Re: mount(2) in 2.3.99pre9!!!

From: Chris Evans (
Date: Thu Jun 01 2000 - 16:03:11 EST

On Thu, 1 Jun 2000, Alexander Viro wrote:

> > Can anyone elaborate on what precisely we allow and why? Despite being
> > named "do_loopback", it looks like a simple aliasing mechanism.
> Yes, it is, and yes, it happened to be simple after some work ;-)
> > At the very least, mount_is_safe() would seem to be missing a check for
> > write permission on the parent directory of the mount-point.
> Hmm... What's the problem with situation when you have write permissions
> on mountpoint but not on its parent? MAY_WRITE on mountpoint is checked,
> unless I've really fscked up. Comments on security implications are more
> than welcome - the variant I've done was, basically, "can we reach the
> thing to be mounted anyway and can we already do whatever we want with the
> contents of the mountpoint?"

Hi Al

Thanks for replying. Thinking about it, it's logical to demand write
access in parent directory (P).

We are essentially replacing a dentry in P with a totally different
one. This is semantically equivalent to

mv(old_dentry, backup_dentry)

And the above sequence certainly requires write permission in P

The only difference is that the mount "bind" case does the above sequence


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