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Date:   Fri, 14 Apr 2023 03:32:11 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Jeff Layton <jlayton@kernel.org>
Cc:     Christian Brauner <brauner@kernel.org>,
        Dave Wysochanski <dwysocha@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-nfs <linux-nfs@vger.kernel.org>,
        David Howells <dhowells@redhat.com>, NeilBrown <neilb@suse.de>,
        Christoph Hellwig <hch@lst.de>
Subject: Re: allowing for a completely cached umount(2) pathwalk
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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?

> 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...

> 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...