Re: [PATCH RFC v2] fhandle: expose u64 mount id to name_to_handle_at(2)

[Christian Brauner <brauner@kernel.org>]

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.