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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 28D6FC433EF for ; Fri, 15 Apr 2022 12:33:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1353707AbiDOMfc (ORCPT ); Fri, 15 Apr 2022 08:35:32 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60336 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1353634AbiDOMfH (ORCPT ); Fri, 15 Apr 2022 08:35:07 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A7F60C8BF1 for ; Fri, 15 Apr 2022 05:31:38 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 2402A61CEE for ; Fri, 15 Apr 2022 12:31:38 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 78417C385A6; Fri, 15 Apr 2022 12:31:35 +0000 (UTC) Date: Fri, 15 Apr 2022 13:31:32 +0100 From: Catalin Marinas To: Herbert Xu Cc: Ard Biesheuvel , Will Deacon , Marc Zyngier , Arnd Bergmann , Greg Kroah-Hartman , Andrew Morton , Linus Torvalds , Linux Memory Management List , Linux ARM , Linux Kernel Mailing List , "David S. Miller" Subject: Re: [PATCH 07/10] crypto: Use ARCH_DMA_MINALIGN instead of ARCH_KMALLOC_MINALIGN Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Apr 15, 2022 at 10:51:40AM +0100, Catalin Marinas wrote: > On Fri, Apr 15, 2022 at 03:51:54PM +0800, Herbert Xu wrote: > > On Fri, Apr 15, 2022 at 09:49:12AM +0200, Ard Biesheuvel wrote: > > > I'm not sure I understand what would go wrong if that assumption no > > > longer holds. > > > > It's very simple, we don't do anything to the pointer returned > > by kmalloc before returning it as a tfm or other object with > > an alignment of CRYPTO_MINALIGN. IOW if kmalloc starts returning > > pointers that are not aligned to CRYPTO_MINALIGN then we'd be > > lying to the compiler. > > I agree that it would be lying to the compiler, but I don't think this > matters for arm64 where the CPU can do unaligned accesses just fine. We > don't even end up with unaligned accesses here. Let's say we have: > > struct x { > ... > } __attribute__ ((__aligned__ (128))); > > and the kmalloc(sizeof(struct x)) returns a 64-byte aligned pointer. This needs a clarification. For the above structure, kmalloc() will return a 128-byte aligned pointer since sizeof(x) is a multiple of 128. The potential problem is if you have something like: kmalloc(sizeof(struct x) + 64); The above could end up as a kmalloc(192) which is available with an ARCH_KMALLOC_MINALIGN of 64. If that's a real use-case, I can change the slab patch to not create the 192 (or 48 if we go for an even smaller ARCH_KMALLOC_MINALIGN) caches and we'd always have ARCH_DMA_MINALIGN guarantee if the structure itself is correctly aligned. No lying to the compiler. -- Catalin