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

[Christoph Hellwig <hch@infradead.org>]

On Mon, May 27, 2024 at 03:34:30PM +0200, Jan Kara wrote:
> So I was wondering how this is actually working in practice. Checking the
> code, NFS server is (based on configuration in /etc/exports) either using
> device number as the filesystem identifier or fsid / uuid as specified in
> /etc/exports.

Yes, it's a rather suboptimal implementation.

> So returning the 64-bit mount ID from name_to_handle_at() weasels out of
> these "how to identify arbitrary superblock" problems by giving userspace a
> reasonably reliable way to generate this superblock identifier itself. I'm
> fully open to less errorprone API for this but at this point I don't see it
> so changing the mount ID returned from name_to_handle_at() to 64-bit unique
> one seems like a sane practical choice to me...

Well, how about we fix the thing for real:

 - allow file systems to provide a uniqueu identifier of at least
   uuid size (16 bytes) in the superblock or through an export operation
 - for non-persistent file systems allow to generate one at boot time
   using the normal uuid generation helpers
 - add a new flag to name_to_handle_at/open_by_handle_at to include it
   in the file handle, and thus make the file handle work more like
   the normal file handle
 - add code to nfsd to directly make use of this

This would solve all the problems in this proposal as well as all the
obvious ones it doesn't solve.