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Date:   Mon, 17 Jan 2022 17:37:57 -0500
From:   "J. Bruce Fields" <bfields@fieldses.org>
To:     Chris Chilvers <chilversc@gmail.com>
Cc:     linux-nfs@vger.kernel.org
Subject: Re: [bug report] Resolving symlinks ignores rootdir setting
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On Fri, Jan 14, 2022 at 03:57:03PM +0000, Chris Chilvers wrote:
> > I'm probably just reading too quickly, but I'm not seeing how this
> > explains the problems with your original configuration.
> >
> > Is it that /sr/nfs/bin on you system is a symlink?  (And what exactly is
> > the content of that symlink?)
> 
> No, all the directories under /srv/nfs are ordinary directories. The symlinks
> are in the root of the real file system.

Oh, got it, thanks for the explanation.

> So I have:
>   /bin -> /usr/bin
>   /software -> /usr/lib (created to test the hypothesis)
>   /srv/nfs/
>   /srv/nfs/assets
>   /srv/nfs/bin
>   /srv/nfs/software
> 
> Because exportfs is not taking into account the rootdir setting, when it tries
> to resolve the path from /etc/exports, it sees the path /bin, and matches that
> with the real /bin, and resolves it to /usr/bin.
> 
> Later exportfs errors when trying to resolve the real path (e_realpath) in the
> exportent_mkrealpath function, as this function does prepend the rootdir. This
> causes exportfs to look for /srv/nfs/usr/bin instead of the expected
> /srv/nfs/bin.
> 
> At best this causes an error, but if that does happen to match an existing path
> under /srv/nfs then it would export the wrong directory.
> 
> This will happen for any export that happens to match an existing symlink.
> 
> There are two issues though that I'm not sure how best to handle:
> * Should the symlink be considered relative to the local system or relative to
>   the rootdir?
> * What happens if the symlink points to a directory outside the rootdir?

Yes, I'm not sure off hand what the right behavior is, but the existing
behavior certainly seems to violate the principal of least surprise.

--b.

> 
> In my case I think the symlink would need to be resolved relative to the
> rootdir. This is because we're re-exporting an existing NFS server, so if the
> existing NFS server is exporting some kind of symlink tree, the symlinks would
> be relative to the existing mounts.
> 
> One option that would be valid for my case would be an setting to disable
> symlink resolution and just consider it an error if an export is a symlink.
> Since I'm re-exporting an existing NFS server, they can't be symlinks anyway.
> 
> -- Chris