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 546D8E743C2 for ; Thu, 28 Sep 2023 21:27:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232382AbjI1V1c (ORCPT ); Thu, 28 Sep 2023 17:27:32 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49892 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232287AbjI1V13 (ORCPT ); Thu, 28 Sep 2023 17:27:29 -0400 Received: from outgoing.mit.edu (outgoing-auth-1.mit.edu [18.9.28.11]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CA08B19E for ; Thu, 28 Sep 2023 14:27:27 -0700 (PDT) Received: from cwcc.thunk.org (pool-173-48-111-87.bstnma.fios.verizon.net [173.48.111.87]) (authenticated bits=0) (User authenticated as tytso@ATHENA.MIT.EDU) by outgoing.mit.edu (8.14.7/8.12.4) with ESMTP id 38SLQvsp021536 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Thu, 28 Sep 2023 17:26:58 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=mit.edu; s=outgoing; t=1695936425; bh=w4hql40FuyyfsTNK9D530X9ExV2y5b7TU7/1a0HHsnU=; h=Date:From:Subject:Message-ID:MIME-Version:Content-Type; b=NGPOVv6aP7Wgx4/k9ruqGjDYWmjUQVAfEL5fppG6yE2RNxUM6uiyoWrXBZfGmZyL2 gb1zEZjWcSSCFd+NWg1/BYhZ5jeEptXNin1ZAM6seg38Angccj8YOPtyzxOx/hDFrb Q+ePr/t98/fHq2BcSzJCThnFTNb50fVmlIrGfb3oMvB11ToZpH+p8R2mhaDk4/44dl uEHGVh6Q6Ih5iyFuN8+PasXijpXT1PXJm+5Rkla8PX5Glna1OAsFe3wre6CQl62oa4 RAr1Z2b44rKF067M11yaRPI56z2wMDur4vLyEnjrhgmaYzW5D9XQ8I+PJlbP887wDq NXSNNWLPWiTbQ== Received: by cwcc.thunk.org (Postfix, from userid 15806) id 06AD715C0266; Thu, 28 Sep 2023 17:26:57 -0400 (EDT) Date: Thu, 28 Sep 2023 17:26:56 -0400 From: "Theodore Ts'o" To: Jeff Layton Cc: "Darrick J. Wong" , Arnd Bergmann , Alexander Viro , Christian Brauner , Linus Torvalds , David Sterba , Amir Goldstein , "Eric W. Biederman" , Kees Cook , Jeremy Kerr , Michael Ellerman , Nicholas Piggin , Christophe Leroy , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Christian Borntraeger , Sven Schnelle , Greg Kroah-Hartman , Arve =?iso-8859-1?B?SGr4bm5lduVn?= , Todd Kjos , Martijn Coenen , Joel Fernandes , Carlos Llamas , Suren Baghdasaryan , Mattia Dongili , Dennis Dalessandro , Jason Gunthorpe , Leon Romanovsky , Brad Warrum , Ritu Agarwal , Hans de Goede , Ilpo =?iso-8859-1?Q?J=E4rvinen?= , Mark Gross , Jiri Slaby , Eric Van Hensbergen , Latchesar Ionkov , Dominique Martinet , Christian Schoenebeck , David Sterba , David Howells , Marc Dionne , Ian Kent , Luis de Bethencourt , Salah Triki , "Tigran A. Aivazian" , Chris Mason , Josef Bacik , Xiubo Li , Ilya Dryomov , Jan Harkes , coda@cs.cmu.edu, Joel Becker , Christoph Hellwig , Nicolas Pitre , "Rafael J . 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Hallyn" , Stephen Smalley , Eric Paris , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, platform-driver-x86@vger.kernel.org, linux-rdma@vger.kernel.org, linux-serial@vger.kernel.org, linux-usb@vger.kernel.org, v9fs@lists.linux.dev, linux-afs@lists.infradead.org, autofs@vger.kernel.org, linux-btrfs@vger.kernel.org, ceph-devel@vger.kernel.org, codalist@telemann.coda.cs.cmu.edu, linux-efi@vger.kernel.org, linux-erofs@lists.ozlabs.org, linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, gfs2@lists.linux.dev, linux-um@lists.infradead.org, linux-mtd@lists.infradead.org, jfs-discussion@lists.sourceforge.net, linux-nfs@vger.kernel.org, linux-nilfs@vger.kernel.org, linux-ntfs-dev@lists.sourceforge.net, ntfs3@lists.linux.dev, ocfs2-devel@lists.linux.dev, linux-karma-devel@lists.sourceforge.net, devel@lists.orangefs.org, linux-unionfs@vger.kernel.org, linux-hardening@vger.kernel.org, reiserfs-devel@vger.kernel.org, linux-cifs@vger.kernel.org, samba-technical@lists.samba.org, linux-trace-kernel@vger.kernel.org, linux-xfs@vger.kernel.org, bpf@vger.kernel.org, Netdev , apparmor@lists.ubuntu.com, linux-security-module@vger.kernel.org, selinux@vger.kernel.org Subject: Re: [PATCH 86/87] fs: switch timespec64 fields in inode to discrete integers Message-ID: <20230928212656.GC189345@mit.edu> References: <20230928110554.34758-1-jlayton@kernel.org> <20230928110554.34758-2-jlayton@kernel.org> <6020d6e7-b187-4abb-bf38-dc09d8bd0f6d@app.fastmail.com> <20230928171943.GK11439@frogsfrogsfrogs> <6a6f37d16b55a3003af3f3dbb7778a367f68cd8d.camel@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <6a6f37d16b55a3003af3f3dbb7778a367f68cd8d.camel@kernel.org> Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Thu, Sep 28, 2023 at 01:40:55PM -0400, Jeff Layton wrote: > > Correct. We'd lose some fidelity in currently stored timestamps, but as > Linus and Ted pointed out, anything below ~100ns granularity is > effectively just noise, as that's the floor overhead for calling into > the kernel. It's hard to argue that any application needs that sort of > timestamp resolution, at least with contemporary hardware. > > Doing that would mean that tests that store specific values in the > atime/mtime and expect to be able to fetch exactly that value back would > break though, so we'd have to be OK with that if we want to try it. The > good news is that it's relatively easy to experiment with new ways to > store timestamps with these wrappers in place. The reason why we store 1ns granularity in ext4's on-disk format (and accept that we only support times only a couple of centuries into the future, as opposed shooting for an on-disk format good for several millennia :-), was in case there was userspace that might try to store a very fine-grained timestamp and want to be able to get it back bit-for-bit identical. For example, what if someone was trying to implement some kind of steganographic scheme where they going store a secret message (or more likely, a 256-bit AES key) in the nanosecond fields of the file's {c,m,a,cr}time timestamps, "hiding in plain sight". Not that I think that we have to support something like that, since the field is for *timestamps* not cryptographic bits, so if we break someone who is doing that, do we care? I don't think anyone will complain about breaking the userspace API --- especially since if, say, the CIA was using this for their spies' drop boxes, they probably wouldn't want to admit it. :-) - Ted