Re: allowing for a completely cached umount(2) pathwalk

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2023-04-14 at 03:32 +0100, Al Viro wrote:
> On Thu, Apr 13, 2023 at 06:00:42PM -0400, Jeff Layton wrote:
> 
> > It describes a situation where there are nested NFS mounts on a client,
> > and one of the intermediate mounts ends up being unexported from the
> > server. In a situation like this, we end up being unable to pathwalk
> > down to the child mount of these unreachable dentries and can't unmount
> > anything, even as root.
> 
> So umount -l the stuck sucker.  What's the problem with that?
> 

You mean lazy umount the parent that is stuck? What happens to the child
mount in that case? Is it also eventually cleaned up?

> > 2/ disallow ->lookup operations: a umount is about removing an existing
> > mount, so the dentries had better already be there.
> 
> That changes the semantics; as it is, you need exec permissions on the
> entire path...
> 

Yep. But, I think it makes some sense to do so in the context of a
umount. Mostly, umount is done by a privileged user anyway so avoiding
permission checks isn't too great a stretch, IMO.

> > Is this a terrible idea? Are there potentially problems with
> > containerized setups if we were to do something like this? Are there
> > better ways to solve this problem (and others like it)? Maybe this would
> > be best done with a new UMOUNT_CACHED flag for umount2()?
> 
> We already have lazy umount.  And what will you do to symlinks you run
> into along the way?  They *are* traversed; requiring the caller to
> canonicalize them will only shift the problem to userland...

Yeah, I hadn't considered symlinks here. Still, if we have a cached
symlink dentry, wouldn't we also already have the symlink target in
cache too? Or is that not guaranteed?

-- 
Jeff Layton <jlayton@kernel.org>