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

[Christian Brauner]

On Mon, May 27, 2024 at 04:47:56AM -0700, Christoph Hellwig wrote:
> On Sun, May 26, 2024 at 12:01:08PM -0700, Aleksa Sarai wrote:
> > The existing interface already provides a mount ID which is not even
> > safe without rebooting.
> 
> And that seems to be a big part of the problem where the Linux by handle
> syscall API deviated from all know precedence for no good reason.  NFS
> file handles which were the start of this do (and have to) encode a
> persistent file system identifier.  As do the xfs handles (although they
> do the decoding in the userspace library on Linux for historic reasons),
> as do the FreeBSD equivalents to these syscalls.
> 
> > An alternative would be to return something unique to the filesystem
> > superblock, but as far as I can tell there is no guarantee that every
> > Linux filesystem's fsid is sufficiently unique to act as a globally
> > unique identifier. At least with a 64-bit mount ID and statmount(2),
> > userspace can decide what information is needed to get sufficiently
> > unique information about the source filesystem.
> 
> Well, every file system that supports export ops already needs a
> globally unique ID for NFS to work properly.  We might not have good
> enough interfaces for that, but that shouldn't be too hard.

I see not inherent problem with exposing the 64 bit mount id through
name_to_handle_at() as we already to expose the old one anyway.

But I agree that it is useful if we had the guarantee that file handles
are unique in the way you describe. As it currently stands that doesn't
seem to be the case and userspace doesn't seem to have a way of figuring
out if the handle provided by name_to_handle_at() is indeed unique as
you describe and can be reliably passed to open_by_handle_at().

Yes, we should fix it but that's really orthogonal to the mount id. It
is separately useful and we already do expose it anyway.