Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754801AbdCINuK (ORCPT ); Thu, 9 Mar 2017 08:50:10 -0500 Received: from mail-oi0-f47.google.com ([209.85.218.47]:34849 "EHLO mail-oi0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753928AbdCINuH (ORCPT ); Thu, 9 Mar 2017 08:50:07 -0500 MIME-Version: 1.0 In-Reply-To: References: <20170305211254.GA3220@khazad-dum.debian.net> From: Tomas Winkler Date: Thu, 9 Mar 2017 15:40:37 +0200 Message-ID: Subject: Re: Arrays of variable length To: =?UTF-8?B?TcOlbnMgUnVsbGfDpXJk?= Cc: Henrique de Moraes Holschuh , "linux-kernel@vger.kernel.org" , linux-sparse@vger.kernel.org, Herbert Xu , Al Viro Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by mail.home.local id v29DoE0i016632 Content-Length: 2046 Lines: 49 On Thu, Mar 9, 2017 at 3:02 PM, Måns Rullgård wrote: > Tomas Winkler writes: > >> On Mon, Mar 6, 2017 at 2:31 AM, Måns Rullgård wrote: >>> Henrique de Moraes Holschuh writes: >>> >>>> On Sun, 05 Mar 2017, Måns Rullgård wrote: >>>>> Tomas Winkler writes: >>>>> > Sparse complains for arrays declared with variable length >>>>> > >>>>> > 'warning: Variable length array is used' >>>>> > >>>>> > Prior to c99 this was not allowed but lgcc (c99) doesn't have problem >>>>> > with that https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html. >>>>> > And also Linux kernel compilation with W=1 doesn't complain. >>>>> > >>>>> > Since sparse is used extensively would like to ask what is the correct >>>>> > usage of arrays of variable length >>>>> > within Linux Kernel. >>>>> >>>>> Variable-length arrays are a very bad idea. Don't use them, ever. >>>>> If the size has a sane upper bound, just use that value statically. >>>>> Otherwise, you have a stack overflow waiting to happen and should be >>>>> using some kind of dynamic allocation instead. >>>>> >>>>> Furthermore, use of VLAs generally results in less efficient code. For >>>>> instance, it forces gcc to waste a register for the frame pointer, and >>>>> it often prevents inlining. >>>> >>>> Well, if we're going to forbid VLAs in the kernel, IMHO the kernel build >>>> system should call gcc with -Werror=vla to get that point across early, >>>> and flush out any offenders. >>> >>> If it were up to me, that's exactly what I'd do. >> >>> >> Some parts of the kernel depends on VLA such as ___ON_STACK macros in >> include/crypto/hash.h >> It's actually pretty neat implementation, maybe it's too harsh to >> disable VLA completely. > > And what happens if the requested size is insane? One option is to add '-Wvla-larger-than=n' other option is to selectively shut down the warning on ON_STACK macros using #pragma warning(disable:) though this looks rather ugly. Just a thought Tomas