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=-2.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_MUTT autolearn=ham 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 C6DDEC072B1 for ; Thu, 30 May 2019 06:44:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5BB4A25ADB for ; Thu, 30 May 2019 06:44:26 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="m97JkP1G" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727442AbfE3GoZ (ORCPT ); Thu, 30 May 2019 02:44:25 -0400 Received: from mail-wr1-f67.google.com ([209.85.221.67]:43977 "EHLO mail-wr1-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725961AbfE3GoZ (ORCPT ); Thu, 30 May 2019 02:44:25 -0400 Received: by mail-wr1-f67.google.com with SMTP id l17so3326306wrm.10; Wed, 29 May 2019 23:44:24 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=T7CPnq8vpZJt0is+qvL5wzl30i9DGzJv4b/+CGnwzds=; b=m97JkP1GLbD/PuR+eESG85L165lqMk8tf6H+lGUWNGF1TRxmhNU7cd4kg+81SooIev wvWKQhnHEmTexB5Pvi6rdpPj3947guoEPDuftnx5+krcfLlDNBqblbJ8rtdBeRsKkkD9 XiZSCn0aRHmBPHBtm5TfQqrvWcAvNIYdOz+z+yexhhKFz2a117kB5ZRt8PstVctGAPS2 dBFJ326rjiwOrw96vSseAeWBLjyBTHhH2/2HmsT9TgXz/U3SXw5zbrVhs7KditZeGwWB tJYEVrRzdkW7/VtYULggz0tDFWezUR52Ox1eLRVwELdn/cc0MHZGSFH0Y9VweoHTwk7q jwdw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=T7CPnq8vpZJt0is+qvL5wzl30i9DGzJv4b/+CGnwzds=; b=ESDWh6e3T3NHCGpUsBAD4FWcKRhb8LgE9ZKIfdvzvzDt6uBmMATUhn4FoJ3ZKfGQOC /114qZlS1CF41jnk+ynUckGaLYagz3+cxdg45Pfe8p44gtLfRBbTcW/u2OlfPnRqTtCu 7zD5FpZdaBpKtxO+iWRf5dOmkPrEwYTqg2MtjaKIO0blnbmcv6vcp4/PWdpfckvgxSho ycNS0hEhbMpbv21hot/4d8OfBO+QkOfe8i45D353n3ZGG651jQwmfClFi+AJJudg6tT5 5FO8RpRunydfOK98fY4txv7diQqtHn71wXj5V66qShSXlNlyQSmFUz8txD9fOcP1qXQb iLGQ== X-Gm-Message-State: APjAAAXljXezZ7IHs20m3xXXD/5TVB8H8Kvw/HvymYzLaJcrgla2DPqZ hJKMeYKOnYD2WxSt6/3h6AKKlYw= X-Google-Smtp-Source: APXvYqyFVthdHiovnywHS5iWPKpmf+9FbYkOsj4Kz+7QA5aGnm6A4zlfxX8sK+qvOlMlF97syahltg== X-Received: by 2002:adf:ee0c:: with SMTP id y12mr1395065wrn.34.1559198663810; Wed, 29 May 2019 23:44:23 -0700 (PDT) Received: from avx2 ([46.53.251.224]) by smtp.gmail.com with ESMTPSA id n5sm1631539wrj.27.2019.05.29.23.44.22 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 29 May 2019 23:44:23 -0700 (PDT) Date: Thu, 30 May 2019 09:44:21 +0300 From: Alexey Dobriyan To: Andrew Morton Cc: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, Kees Cook Subject: Re: [PATCH] elf: align AT_RANDOM bytes Message-ID: <20190530064421.GA5148@avx2> References: <20190529213708.GA10729@avx2> <20190529152020.c9d0ed1c6194328f751fe0f9@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190529152020.c9d0ed1c6194328f751fe0f9@linux-foundation.org> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 29, 2019 at 03:20:20PM -0700, Andrew Morton wrote: > On Thu, 30 May 2019 00:37:08 +0300 Alexey Dobriyan wrote: > > > AT_RANDOM content is always misaligned on x86_64: > > > > $ LD_SHOW_AUXV=1 /bin/true | grep AT_RANDOM > > AT_RANDOM: 0x7fff02101019 > > > > glibc copies first few bytes for stack protector stuff, aligned > > access should be slightly faster. > > I just don't understand the implications of this. Is there > (badly-behaved) userspace out there which makes assumptions about the > current alignment? I don't think so: glibc has getauxval(AT_RANDOM) and userspace should use whatever it returns as "char[16]" base pointer; > How much faster, anyway? How frequently is the AT_RANDOM record > accessed? I don't think it is measureable :-\ It is accessed twice per execve: first by the kernel putting data there, second by glibc fetching first sizeof(uintptr_t) bytes for stack canary. Here is stack layout at the beginning of execution: ....10 e8 03 00 00 00 00 00 00 17 00 00 00 00 00 00 00 |................| ....20 00 00 00 00 00 00 00 00 19 00 00 00 00 00 00 00 |................| AT_RANDOM=25 ....30 79 dd ff ff ff 7f 00 00 1a 00 00 00 00 00 00 00 |y...............| AT_RANDOM pointer ....40 00 00 00 00 00 00 00 00 1f 00 00 00 00 00 00 00 |................| ....50 e2 ef ff ff ff 7f 00 00 0f 00 00 00 00 00 00 00 |................| ....60 89 dd ff ff ff 7f 00 00 00 00 00 00 00 00 00 00 |................| AT_RANDOM bytes (misaligned) ....70 00 00 00 00 00 00 00 00 00|a2 ef 76 37 0c 0c 69 |...........v7..i| ....80 04 32 68 e4 68 2d 53 cf a5|78 38 36 5f 36 34 00 |.2h.h-S..x86_64.| AT_RANDOM------------------|"x86_64" (misaligned) argv[0], envp[0] ....90 00 00 00|2f 68 6f 6d 65 2f 61 64 2f 73 2d 74 65 |.../home/ad/s-te| ....a0 73 74 2f 61 2e 6f 75 74 00 47 53 5f 4c 49 42 3d |st/a.out.GS_LIB=| > I often have questions such as these about your performance/space > tweaks :(. Please try to address them as a matter of course when > preparing changelogs? OK.