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 32865FA373E for ; Tue, 25 Oct 2022 07:47:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230040AbiJYHrA (ORCPT ); Tue, 25 Oct 2022 03:47:00 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49048 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232037AbiJYHqu (ORCPT ); Tue, 25 Oct 2022 03:46:50 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 49FB67B1F1 for ; Tue, 25 Oct 2022 00:46:49 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 02848B81A99 for ; Tue, 25 Oct 2022 07:46:48 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 621DFC433D6; Tue, 25 Oct 2022 07:46:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1666684006; bh=nBoTP4tEFx4CkrBDxFl7aUbkBwpWy6P5VCH8o58NWSE=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=oTmo+eYuA9oQI7SHJpztmHDd114YnLxzMHQPu828gkPfvBE4MiEAMence3z6527cx hokxbS7PyspkI4MgyHVgwLjEvCRX0TNEmqVxbRZmp52XMxaiXqLyOibTg8xbXlfcvy UQi07rJupICNhE2cbAwqBXIiT3y24s2J+N9x4HJg= Date: Tue, 25 Oct 2022 09:47:39 +0200 From: Greg Kroah-Hartman To: "Rafael J. Wysocki" Cc: Kees Cook , Andy Shevchenko , Sakari Ailus , linux-kernel@vger.kernel.org, David Laight Subject: Re: [PATCH 1/1] linux/container_of.h: Warn about loss of constness Message-ID: References: <20221024082610.74990-1-sakari.ailus@linux.intel.com> <202210241035.1487F523@keescook> 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 Mon, Oct 24, 2022 at 07:51:11PM +0200, Rafael J. Wysocki wrote: > On Mon, Oct 24, 2022 at 7:39 PM Kees Cook wrote: > > > > On Mon, Oct 24, 2022 at 12:00:16PM +0300, Andy Shevchenko wrote: > > > + Kees > > > > > > On Mon, Oct 24, 2022 at 10:45:25AM +0200, Greg Kroah-Hartman wrote: > > > > On Mon, Oct 24, 2022 at 10:43:52AM +0200, Greg Kroah-Hartman wrote: > > > > > On Mon, Oct 24, 2022 at 11:26:10AM +0300, Sakari Ailus wrote: > > > > > > container_of() casts the original type to another which leads to the loss > > > > > > of the const qualifier if it is not specified in the caller-provided type. > > > > > > This easily leads to container_of() returning a non-const pointer to a > > > > > > const struct which the C compiler does not warn about. > > > > > > ... > > > > > > > > > * @type: the type of the container struct this is embedded in. > > > > > > * @member: the name of the member within the struct. > > > > > > * > > > > > > + * WARNING: as container_of() casts the given struct to another, also the > > > > > > > > Wrong function name here. > > > > > > > > > > + * possible const qualifier of @ptr is lost unless it is also specified in > > > > > > + * @type. This is not a problem if the containing object is not const. Use with > > > > > > + * care. > > > > > > > > > > Same comments here. > > > > > > > > Wait, no one uses this macro, so why not just remove it entirely? > > > > > > Kees, do you know why and what for we have container_of_safe()? > > > > It looks like it was designed to handle the cases where the pointer was > > ERR_OR_NULL: > > > > IS_ERR_OR_NULL(__mptr) ? ERR_CAST(__mptr) : \ > > ((type *)(__mptr - offsetof(type, member))); }) > > > > i.e. just pass through the NULL/ERR instead of attempting the cast, > > which would fail spectacularly. :) > > > > It seems like this version should actually be used everywhere instead of > > nowhere... (i.e. just drop container_of() and rename container_of_safe() > > to container_of()) > > As a rule, though, users of container_of() don't check the pointer > returned by it against NULL, so I'm not sure how much of an > improvement that would be. Nor should they. This is just tiny pointer math, that always assumes a valid pointer is passed in. It should never be used in any code path where a valid pointer is NOT passed into it. thanks, greg k-h