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Date:   Thu, 12 Oct 2023 10:26:24 +0200
From:   Christian Brauner <brauner@kernel.org>
To:     Miklos Szeredi <miklos@szeredi.hu>
Cc:     Amir Goldstein <amir73il@gmail.com>,
        Sebastian Wick <sebastian.wick@redhat.com>,
        Ryan Hendrickson <ryan.hendrickson@alum.mit.edu>,
        linux-unionfs@vger.kernel.org, Karel Zak <kzak@redhat.com>
Subject: Re: [regression?] escaping commas in overlayfs mount options
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> > Christian,
> >
> > Do you know any userspace that already uses your new append prefixes?
> > Do we have any good reason to support "lowerdir_first"
> > so a lower dir stack could be reset before creating the sb?
> 
> If that is a requirement, I suggest extending fsconfig(2) to allow
> resetting an option.

Overlayfs does already support this. If you pass:
fsconfig(FSCONFIG_SET_STRING, "lowerdir", "", ...)
then the lower layer stack is reset. I've implemented it that way in
ovl_parse_param_lowerdir().

> 
> > > > > >
> > > > > > Anyway, let's focus on what you would like best.
> > > > > > If you prefer to just fix the regression, it is doable.
> > > > > > If you prefer the upperdirfd, workdirfd, lowerdirfd API, I think we can
> > > > > > find a volunteer to write it up.
> 
> Can't the existing option names be overloaded if a separate cmd
> (FSCONFIG_SET_PATH or FSCONFIG_SET_PATH_EMPTY) is used in fsconfig()?

Yes, they can and filesystems do do that today depending on whether they
want to e.g., take an fd or a path or something.