From: Al Viro <viro@ZenIV.linux.org.uk>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Christoph Hellwig <hch@infradead.org>,
Andrew Morton <akpm@linux-foundation.org>,
Robo Bot <apw@canonical.com>, Felix Fietkau <nbd@openwrt.org>,
Neil Brown <neilb@suse.de>, Jordi Pujol <jordipujolp@gmail.com>,
ezk@fsl.cs.sunysb.edu, David Howells <dhowells@redhat.com>,
Sedat Dilek <sedat.dilek@googlemail.com>,
"J. R. Okajima" <hooanon05@yahoo.co.jp>
Subject: Re: [PATCH 00/13] overlay filesystem: request for inclusion (v16)
Date: Thu, 14 Mar 2013 22:59:40 +0000 [thread overview]
Message-ID: <20130314225940.GQ21522@ZenIV.linux.org.uk> (raw)
In-Reply-To: <CAJfpegsCirmYYimr6Nn4Q4Kdc7rMYaxWhk+fsmp+BLp6mjb_MQ@mail.gmail.com>
On Thu, Mar 14, 2013 at 11:37:50AM +0100, Miklos Szeredi wrote:
> > As for whiteouts... I think we ought to pull these bits of unionmoun
> > queue into the common stem and add the missing filesystems to them;
> > ext* and ufs are trivial (keep in mind that FFS derivatives, including
> > ext*, have d_type in directory entry and type 14 (DT_WHT) is there
> > precisely for that purpose). btrfs also has "dir_type" thing - 8bit
> > field...
>
> What about userspace interfaces? Are we allowed to extend d_type and
> st_mode without breaking things?
Huh?
* from st_mode point of view, it's not going to conflict with
anything; FFS "entry type" matches bits 12..15 of mode_t, and the value
picked by whoever had first implemented whiteouts had been chosen so
that it would not clash with any existing values. We have
#define S_IFMT 00170000
#define S_IFSOCK 0140000
#define S_IFLNK 0120000
#define S_IFREG 0100000
#define S_IFBLK 0060000
#define S_IFDIR 0040000
#define S_IFCHR 0020000
#define S_IFIFO 0010000
and this sucker would've been 0160000; new filesystem object types are not
frequently introduced, to put it mildly, so I wouldn't expect clashes.
Especially since any such clash would hit Solaris and *BSD (including
MacOS X) as well - all of them use that value for whiteouts.
* for practically all syscalls these guys are treated as file not
being there; e.g. mkdir() on such name results in whiteout silently
replaced with references to newly created subdirectory, etc.
* BSDs have one major exposure of those guys to userland - their
getdents(2) shows whiteouts (with d_type equal to DT_WHT and d_fileno - to 1).
Other than that (and we obviously do _not_ want that as default behaviour;
maybe a new O_<something> to produce that), there's practically nothing -
undelete(2) removes a whiteout in attempt to expose the object masked by
it, mknod(2) with S_IFWHT as type does create one. That's it.
IOW, I don't see how we'd get any kind of userland problems with those. And
this is not a new API - a bunch of BSD-derived Unices starting with, IIRC,
SunOS 3 and 4.3BSD, as well as their bastard progeny (Solaris, MacOS X)
had that since mid-80s...
next prev parent reply other threads:[~2013-03-14 22:59 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-03-12 15:41 [PATCH 00/13] overlay filesystem: request for inclusion (v16) Miklos Szeredi
2013-03-12 15:41 ` [PATCH 01/13] vfs-add-i_op-dentry_open Miklos Szeredi
2013-03-12 15:41 ` [PATCH 02/13] vfs-export-do_splice_direct-to-modules Miklos Szeredi
2013-03-12 15:41 ` [PATCH 03/13] vfs-introduce-clone_private_mount Miklos Szeredi
2013-03-12 15:41 ` [PATCH 04/13] overlay filesystem Miklos Szeredi
2013-03-12 15:41 ` [PATCH 05/13] overlayfs-add-statfs-support Miklos Szeredi
2013-03-12 15:41 ` [PATCH 06/13] overlayfs-implement-show_options Miklos Szeredi
2013-03-12 15:41 ` [PATCH 07/13] overlay-overlay-filesystem-documentation Miklos Szeredi
2013-03-12 15:41 ` [PATCH 08/13] fs-limit-filesystem-stacking-depth Miklos Szeredi
2013-03-12 15:41 ` [PATCH 09/13] overlayfs-fix-possible-leak-in-ovl_new_inode Miklos Szeredi
2013-03-12 15:41 ` [PATCH 10/13] overlayfs-create-new-inode-in-ovl_link Miklos Szeredi
2013-03-12 15:41 ` [PATCH 11/13] vfs-export-inode_permission-to-modules Miklos Szeredi
2013-03-12 15:41 ` [PATCH 12/13] ovl-switch-to-inode_permission Miklos Szeredi
2013-03-12 15:41 ` [PATCH 13/13] overlayfs-copy-up-i_uid-i_gid-from-the-underlying-inode Miklos Szeredi
2013-03-12 16:49 ` [PATCH 00/13] overlay filesystem: request for inclusion (v16) Sedat Dilek
2013-03-12 20:00 ` Miklos Szeredi
2013-03-12 17:22 ` J. R. Okajima
2013-03-12 20:01 ` Miklos Szeredi
2013-03-12 21:50 ` Linus Torvalds
2013-03-12 22:23 ` Al Viro
2013-03-13 9:42 ` Sedat Dilek
2013-03-13 15:24 ` Miklos Szeredi
2013-03-13 18:52 ` Al Viro
2013-03-13 22:09 ` Miklos Szeredi
2013-03-13 23:19 ` Al Viro
2013-03-14 10:37 ` Miklos Szeredi
2013-03-14 12:57 ` DT_WHT (Re: [PATCH 00/13] overlay filesystem: request for inclusion (v16)) J. R. Okajima
2013-03-14 22:59 ` Al Viro [this message]
2013-03-18 11:27 ` [PATCH 00/13] overlay filesystem: request for inclusion (v16) Miklos Szeredi
2013-03-13 10:09 ` Sedat Dilek
2013-03-13 10:57 ` Miklos Szeredi
2013-03-20 19:41 George Spelvin
2013-03-20 21:32 ` Al Viro
2013-03-22 5:15 ` Rob Landley
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