[PATCH v3 09/25] fs: add is_userns_visible() helper

[Andy Lutomirski]

On Wed, Feb 19, 2020 at 4:06 AM Christian Brauner
<christian.brauner at ubuntu.com> wrote:
>
> On Tue, Feb 18, 2020 at 08:42:33PM -0600, Serge Hallyn wrote:
> > On Tue, Feb 18, 2020 at 03:33:55PM +0100, Christian Brauner wrote:
> > > Introduce a helper which makes it possible to detect fileystems whose
> > > superblock is visible in multiple user namespace. This currently only
> > > means proc and sys. Such filesystems usually have special semantics so their
> > > behavior will not be changed with the introduction of fsid mappings.
> >
> > Hi,
> >
> > I'm afraid I've got a bit of a hangup about the terminology here.  I
> > *think* what you mean is that SB_I_USERNS_VISIBLE is an fs whose uids are
> > always translated per the id mappings, not fsid mappings.  But when I see
>
> Correct!
>
> > the name it seems to imply that !SB_I_USERNS_VISIBLE filesystems can't
> > be seen by other namespaces at all.
> >
> > Am I right in my first interpretation?  If so, can we talk about the
> > naming?
>
> Yep, your first interpretation is right. What about: wants_idmaps()

Maybe fsidmap_exempt()?

I still haven't convinced myself that any of the above is actually
correct behavior, especially when people do things like creating
setuid binaries.