From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mails.dpdk.org (mails.dpdk.org [217.70.189.124]) by inbox.dpdk.org (Postfix) with ESMTP id BB6C243E38; Wed, 10 Apr 2024 19:04:46 +0200 (CEST) Received: from mails.dpdk.org (localhost [127.0.0.1]) by mails.dpdk.org (Postfix) with ESMTP id A6E3840648; Wed, 10 Apr 2024 19:04:46 +0200 (CEST) Received: from linux.microsoft.com (linux.microsoft.com [13.77.154.182]) by mails.dpdk.org (Postfix) with ESMTP id 4974A40144; Wed, 10 Apr 2024 19:04:45 +0200 (CEST) Received: by linux.microsoft.com (Postfix, from userid 1086) id 83C2B20EB21D; Wed, 10 Apr 2024 10:04:44 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com 83C2B20EB21D DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1712768684; bh=etTMN8Az3S0TLde3zog8ZPpKFGuoIA70Os1TLJQuQU4=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=oHRQEuOsLhq77+A1Ld7vaAxh/x7Ivp6vCYeKuXle3/G19J98voPwzmOe/gXEGn3Jf 3fS33C3nOiLOekSpPCl5LVMEqDljEn+3uVSJpa+CyuLZoLXuCgz8rDktuII9C4qHVj miopn8xc6+QdZ3xHhDspmvF1si7SMzZfWYgiz8ZI= Date: Wed, 10 Apr 2024 10:04:44 -0700 From: Tyler Retzlaff To: Mattias =?iso-8859-1?Q?R=F6nnblom?= Cc: Morten =?iso-8859-1?Q?Br=F8rup?= , Stephen Hemminger , techboard@dpdk.org, dev@dpdk.org, Bruce Richardson , Thomas Monjalon Subject: Re: [PATCH 0/4] RFC samples converting VLA to alloca Message-ID: <20240410170444.GB29636@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net> References: <20231107193220.GA15232@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net> <1712250913-1977-1-git-send-email-roretzla@linux.microsoft.com> <98CBD80474FA8B44BF855DF32C47DC35E9F379@smartserver.smartshare.dk> <20240407100306.36c9688f@hermes.local> <20240408152703.GA25804@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net> <98CBD80474FA8B44BF855DF32C47DC35E9F380@smartserver.smartshare.dk> <28162ea3-7679-49a5-ac44-869718d32f53@lysator.liu.se> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <28162ea3-7679-49a5-ac44-869718d32f53@lysator.liu.se> User-Agent: Mutt/1.5.21 (2010-09-15) X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org On Wed, Apr 10, 2024 at 09:32:10AM +0200, Mattias Rönnblom wrote: > On 2024-04-08 17:53, Morten Brørup wrote: > >>From: Tyler Retzlaff [mailto:roretzla@linux.microsoft.com] > >>Sent: Monday, 8 April 2024 17.27 > >> > >>For next technboard meeting. > >> > >>On Sun, Apr 07, 2024 at 10:03:06AM -0700, Stephen Hemminger wrote: > >>>On Sun, 7 Apr 2024 13:07:06 +0200 > >>>Morten Brørup wrote: > >>> > >>>>>From: Mattias Rönnblom [mailto:hofors@lysator.liu.se] > >>>>>Sent: Sunday, 7 April 2024 11.32 > >>>>> > >>>>>On 2024-04-04 19:15, Tyler Retzlaff wrote: > >>>>>>This series is not intended for merge. It insteat provides examples > >>>>>of > >>>>>>converting use of VLAs to alloca() would look like. > >>>>>> > >>>>>>what's the advantages of VLA over alloca()? > >>>>>> > >>>>>>* sizeof(array) works as expected. > >>>>>> > >>>>>>* multi-dimensional arrays are still arrays instead of pointers to > >>>>>> dynamically allocated space. this means multiple subscript syntax > >>>>>> works (unlike on a pointer) and calculation of addresses into > >>>>>allocated > >>>>>> space in ascending order is performed by the compiler instead of > >>>>>manually. > >>>>>> > >>>>> > >>>>>alloca() is a pretty obscure mechanism, and also not a part of the C > >>>>>standard. VLAs are C99, and well-known and understood, and very > >>>>>efficient. > >>>> > >>>>The RFC fails to mention why we need to replace VLAs with something else: > >>>> > >>>>VLAs are C99, but not C++; VLAs were made optional in C11. > >>>> > >>>>MSVC doesn't support VLAs, and is not going to: > >>>>https://devblogs.microsoft.com/cppblog/c11-and-c17-standard-support- > >>arriving-in-msvc/#variable-length-arrays > >>>> > >>>> > >>>>I dislike alloca() too, and the notes section in the alloca(3) man page > >>even discourages the use of alloca(): > >>>>https://man7.org/linux/man-pages/man3/alloca.3.html > >>>> > >>>>But I guess alloca() is the simplest replacement for VLAs. > >>>>This RFC patch series opens the discussion for alternatives in different > >>use cases. > >>>> > >>> > >>>The other issue with VLA's is that if the number is something that can be > >>externally > >>>input, then it can be a source of stack overflow bugs. That is why the Linux > >>kernel > >>>has stopped using them; for security reasons. DPDK has much less of a > >>security > >>>trust domain. Mostly need to make sure that no data from network is being > >>>used to compute VLA size. > >>> > >> > >>Looks like we need to discuss this at the next techboard meeting. > >> > >>* MSVC doesn't support C11 optional VLAs (and never will). > >>* alloca() is an alternative that is available on all platforms/toolchain > >> combinations. > >>* it's reasonable for some VLAs to be turned into regular arrays but it > >> would be unsatisfactory to be stuck waiting discussions of defining new > >> constant expression macros on a per-use basis. > > > >We must generally stop using VLAs, for many reasons. > > What reasons would that be? And which of those reasons are not also > reasons to stop using alloca(). truncated the sentence, probably should have said where static array is not practical.