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=-11.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=ham 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 468A4C2D0DB for ; Thu, 23 Jan 2020 18:56:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1F76A21569 for ; Thu, 23 Jan 2020 18:56:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1579805774; bh=ntVLWfAQ4HN6TI0EcJB4auZ7w0f1L4NqXYySckaiR44=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=WGEp2I+wQ2vH+V9VxdBbiIkk+En0orblaXxlHJvvRsmpQBZEyuWrU+PrQUIf/Qbvz btS4whZ+xiAIg4PQ3sWmcSmV7ElpJkKFtuSuuKIF+MvvaxapXgxoVBFG/0CFWDqZWv Brwlqw5/PMfbDSgpKXZZc78joBkljUrvws+kOzio= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728904AbgAWS4N (ORCPT ); Thu, 23 Jan 2020 13:56:13 -0500 Received: from mail.kernel.org ([198.145.29.99]:55104 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727022AbgAWS4N (ORCPT ); Thu, 23 Jan 2020 13:56:13 -0500 Received: from localhost (83-86-89-107.cable.dynamic.v4.ziggo.nl [83.86.89.107]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 93C9922464; Thu, 23 Jan 2020 18:56:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1579805772; bh=ntVLWfAQ4HN6TI0EcJB4auZ7w0f1L4NqXYySckaiR44=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=cxpNMLpN+juRbN63ywA8YrYyxbcr5NlorR9Fc0aaadi3UqBC4eT0sF0OE5ZeYrBQi cDV8M/YXdywC01dAOolO2rFCg7Dhr9k8l2PFq16zFyo+kd9BuTmW0QJE9DLIN3zThk 6YYsC6IONp+Qwc5p2ijMwlXR6j477W16aMKHAOfE= Date: Thu, 23 Jan 2020 19:56:09 +0100 From: Greg Kroah-Hartman To: "Gustavo A. R. Silva" Cc: Clemens Ladisch , Arnd Bergmann , linux-kernel@vger.kernel.org Subject: Re: [PATCH] char: hpet: Use flexible-array member Message-ID: <20200123185609.GA2075008@kroah.com> References: <20200120235326.GA29231@embeddedor.com> <20200123182545.GA1954152@kroah.com> <7bf963ff-94b1-7efe-747e-4153081d1947@embeddedor.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7bf963ff-94b1-7efe-747e-4153081d1947@embeddedor.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 23, 2020 at 12:46:42PM -0600, Gustavo A. R. Silva wrote: > > > On 1/23/20 12:25, Greg Kroah-Hartman wrote: > > On Mon, Jan 20, 2020 at 05:53:26PM -0600, Gustavo A. R. Silva wrote: > >> Old code in the kernel uses 1-byte and 0-byte arrays to indicate the > >> presence of a "variable length array": > >> > >> struct something { > >> int length; > >> u8 data[1]; > >> }; > >> > >> struct something *instance; > >> > >> instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL); > >> instance->length = size; > >> memcpy(instance->data, source, size); > >> > >> There is also 0-byte arrays. Both cases pose confusion for things like > >> sizeof(), CONFIG_FORTIFY_SOURCE, etc.[1] Instead, the preferred mechanism > >> to declare variable-length types such as the one above is a flexible array > >> member[2] which need to be the last member of a structure and empty-sized: > >> > >> struct something { > >> int stuff; > >> u8 data[]; > >> }; > >> > >> Also, by making use of the mechanism above, we will get a compiler warning > >> in case the flexible array does not occur last in the structure, which > >> will help us prevent some kind of undefined behavior bugs from being > >> unadvertenly introduced[3] to the codebase from now on. > >> > >> [1] https://github.com/KSPP/linux/issues/21 > >> [2] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html > >> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour") > >> > >> Signed-off-by: Gustavo A. R. Silva > >> --- > >> drivers/char/hpet.c | 2 +- > >> 1 file changed, 1 insertion(+), 1 deletion(-) > >> > >> diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c > >> index 9ac6671bb514..aed2c45f7968 100644 > >> --- a/drivers/char/hpet.c > >> +++ b/drivers/char/hpet.c > >> @@ -110,7 +110,7 @@ struct hpets { > >> unsigned long hp_delta; > >> unsigned int hp_ntimer; > >> unsigned int hp_which; > >> - struct hpet_dev hp_dev[1]; > >> + struct hpet_dev hp_dev[]; > > > > Are you sure the allocation size is the same again? Much like the > > n_hdlc patch was, I think you need to adjust the variable size here. > > Maybe, it's a bit of a pain to figure out at a quick glance, I just want > > to make sure you at least do look at that :) > > > > Yep. The allocation thing was already handled almost a year ago by the > following patch, and it didn't require to increase the size at that time: > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=401c9bd10beef4b030eb9e34d16b5341dc6c683b Great, thanks for verifying, I'll go queue this up now. greg k-h