[{"id":1793462,"web_url":"http://patchwork.ozlabs.org/comment/1793462/","msgid":"<20171024212119.GB1611@linux.intel.com>","list_archive_url":null,"date":"2017-10-24T21:21:19","subject":"Re: [PATCH 01/17] mm: introduce MAP_SHARED_VALIDATE, a mechanism to\n\tsafely define new mmap flags","submitter":{"id":46514,"url":"http://patchwork.ozlabs.org/api/people/46514/","name":"Ross Zwisler","email":"ross.zwisler@linux.intel.com"},"content":"On Tue, Oct 24, 2017 at 05:23:58PM +0200, Jan Kara wrote:\n> From: Dan Williams \n> \n> The mmap(2) syscall suffers from the ABI anti-pattern of not validating\n> unknown flags. However, proposals like MAP_SYNC need a mechanism to\n> define new behavior that is known to fail on older kernels without the\n> support. Define a new MAP_SHARED_VALIDATE flag pattern that is\n> guaranteed to fail on all legacy mmap implementations.\n> \n> It is worth noting that the original proposal was for a standalone\n> MAP_VALIDATE flag. However, when that could not be supported by all\n> archs Linus observed:\n> \n> I see why you *think* you want a bitmap. You think you want\n> a bitmap because you want to make MAP_VALIDATE be part of MAP_SYNC\n> etc, so that people can do\n> \n> ret = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED\n> \t\t | MAP_SYNC, fd, 0);\n> \n> and \"know\" that MAP_SYNC actually takes.\n> \n> And I'm saying that whole wish is bogus. You're fundamentally\n> depending on special semantics, just make it explicit. It's already\n> not portable, so don't try to make it so.\n> \n> Rename that MAP_VALIDATE as MAP_SHARED_VALIDATE, make it have a value\n> of 0x3, and make people do\n> \n> ret = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED_VALIDATE\n> \t\t | MAP_SYNC, fd, 0);\n> \n> and then the kernel side is easier too (none of that random garbage\n> playing games with looking at the \"MAP_VALIDATE bit\", but just another\n> case statement in that map type thing.\n> \n> Boom. Done.\n> \n> Similar to ->fallocate() we also want the ability to validate the\n> support for new flags on a per ->mmap() 'struct file_operations'\n> instance basis. Towards that end arrange for flags to be generically\n> validated against a mmap_supported_flags exported by 'struct\n> file_operations'. By default all existing flags are implicitly\n> supported, but new flags require MAP_SHARED_VALIDATE and\n> per-instance-opt-in.\n> \n> Cc: Jan Kara \n> Cc: Arnd Bergmann \n> Cc: Andy Lutomirski \n> Cc: Andrew Morton \n> Suggested-by: Christoph Hellwig \n> Suggested-by: Linus Torvalds \n> Signed-off-by: Dan Williams \n> Signed-off-by: Jan Kara \n\nLooks great.\n\nReviewed-by: Ross Zwisler ","headers":{"Return-Path":"","X-Original-To":"patchwork-incoming@ozlabs.org","Delivered-To":"patchwork-incoming@ozlabs.org","Authentication-Results":"ozlabs.org;\n\tspf=none (mailfrom) smtp.mailfrom=vger.kernel.org\n\t(client-ip=209.132.180.67; helo=vger.kernel.org;\n\tenvelope-from=linux-ext4-owner@vger.kernel.org;\n\treceiver=)","Received":["from vger.kernel.org (vger.kernel.org [209.132.180.67])\n\tby ozlabs.org (Postfix) with ESMTP id 3yM5mp6G5xz9t2h\n\tfor ;\n\tWed, 25 Oct 2017 08:21:30 +1100 (AEDT)","(majordomo@vger.kernel.org) by vger.kernel.org via listexpand\n\tid S1751336AbdJXVV0 (ORCPT );\n\tTue, 24 Oct 2017 17:21:26 -0400","from mga09.intel.com ([134.134.136.24]:9667 \"EHLO mga09.intel.com\"\n\trhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP\n\tid S1751267AbdJXVVZ (ORCPT );\n\tTue, 24 Oct 2017 17:21:25 -0400","from fmsmga004.fm.intel.com ([10.253.24.48])\n\tby orsmga102.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384;\n\t24 Oct 2017 14:21:20 -0700","from theros.lm.intel.com (HELO linux.intel.com) ([10.232.112.77])\n\tby fmsmga004.fm.intel.com with ESMTP; 24 Oct 2017 14:21:19 -0700"],"X-ExtLoop1":"1","X-IronPort-AV":"E=Sophos;i=\"5.43,429,1503385200\"; d=\"scan'208\";a=\"327237420\"","Date":"Tue, 24 Oct 2017 15:21:19 -0600","From":"Ross Zwisler ","To":"Jan Kara ","Cc":"Dan Williams ,\n\tRoss Zwisler ,\n\tChristoph Hellwig ,\n\tlinux-ext4@vger.kernel.org, linux-nvdimm@lists.01.org,\n\tlinux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org,\n\tlinux-api@vger.kernel.org, linux-mm@kvack.org,\n\tArnd Bergmann , Andy Lutomirski ,\n\tAndrew Morton ","Subject":"Re: [PATCH 01/17] mm: introduce MAP_SHARED_VALIDATE, a mechanism to\n\tsafely define new mmap flags","Message-ID":"<20171024212119.GB1611@linux.intel.com>","Mail-Followup-To":"Ross Zwisler ,\n\tJan Kara , Dan Williams ,\n\tChristoph Hellwig , linux-ext4@vger.kernel.org,\n\tlinux-nvdimm@lists.01.org, linux-fsdevel@vger.kernel.org,\n\tlinux-xfs@vger.kernel.org, linux-api@vger.kernel.org,\n\tlinux-mm@kvack.org, Arnd Bergmann ,\n\tAndy Lutomirski ,\n\tAndrew Morton ","References":"<20171024152415.22864-1-jack@suse.cz>\n\t<20171024152415.22864-2-jack@suse.cz>","MIME-Version":"1.0","Content-Type":"text/plain; charset=us-ascii","Content-Disposition":"inline","In-Reply-To":"<20171024152415.22864-2-jack@suse.cz>","User-Agent":"Mutt/1.9.1 (2017-09-22)","Sender":"linux-ext4-owner@vger.kernel.org","Precedence":"bulk","List-ID":"","X-Mailing-List":"linux-ext4@vger.kernel.org"}}]