Re: [RFC PATCH] Show data flow for file copyup in unions

[Valerie Aurora <vaurora@redhat.com>]

On Wed, Mar 17, 2010 at 07:51:31AM +0900, J. R. Okajima wrote:
> 
> Valerie Aurora:
> > 1. Don't copyup if the operation would fail (e.g., open(O_WRONLY) on a
> > file with mode 444).  It's inefficient and a possible security hole to
> > copy up a file if no write (or maybe even read) can occur anyway.
> 
> Just a question.
> How about this case?
> When the file is writable (0644 or something) but its parent directory
> is readonly (0555), do you think the file should be copied-up?

This problem comes up with readdir() too, since we copy up all the
directory entries from the lower layer on what is ostensibly a
read-only access.

I think what people will expect is that we copy up in that case.  I
can think of ways this can go wrong, but perhaps that should be an
explicit requirement on the top-layer file system, that it can handle
create/unlink() in a directory without write permission.

-VAL