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=-17.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_2 autolearn=unavailable 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 717D8C43460 for ; Wed, 14 Apr 2021 06:14:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 568C3611EE for ; Wed, 14 Apr 2021 06:14:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1349125AbhDNGO5 (ORCPT ); Wed, 14 Apr 2021 02:14:57 -0400 Received: from mail.kernel.org ([198.145.29.99]:36270 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1349130AbhDNGOv (ORCPT ); Wed, 14 Apr 2021 02:14:51 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 87A0B60FD8; Wed, 14 Apr 2021 06:14:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1618380870; bh=QUwhG1aEuYUYzpTX5r+cTxe2OtqIgKW7ZzBKXbUv0qE=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=thusl7s/bxHLGdsAmgh+pH+X9REzuDPhBdzVl1RfGWSvGH0esOhhO4XNPANmqOQLX xamcSotJXglVS6YhmGa8nLoVDcdUwC9bE7FdYX4hue7yEiFXhVHGvqenqDq4roJ6Sc A/YaT/EFd2fN8q12udBhBJFgvTs2S7aVxIb5BWB7OGt4h3U1q/VY9pjyQP+55c0wM0 cb76MI26cMoEf8s1YmbLXaz2CzB5NnsRRPIUJdvCYB/GoxzXCFkwc2Sz28M9a/PWXq EBIy9On512F83Z+NtuJzgkiz0e+LXrm9oQqDMTxYRYQNG3ppWwoONaNrqKxXlfs+ph bmu4zyqOrQALQ== Date: Wed, 14 Apr 2021 08:14:22 +0200 From: Mauro Carvalho Chehab To: Yury Norov Cc: linux-api@vger.kernel.org, linux-arch@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, "Alexander A. Klimov" , =?UTF-8?B?QW5kcsOp?= Almeida , Andrew Morton , Arnd Bergmann , David Sterba , Joe Perches , Jonathan Corbet , Mike Rapoport Subject: Re: [PATCH] Documentation: syscalls: add a note about ABI-agnostic types Message-ID: <20210414081422.5a9d0c4b@coco.lan> In-Reply-To: <20210414044020.GA44464@yury-ThinkPad> References: <20210409204304.1273139-1-yury.norov@gmail.com> <20210414044020.GA44464@yury-ThinkPad> X-Mailer: Claws Mail 3.17.8 (GTK+ 2.24.33; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org Em Tue, 13 Apr 2021 21:40:20 -0700 Yury Norov escreveu: > Ping? > > On Fri, Apr 09, 2021 at 01:43:04PM -0700, Yury Norov wrote: > > Recently added memfd_secret() syscall had a flags parameter passed > > as unsigned long, which requires creation of compat entry for it. > > It was possible to change the type of flags to unsigned int and so > > avoid bothering with compat layer. > > > > https://www.spinics.net/lists/linux-mm/msg251550.html > > > > Documentation/process/adding-syscalls.rst doesn't point clearly about > > preference of ABI-agnostic types. This patch adds such notification. > > > > Signed-off-by: Yury Norov > > --- > > Documentation/process/adding-syscalls.rst | 7 +++++++ > > 1 file changed, 7 insertions(+) > > > > diff --git a/Documentation/process/adding-syscalls.rst b/Documentation/process/adding-syscalls.rst > > index 9af35f4ec728..46add16edf14 100644 > > --- a/Documentation/process/adding-syscalls.rst > > +++ b/Documentation/process/adding-syscalls.rst > > @@ -172,6 +172,13 @@ arguments (i.e. parameter 1, 3, 5), to allow use of contiguous pairs of 32-bit > > registers. (This concern does not apply if the arguments are part of a > > structure that's passed in by pointer.) > > > > +Whenever possible, try to use ABI-agnostic types for passing parameters to > > +a syscall in order to avoid creating compat entry for it. Linux supports two > > +ABI models - ILP32 and LP64. > > + The types like ``void *``, ``long``, ``size_t``, > > +``off_t`` have different size in those ABIs; In the case of pointers, the best is to use __u64. The pointer can then be read on Kernelspace with something like this: static inline void __user *media_get_uptr(__u64 arg) { return (void __user *)(uintptr_t)arg; } > > types like ``char`` and ``int`` > > +have the same size and don't require a compat layer support. For flags, it's > > +always better to use ``unsigned int``. > > + I don't think this is true for all compilers on userspace, as the C standard doesn't define how many bits an int/unsigned int has. So, even if this is today's reality, things may change in the future. For instance, I remember we had to replace "int" and "enum" by "__u32" and "long" by "__u64" at the media uAPI in the past, when we start seeing x86_64 Kernels with 32-bits userspace and when cameras started being supported on arm32. We did have some real bugs with "enum", as, on that time, some compilers (gcc, I guess) were optimizing them to have less than 32 bits on certain architectures, when it fits. Thanks, Mauro