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

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Message-ID: <ZlS0_DWzGk24GYZA@infradead.org>
Date: Mon, 27 May 2024 09:29:48 -0700
From: "hch@...radead.org" <hch@...radead.org>
To: Trond Myklebust <trondmy@...merspace.com>
Cc: "hch@...radead.org" <hch@...radead.org>, "jack@...e.cz" <jack@...e.cz>,
	"chuck.lever@...cle.com" <chuck.lever@...cle.com>,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"brauner@...nel.org" <brauner@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"alex.aring@...il.com" <alex.aring@...il.com>,
	"cyphar@...har.com" <cyphar@...har.com>,
	"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
	"jlayton@...nel.org" <jlayton@...nel.org>,
	"amir73il@...il.com" <amir73il@...il.com>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>
Subject: Re: [PATCH RFC v2] fhandle: expose u64 mount id to
 name_to_handle_at(2)

On Mon, May 27, 2024 at 03:38:40PM +0000, Trond Myklebust wrote:
> > It
> > does not matter what mount you use to access it.
> 
> Sure. However if you are providing a path argument, then presumably you
> need to know which file system (aka super_block) it eventually resolves
> to.

Except that you can't, at least not without running into potential
races.  The only way to fix a race vs unmount/remount is to include
the fsid part in the kernel generated file handle.

> 
> If your use case isn't NFS servers, then what use case are you
> targeting, and how do you expect those applications to use this API?

The main user of the open by handle syscalls seems to be fanotify
magic.