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SubjectRe: [PATCH 00/13] overlay filesystem: request for inclusion (v16)
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...


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