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 3832DC2D0DB for ; Thu, 23 Jan 2020 18:25:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0E0CF21569 for ; Thu, 23 Jan 2020 18:25:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1579803950; bh=//Gx7TpRR9XJE4SitAtv6bFOO1oNBBqtuf9aoscDo6s=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=uinpLSD5emkNh5ofAHqsbFdznBa/aMOAO4YVC0qg2Vq6WX8N7aa0STw4WUgTyGS+E c1PQcHUnk314w9BQSchq9EN16ZNtXL7h3Ijtc6UahhYD1VVmY6AXIhGKR3L9kFPUrs z8eJIb5DXhK+xpTlsKIh9kEXKe1hDpJ4xkBo+MBQ= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728988AbgAWSZt (ORCPT ); Thu, 23 Jan 2020 13:25:49 -0500 Received: from mail.kernel.org ([198.145.29.99]:40222 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727278AbgAWSZt (ORCPT ); Thu, 23 Jan 2020 13:25:49 -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 AF9462077C; Thu, 23 Jan 2020 18:25:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1579803948; bh=//Gx7TpRR9XJE4SitAtv6bFOO1oNBBqtuf9aoscDo6s=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=GD7H1bVYa9bikI7RV+4cY6E4eQZ1G0GKSNpUZS3jSY39neata6PAyGeiPm+5Rp04l OzAj+T3u9o7SJv+LrYPbVDTwTt2yLKAHKTm399caH9wSWGvdEtR0FndfEgEC4a94qq 7efA7P/zX19EtOMiNR+Z/tqiklYAq419vY282FHc= Date: Thu, 23 Jan 2020 19:25:45 +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: <20200123182545.GA1954152@kroah.com> References: <20200120235326.GA29231@embeddedor.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200120235326.GA29231@embeddedor.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 :) thanks, greg k-h