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

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Date: Tue, 28 May 2024 06:22:23 -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 Tue, May 28, 2024 at 02:04:16PM +0200, Christian Brauner wrote:
> Can you please explain how opening an fd based on a handle returned from
> name_to_handle_at() and not using a mount file descriptor for
> open_by_handle_at() would work?

Same as NFS file handles:

name_to_handle_at returns a handle that includes a file system
identifier.

open_by_handle_at looks up the superblock based on that identifier.

For the identifier I could imagin three choices:

 1) use the fsid as returned in statfs and returned by fsnotify.
    The downside is that it is "only" 64-bit.  The upside is that
    we have a lot of plumbing for it
 2) fixed 128-bit identifier to provide more entropy
 3) a variable length identifier, which is more similar to NFS,
    but also a lot more complicated

We'd need a global lookup structure to find the sb by id.  The simplest
one would be a simple linear loop over super_blocks which isn't terribly
efficient, but probably better than whatever userspace is doing to
find a mount fd right now.

Let me cook up a simple prototype for 1) as it shouldn't be more than
a few hundred lines of code.