Re: [PATCH 0/4] exportfs: add flag to allow marking export operations as only supporting file handles

[Christoph Hellwig <hch@infradead.org>]

On Sat, Dec 07, 2024 at 09:49:02AM +0100, Amir Goldstein wrote:
> > /* file handles can be used by a process on another node */
> > #define EXPORT_OP_ALLOW_REMOTE_NODES    (...)
> 
> This has a sound of security which is incorrect IMO.
> The fact that we block nfsd export of cgroups does not prevent
> any type of userland file server from exporting cgroup file handles.

So what is the purpose of the flag?  Asking for a coherent name and
description was the other bigger ask for me.

> Maybe opt-out of nfsd export is a little less safer than opt-in, but
> 1. opt-out is and will remain the rare exception for export_operations
> 2. at least the flag name EXPORT_OP_LOCAL_FILE_HANDLE
>     is pretty clear IMO

Even after this thread I have absolutely no idea what problem it tries
to solve.  Maybe that's not just the flag names fault, and not of opt-in
vs out, but both certainly don't help.

> Plus, as I wrote in another email, the fact that pidfs is SB_NOUSER,
> so userspace is not allowed to mount it into the namespace and
> userland file servers cannot export the filesystem itself.
> That property itself (SB_NOUSER), is therefore a good enough indication
> to deny nfsd export of this fs.

So check SB_NOUSER in nfsd and be done with it?