linux-kernel - Re: How to support directory opacity in a filesystem for overlayfs to use?

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Message-ID: <CAJfpegsVd3MyHBuGXApbwY3pYz5H2+2Xxm3O8FrJgboi2S5CWw@mail.gmail.com>
Date: Tue, 14 Jan 2025 17:23:07 +0100
From: Miklos Szeredi <miklos@...redi.hu>
To: David Howells <dhowells@...hat.com>
Cc: mszeredi@...hat.com, linux-unionfs@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: How to support directory opacity in a filesystem for overlayfs to use?

On Tue, 14 Jan 2025 at 16:56, David Howells <dhowells@...hat.com> wrote:
>
> Miklos Szeredi <miklos@...redi.hu> wrote:
>
> > On Tue, 14 Jan 2025 at 16:15, David Howells <dhowells@...hat.com> wrote:
> >
> > > What's the best way for a network filesystem to make a native
> > > directory-is-opaque flag available to the system?  Is it best to catch
> > > setxattr/getxattr/removexattr("overlay.opaque") and translate these into the
> > > RPCs to wrangle the flag?
> >
> > I don't know.  Out of curiosity, which filesystem is it?
>
> One of the varieties of AFS.  Unfortunately, xattrs aren't a thing and can't
> easily be added because of the volume transfer and backup protocols and
> formats.
>
> > There's "trusted.overlay.opaque" and "user.overlay.opaque" and are
> > used in different scenarios.   There was also talk of making the
> > "trusted." namespace nest inside user namespaces, but apparently it's
> > not so important.
> >
> > Which one would you like to emulate?
>
> Um - I don't know the difference to answer that question.

"trusted." needs CAP_SYS_ADMIN in the init user ns, while "user."
needs write access on the object, which for an overlayfs mount in a
user namespace practically means CAP_DAC_OVERRIDE in the user ns.

So for plain, privileged overlayfs you'd want to implement
"trusted.overlay.opaque".  I don't have a better idea, than to add the
xattr callbacks to the filesystem and return -EOPNOTSUPP for
everything else.

Thanks,
Miklos