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=-13.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 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 A74C9C38A24 for ; Thu, 7 May 2020 18:55:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7A31924954 for ; Thu, 7 May 2020 18:55:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1588877718; bh=dd1tg8D5OHCVId8oTy4YDKOQkz34ytpBDxTe3n03A5w=; h=Date:From:To:Cc:Subject:List-ID:From; b=bHICeASqfcSWCZAZcCQ2g9eVoDOwIhaSWX815ZY4Bx/XpYFbqu33MgwuY4OtaKjvM hLxUzZVlsGd16yD4uAASWFkU2Sye9Pk+vWSpROKpon1ePY24DW3DbNyRk7IBkm57C1 wY2R5A20yjAiZCIv8BUjRpNojVBIep4JTuoao4/M= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728629AbgEGSzR (ORCPT ); Thu, 7 May 2020 14:55:17 -0400 Received: from mail.kernel.org ([198.145.29.99]:57212 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727904AbgEGSzQ (ORCPT ); Thu, 7 May 2020 14:55:16 -0400 Received: from embeddedor (unknown [189.207.59.248]) (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 21677221F7; Thu, 7 May 2020 18:55:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1588877715; bh=dd1tg8D5OHCVId8oTy4YDKOQkz34ytpBDxTe3n03A5w=; h=Date:From:To:Cc:Subject:From; b=E/S6rwWxvQ/dwNe8ctjaZopkyWPs7Lf5pBQRuFtUtqYB8CcatdbEFdzcrP9rV9Xik LCbqmFUErBbx9X8mNniW1gobQ5gNTep8MybRDfJiG2yiDPGkvIssMq0Djj2MjRzEMq m1IiqkQwIWXMZO7V29rv3X12QFFXwQQBLLgGJLpU= Date: Thu, 7 May 2020 13:59:42 -0500 From: "Gustavo A. R. Silva" To: Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: [PATCH] mm: Replace zero-length array with flexible-array Message-ID: <20200507185942.GA15193@embeddedor> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The current codebase makes use of the zero-length array language extension to the C90 standard, but the preferred mechanism to declare variable-length types such as these ones is a flexible array member[1][2], introduced in C99: struct foo { int stuff; struct boo array[]; }; 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 inadvertently introduced[3] to the codebase from now on. Also, notice that, dynamic memory allocations won't be affected by this change: "Flexible array members have incomplete type, and so the sizeof operator may not be applied. As a quirk of the original implementation of zero-length arrays, sizeof evaluates to zero."[1] sizeof(flexible-array-member) triggers a warning because flexible array members have incomplete type[1]. There are some instances of code in which the sizeof operator is being incorrectly/erroneously applied to zero-length arrays and the result is zero. Such instances may be hiding some bugs. So, this work (flexible-array member conversions) will also help to get completely rid of those sorts of issues. This issue was found with the help of Coccinelle. [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html [2] https://github.com/KSPP/linux/issues/21 [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour") Signed-off-by: Gustavo A. R. Silva --- include/linux/mm.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/include/linux/mm.h b/include/linux/mm.h index 5a323422d783..9831bb55caf3 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -1718,7 +1718,7 @@ struct frame_vector { unsigned int nr_frames; /* Number of frames stored in ptrs array */ bool got_ref; /* Did we pin pages by getting page ref? */ bool is_pfns; /* Does array contain pages or pfns? */ - void *ptrs[0]; /* Array of pinned pfns / pages. Use + void *ptrs[]; /* Array of pinned pfns / pages. Use * pfns_vector_pages() or pfns_vector_pfns() * for access */ };