Re: [PATCH] xfs: use a unique and persistent value for f_fsid

[Christoph Hellwig <hch@infradead.org>]

On Wed, Mar 24, 2021 at 08:53:25AM +0200, Amir Goldstein wrote:
> > This also means that userspace can be entirely filesystem agnostic
> > and it doesn't need to rely on parsing proc files to translate
> > ephemeral mount IDs to paths, statvfs() and hoping that f_fsid is
> > stable enough that it doesn't get the destination wrong.  It also
> > means that fanotify UAPI probably no longer needs to supply a
> > f_fsid with the filehandle because it is built into the
> > filehandle....
> >
> 
> That is one option. Let's call it the "bullet proof" option.
> 
> Another option, let's call it the "pragmatic" options, is that you accept
> that my patch shouldn't break anything and agree to apply it.

Your patch may very well break something.  Most Linux file systems do
store the dev_t in the fsid and userspace may for whatever silly
reasons depend on it.

Also trying to use the fsid for anything persistent is plain stupid,
64-bits are not enough entropy for such an identifier.  You at least
need a 128-bit UUID-like identifier for that.

So I think this whole discussion is going in the wrong direction.
Is exposing a stable file system identifier useful?  Yes, for many
reasons.  Is repurposing the fsid for that a good idea?  Hell no.