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Date: Tue, 28 May 2024 09:12:33 +0200
From: Christian Brauner <brauner@kernel.org>
To: "hch@infradead.org" <hch@infradead.org>
Cc: Trond Myklebust <trondmy@hammerspace.com>, 
	"jack@suse.cz" <jack@suse.cz>, "chuck.lever@oracle.com" <chuck.lever@oracle.com>, 
	"linux-api@vger.kernel.org" <linux-api@vger.kernel.org>, "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>, 
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "alex.aring@gmail.com" <alex.aring@gmail.com>, 
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	"jlayton@kernel.org" <jlayton@kernel.org>, "amir73il@gmail.com" <amir73il@gmail.com>, 
	"linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH RFC v2] fhandle: expose u64 mount id to
 name_to_handle_at(2)
Message-ID: <20240528-fraglich-abmildern-cca211d1791c@brauner>
References: <20240523-exportfs-u64-mount-id-v2-1-f9f959f17eb1@cyphar.com>
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 <86065f6a4f3d2f3d78f39e7a276a2d6e25bfbc9d.camel@hammerspace.com>
 <ZlS0_DWzGk24GYZA@infradead.org>
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On Mon, May 27, 2024 at 09:29:48AM -0700, hch@infradead.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.