Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932460AbdCIORe convert rfc822-to-8bit (ORCPT ); Thu, 9 Mar 2017 09:17:34 -0500 Received: from unicorn.mansr.com ([81.2.72.234]:45392 "EHLO unicorn.mansr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754092AbdCIOR3 (ORCPT ); Thu, 9 Mar 2017 09:17:29 -0500 From: =?iso-8859-1?Q?M=E5ns_Rullg=E5rd?= To: Tomas Winkler Cc: Henrique de Moraes Holschuh , "linux-kernel\@vger.kernel.org" , linux-sparse@vger.kernel.org, Herbert Xu , Al Viro Subject: Re: Arrays of variable length References: <20170305211254.GA3220@khazad-dum.debian.net> Date: Thu, 09 Mar 2017 14:16:50 +0000 In-Reply-To: (Tomas Winkler's message of "Thu, 9 Mar 2017 15:40:37 +0200") Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2053 Lines: 51 Tomas Winkler writes: > On Thu, Mar 9, 2017 at 3:02 PM, M?ns Rullg?rd wrote: >> Tomas Winkler writes: >> >>> On Mon, Mar 6, 2017 at 2:31 AM, M?ns Rullg?rd wrote: >>>> Henrique de Moraes Holschuh writes: >>>> >>>>> On Sun, 05 Mar 2017, M?ns Rullg?rd 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' If you know the upper bound, why use VLAs in the first place? -- M?ns Rullg?rd