Re: Union mounts and fchown/fchmod/utimensat

[Jamie Lokier <jamie@shareable.org>]

Valerie Aurora wrote:
> I'm working on the assumption that this is okay.  Basically, an
> in-kernel copyup has the same effect on an application with an
> O_RDONLY fd as a userland copy-up to a temporary file followed by a
> rename() to the original name.  Editors, among other applications, use
> this sequence all the time.  If it breaks under union mount copyup, it
> will also break if you edit the file with vi.

Ouch.  That's fine for rename-over style updates, but if an
application depends on in-place updates, or if it depends on file
locking to serialise something (using O_RDONLY for read locks and
O_RDWR for write locks), it will get confused.

Now, that may be acceptable compromise non-POSIX behaviour.

But if it only works with rename-over style updates.... Why have
in-kernel copyup at all?  Why not simply forbid writes to lower files?
Rename-over updates will be fine with that, and there'll be no
unexpected long delays to do copyup of a huge file.

-- Jamie