From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753357AbdF2Of7 (ORCPT ); Thu, 29 Jun 2017 10:35:59 -0400 Received: from fieldses.org ([173.255.197.46]:50688 "EHLO fieldses.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751974AbdF2Ofw (ORCPT ); Thu, 29 Jun 2017 10:35:52 -0400 Date: Thu, 29 Jun 2017 10:35:51 -0400 From: "J. Bruce Fields" To: "Darrick J. Wong" Cc: William Koh , Andreas Dilger , "Theodore Ts'o" , linux-ext4 , lkml , Kernel Team , linux-fsdevel , Trond Myklebust , xfs Subject: Re: [PATCH] fs: ext4: inode->i_generation not assigned 0. Message-ID: <20170629143551.GB1651@fieldses.org> References: <20A40B3C-E179-432B-B56F-BDAAF0CD2E1F@dilger.ca> <7CD38230-D961-428F-B2E9-2C0E28CAF442@fb.com> <20170629045940.GB5865@birch.djwong.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170629045940.GB5865@birch.djwong.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 28, 2017 at 09:59:40PM -0700, Darrick J. Wong wrote: > AFAICT, i_generation == 0 in XFS and btrfs is just as valid as any other > number. There is no special casing of zero in either filesystem. > > So now, my curiosity intrigued, I surveyed all the Linux filesystems > that can export to NFS. I see that there are actually quite a few fs > (ext[2-4], exofs, efs, fat, jfs, f2fs, isofs, nilfs2, reiserfs, udf, > ufs) that treat zero as a special value meaning "ignore generation > check"; others (xfs, btrfs, fuse, ntfs, ocfs2) that don't consider zero > special and always require a match; and still others (affs, befs, ceph, > gfs2, jffs2, squashfs) that don't check at all. > > That to mean strongly suggests that more research is necessary to figure > out why some of the filesystems that support i_generation reserve zero > as a special value to disable generation checks and why others always > require an exact match. Until we can recapture why things are they way > they are, it doesn't make much sense to have a helper that only applies > to half the filesystems. >>From a quick look at ext2; correct me if I got anything wrong: - it looks like this is *only* used by NFS fh->inode lookups, there's not some other internal use for this special case. - filehandles are never encoded with i_generation 0, so will never be returned to clients. So, this could only ever be used by an NFS client. But the only NFS client that could ever use it would be a non-standard client that knew this special feature of these particular filesystems. Sounds like at most a slightly useful tool for malicious clients attempting filehandle-guessing attacks. I'm probably missing something. --b.