Re: [Qemu-devel] [PATCH] A different way to ask for readonly drive

[Jamie Lokier <jamie@shareable.org>]

Christoph Hellwig wrote:
> On Tue, Dec 15, 2009 at 06:45:01PM +0000, Jamie Lokier wrote:
> > access=rw
> > access=ro
> > access=auto  (default)
> 
> Yes, that sounds like the least clumsy one.  I still think the current
> implementation is a very bad default, though.

Without agreeing or disagreeing over whether it's a bad default :), a
usability problem occurs with the current implementation when you
deliberately "chmod 444" an image to have high confidence that it's
opened read only: When running as root, file permissions are ignored
(except sometimes on NFS).

For that reason I use "chattr +i" on all my read-only image files, to
really make sure that no qemu invocation mistake could accidentally
corrupt valuable images.  That works, but it's not very convenient.

If the "auto" method is kept, I think it would be an improvement if it
checks the file permission itself, and does not even try to open a
file O_RDWR if there are no writable permission bits - so that "chmod
444" has the same "open as read only" effect when qemu is invoked as root.

-- Jamie