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

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Date: Tue, 28 May 2024 09:12:33 +0200
From: Christian Brauner <brauner@...nel.org>
To: "hch@...radead.org" <hch@...radead.org>
Cc: Trond Myklebust <trondmy@...merspace.com>, 
	"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>, 
	"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 09:29:48AM -0700, hch@...radead.org wrote:
> 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.

It's also used by userspace for uniquely identifying cgroups via handles
as cgroups and - even without open_by_handle_at() - to check whether a
file is still valid.

And again a 64bit mount is is a simple way to race-free go to whatever
superblock uuid you want. They cannot be recycled and are unique for the
lifetime of the system.