linux-kernel - Re: [PATCH RFC v2] fhandle: expose u64 mount id to name_to_handle

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Date: Fri, 31 May 2024 01:14:13 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Christian Brauner <brauner@...nel.org>
Cc: Christoph Hellwig <hch@...radead.org>, Jan Kara <jack@...e.cz>,
	Aleksa Sarai <cyphar@...har.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Chuck Lever <chuck.lever@...cle.com>,
	Jeff Layton <jlayton@...nel.org>,
	Amir Goldstein <amir73il@...il.com>,
	Alexander Aring <alex.aring@...il.com>,
	linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH RFC v2] fhandle: expose u64 mount id to
 name_to_handle_at(2)

On Wed, May 29, 2024 at 09:40:01AM +0200, Christian Brauner wrote:
> Yeah, that's exactly what I figured and no that's not something we
> should do.
> 
> Not just can have a really large number of superblocks if you have mount
> namespaces and large container workloads that interface also needs to be
> highly privileged.

Again, that would be the most trivial POC.  We can easily do hash.

> Plus, you do have filesystems like btrfs that can be mounted multiple
> times with the same uuid.

Which doesn't matter.  Just like for NFS file handles the fs identifier
identifier plus the file part of the file handle need to be unique.

> And in general users will still need to be able to legitimately use a
> mount fd and not care about the handle type used with it.

I don't understand what you mean.  If we hand out file handles with
fsid that of course needs to be keyed off a new flag for both
name_to_handle and open_by_hnalde that makes them not interchangable
to handles generated without that flag.