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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id BE9CBC433EF for ; Tue, 9 Nov 2021 19:22:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8F01A611C9 for ; Tue, 9 Nov 2021 19:22:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243127AbhKITZd (ORCPT ); Tue, 9 Nov 2021 14:25:33 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39242 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S243085AbhKITZc (ORCPT ); Tue, 9 Nov 2021 14:25:32 -0500 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D975CC061764 for ; Tue, 9 Nov 2021 11:22:45 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=iJZIo3STfY3lf2haVykR7BVPcS/nIvgKESoo2TuuPPo=; b=Ihy6ocXG5te82fzbfyuXsZFbuv Pt6LWuWw7qsPHsavKWVae0V/EKhdhvCYz73A4h3bvAKsQKdAuio/yRaBdc8maCTEKIi5GQzY3gvxo 3WI+uBu93CVO3Utm6Lc8KXzkES9hiJvHpMLJjkjNc7NE46/QMZVv4Tgncx+hWc0ot9SkNwYq4PVYT tHT5A0lad78z1XnXuuZKoB2MSecTCDMcq/OcGYfxUbuOaXdMqHNuw76B7X3pJZ1JQVKegjfqdByKV o7bN2cnK5Mo2ELf6Dw2Wchxu6lGmI43XpZO9x+iQN3XLrTMWi+h7V0p6L4Hzz96wwjc8A2bqzmEbx qkNjnTQA==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1mkWhT-001HLo-Cu; Tue, 09 Nov 2021 19:22:32 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id E20813000A3; Tue, 9 Nov 2021 20:22:30 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id BDFE7212EE58A; Tue, 9 Nov 2021 20:22:30 +0100 (CET) Date: Tue, 9 Nov 2021 20:22:30 +0100 From: Peter Zijlstra To: Ard Biesheuvel Cc: Linux Kernel Mailing List , Linux ARM , Josh Poimboeuf , Jason Baron , Steven Rostedt , Mark Rutland , Kees Cook Subject: Re: [RFC PATCH 1/7] static_call: get rid of static_call_cond() Message-ID: References: <20211109164549.1724710-1-ardb@kernel.org> <20211109164549.1724710-2-ardb@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 09, 2021 at 07:41:03PM +0100, Ard Biesheuvel wrote: > On Tue, 9 Nov 2021 at 19:38, Peter Zijlstra wrote: > > > > On Tue, Nov 09, 2021 at 05:45:43PM +0100, Ard Biesheuvel wrote: > > > The main reason for the existence of static_call_cond() seems to be that > > > in theory, when using the generic implementation of static calls, it may > > > be possible for a compiler to elide the indirect call entirely if the > > > target is NULL, while still guaranteeing that all side effects of > > > argument evaluation occur as expected. > > > > > > This is rather optimistic: as documented by an existing code comment, > > > both GCC and Clang (version 10) get this wrong, and even if they ever > > > get it right, this is far too subtle to rely on for a code path that is > > > expected to be used only by the 'remaining' architectures once all the > > > best supported ones implement either the out-of-line or inline optimized > > > variety of static calls. > > > > > > Given that having static_call_cond() clutters up the API, and puts the > > > burden on the caller to go and check what kind of static call they are > > > dealing with, let's just get rid of the distinction. > > > > No, static_call_cond() signifies the function can be NULL. Both gcc and > > clang generate correct (but wildly ineffecient) code for this. Without > > static_call_cond() the generic implementation will do a NULL deref. > > > > That is, static_call_cond() does properly encapuslate: > > > > func = READ_ONCE(key.func); > > if (func) > > func(ARGS); > > > > You can't take that out. > > I actually address that in the patch. > > AIUI, the compiler generates an indirect call to __static_call_nop(), > right? So why not simply set .func to the address of > __static_call_nop() when NULL is passed to update / the initializer? Ooh, lemme go have a proper look then.