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Date: Wed, 21 Jan 2026 11:12:34 +0100
From: Christoph Hellwig <hch@lst.de>
To: Amir Goldstein <amir73il@gmail.com>
Cc: Christian Brauner <brauner@kernel.org>,
	Chuck Lever <chuck.lever@oracle.com>,
	Jeff Layton <jlayton@kernel.org>, Christoph Hellwig <hch@lst.de>,
	Neil Brown <neilb@suse.de>, Jan Kara <jack@suse.cz>,
	linux-fsdevel@vger.kernel.org, linux-nfs@vger.kernel.org
Subject: Re: [PATCH] nfsd: do not allow exporting of special kernel
 filesystems
Message-ID: <20260121101234.GA22918@lst.de>
References: <20260121085028.558164-1-amir73il@gmail.com>
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On Wed, Jan 21, 2026 at 09:50:27AM +0100, Amir Goldstein wrote:
> pidfs and nsfs recently gained support for encode/decode of file handles
> via name_to_handle_at(2)/opan_by_handle_at(2).
> 
> These special kernel filesystems have custom ->open() and ->permission()
> export methods, which nfsd does not respect and it was never meant to be
> used for exporting those filesystems by nfsd.
> 
> Therefore, do not allow nfsd to export filesystems with custom ->open()
> or ->permission() methods.

Yeah, this was added in and not used in the existing export_ops users.

> +	/*
> +	 * The requirements for a filesystem to be exportable:
> +	 * 1. The filehandle must identify a filesystem by number
> +	 * 2. The filehandle must uniquely identify an inode
> +	 * 3. The filesystem must not have custom filehandle open/perm methods
> +	 * 4. The requested file must not reside on an idmapped mount
>  	 */

Please spell out here why ->open and ->permission are not allowed.
Listing what the code does is generally not that useful, while why
it does that provides value.

While looking this I have to say the API documentation for these
methods in exportfs.h is unfortunately completely useless as well.
It doesn't mention the limitation that it's only used by the
non-exportfs code, and also doesn't mention why a file system
would implement or have to implement them :(  The commit messages
adding them are just as bad as well.