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

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Date: Mon, 27 May 2024 04:47:56 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Aleksa Sarai <cyphar@...har.com>
Cc: Christoph Hellwig <hch@...radead.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	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 Sun, May 26, 2024 at 12:01:08PM -0700, Aleksa Sarai wrote:
> The existing interface already provides a mount ID which is not even
> safe without rebooting.

And that seems to be a big part of the problem where the Linux by handle
syscall API deviated from all know precedence for no good reason.  NFS
file handles which were the start of this do (and have to) encode a
persistent file system identifier.  As do the xfs handles (although they
do the decoding in the userspace library on Linux for historic reasons),
as do the FreeBSD equivalents to these syscalls.

> An alternative would be to return something unique to the filesystem
> superblock, but as far as I can tell there is no guarantee that every
> Linux filesystem's fsid is sufficiently unique to act as a globally
> unique identifier. At least with a 64-bit mount ID and statmount(2),
> userspace can decide what information is needed to get sufficiently
> unique information about the source filesystem.

Well, every file system that supports export ops already needs a
globally unique ID for NFS to work properly.  We might not have good
enough interfaces for that, but that shouldn't be too hard.