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

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Message-ID: <CAOQ4uxjcVuq+PCoMos5Vi=t_S1OgJEM5wQ6Za2Ue9_FOq31m9Q@mail.gmail.com>
Date: Mon, 9 Dec 2024 17:30:24 +0100
From: Amir Goldstein <amir73il@...il.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: "Darrick J. Wong" <djwong@...nel.org>, Christian Brauner <brauner@...nel.org>, 
	Jeff Layton <jlayton@...nel.org>, Erin Shepherd <erin.shepherd@....eu>, 
	Chuck Lever <chuck.lever@...cle.com>, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-nfs@...r.kernel.org, 
	stable <stable@...nel.org>, Greg KH <gregkh@...uxfoundation.org>, 
	Jens Axboe <axboe@...nel.dk>, Shaohua Li <shli@...com>
Subject: Re: [PATCH 0/4] exportfs: add flag to allow marking export operations
 as only supporting file handles

On Mon, Dec 9, 2024 at 2:46 PM Christoph Hellwig <hch@...radead.org> wrote:
>
> On Mon, Dec 09, 2024 at 09:58:58AM +0100, Amir Goldstein wrote:
> > To be clear, exporting pidfs or internal shmem via an anonymous fd is
> > probably not possible with existing userspace tools, but with all the new
> > mount_fd and magic link apis, I can never be sure what can be made possible
> > to achieve when the user holds an anonymous fd.
> >
> > The thinking behind adding the EXPORT_OP_LOCAL_FILE_HANDLE flag
> > was that when kernfs/cgroups was added exportfs support with commit
> > aa8188253474 ("kernfs: add exportfs operations"), there was no intention
> > to export cgroupfs over nfs, only local to uses, but that was never enforced,
> > so we thought it would be good to add this restriction and backport it to
> > stable kernels.
>
> Can you please explain what the problem with exporting these file
> systems over NFS is?  Yes, it's not going to be very useful.  But what
> is actually problematic about it?  Any why is it not problematic with
> a userland nfs server?  We really need to settle that argumet before
> deciding a flag name or polarity.
>

I agree that it is not the end of the world and users do have to explicitly
use fsid= argument to be able to export cgroupfs via nfsd.

The idea for this patch started from the claim that Jeff wrote that cgroups
is not allowed for nfsd export, but I couldn't find where it is not allowed.

I have no issue personally with leaving cgroupfs exportable via nfsd
and changing restricting only SB_NOUSER and SB_KERNMOUNT fs.

Jeff, Chuck, what is your opinion w.r.t exportability of cgroupfs via nfsd?

Thanks,
Amir.