linux-kernel - Re: [PATCH 0/4] pidfs: implement file handle support

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Message-ID: <e280163e-357e-400c-81e1-0149fa5bfc89@e43.eu>
Date: Wed, 13 Nov 2024 11:17:10 +0100
From: Erin Shepherd <erin.shepherd@....eu>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: Christian Brauner <brauner@...nel.org>, Jeff Layton <jlayton@...nel.org>,
 Amir Goldstein <amir73il@...il.com>, linux-kernel@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, christian@...uner.io, paul@...l-moore.com,
 bluca@...ian.org, Chuck Lever <chuck.lever@...cle.com>
Subject: Re: [PATCH 0/4] pidfs: implement file handle support

On 13/11/2024 01:40, Darrick J. Wong wrote:
>> Hmm, I guess I might have made that possible, though I'm certainly not
>> familiar enough with the internals of nfsd to be able to test if I've done
>> so.
> AFAIK check_export() in fs/nfsd/export.c spells this it out:
>
> 	/* There are two requirements on a filesystem to be exportable.
> 	 * 1:  We must be able to identify the filesystem from a number.
> 	 *       either a device number (so FS_REQUIRES_DEV needed)
> 	 *       or an FSID number (so NFSEXP_FSID or ->uuid is needed).
> 	 * 2:  We must be able to find an inode from a filehandle.
> 	 *       This means that s_export_op must be set.
> 	 * 3: We must not currently be on an idmapped mount.
> 	 */
>
> Granted I've been wrong on account of stale docs before. :$
>
> Though it would be kinda funny if you *could* mess with another
> machine's processes over NFS.
>
> --D

To be clear I'm not familiar enough with the workings of nfsd to tell if
pidfs fails those requirements and therefore wouldn't become exportable as
a result of this patch, though I gather from you're message that we're in the
clear?

Regardless I think my question is: do we think either those requirements could
change in the future, or the properties of pidfs could change in the future,
in ways that could accidentally make the filesystem exportable?

I guess though that the same concern would apply to cgroupfs and it hasn't posed
an issue so far.

- Erin