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SubjectRe: [VFS] move active filesystem
On Wed, May 12, 1999 at 11:55:14AM -0400, Alexander Viro wrote:
>
>
> On Wed, 12 May 1999, Matthew Wilcox wrote:
>
> >
> > Hi, Al. You seem to be the most active developer in the VFS, and there
> > seems to be no official maintainer for it, so I thought I'd try feeding
> > this patch to you.
> >
> > Basically all this patch does is allow you to specify a new directory
> > when remounting a filesystem. I've had several people using it, and
> > the one reported bug has been fixed.
>
> [snip the patch]
>
> It looks sane, but... I don't see where it prevents remounting of /usr to
> /usr/local/foo. Could you comment on this?

You are correct, it does not prevent that. My reasoning is that root
has a large number of ways to screw up the system already and detaching
a filesystem from the namespace by a trick like this is pretty minor
in comparison. After all, the filesystem may be reattached later.
A check could be added for the simplest case (preventing mounting a
filesystem on itself, but consider:

/dev/a /
/dev/b /b
/dev/c /b/c

Now remount /b on /b/c/d and it's gone.

Would a check that the current mount point is not a prefix of the new
mount point be sufficient to catch all possible loops?

Another question... is this necessarily a bad thing? Yes, it can get you
into situations that you can't get into now, but you can always return
to a sensible state. I'm having trouble thinking of a use for this that
chroot doesn't give you, to be fair.

--
Matthew Wilcox <willy@bofh.ai>
"Windows and MacOS are products, contrived by engineers in the service of
specific companies. Unix, by contrast, is not so much a product as it is a
painstakingly compiled oral history of the hacker subculture." - N Stephenson

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