit noreturn" property. I meant functions that are noreturn intrinsically. The trivial example: void f(void) { for (;;) ; } > I agree we would need to know > about those functions (or, as Michael suggested, their call sites) as > well. Many "potentially does not return" functions (there are very many such functions!) turn into "never returns" functions, for some inputs (or something in the environment). If the compiler specialises a code path that does not return, you'll not see that marked up any way. Of course such a path should not be taken in the kernel, normally :-) > > > This information is needed because the > > > code after the call to such a function is optimized out as > > > unreachable and objtool has no way of knowing that. > > > > Since June we (GCC) have -funreachable-traps. This creates a trap insn > > wherever control flow would otherwise go into limbo. > > Ah, that's interesting, though I'm not sure if we'd be able to > distinguish between "call doesn't return" traps and other traps or > reasons for UD2. The trap handler can see where the trap came from. And then look up that address in some tables or such. Just like __bug_table? Segher