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Re: kern/46626: EROFS enforcement in namei breaks union mounts
The following reply was made to PR kern/46626; it has been noted by GNATS.
From: David Laight <david%l8s.co.uk@localhost>
Subject: Re: kern/46626: EROFS enforcement in namei breaks union mounts
Date: Mon, 25 Jun 2012 07:55:19 +0100
On Sun, Jun 24, 2012 at 11:15:00PM +0000, dholland%NetBSD.org@localhost wrote:
> As I understand the way union mounts are supposed to work based on
> Plan 9, for any operation, the layer in a union stack where the named
> target exists should be selected to perform the operation. If the
> named target does not exist, the top layer should be used. Having a
> read-only upper layer arbitrarily intercept and deny all write
> operations breaks these semantics.
Letting a rename move files between layers does seem a little wrong!
> (There's another wrinkle with rename, which is that if the to-file
> doesn't exist, the layer of the from-file should be selected.
> Otherwise, most rename operations will give EXDEV. This is not easy to
> arrange, but it's a different problem.)
I think the layer of the from-file should always be selected.
This might mean you can move a file so that it is hidden! but
you really don't want to move a file from a low-layer into a
higher layer just because the file exists in the higher layer.
Does the NetBSD code auto-created (and auto-delete) directories
in the top layer?
Somewhere I've probably got the sources for a 32-layer union
mount filesystem (for SYSV) - which I fixed somewhat!
Unless a specific layer was specified (append "\\n" to the name)
all creates were done in layer 0, everything else just did a
scan through the directories then 'handed off' the request to
the located vnode.
(Directory reads are hard, IIRC I left them O(N^2)).
It was a shame that the kernel exec code didn't allow for VOP_OPEN()
changing the vnode - otherwise the whole fs would only have had
to process VOP_LOOKUP() and directory requests!
David Laight: david%l8s.co.uk@localhost
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