Re: User-visible context-mount API

[Adding util-linux@vger and Michael Kerrisk]

On Wed, Jan 17, 2018 at 5:17 AM, Al Viro [off-list ref] wrote:

On Tue, Jan 16, 2018 at 05:41:46PM +0100, Miklos Szeredi wrote:

quoted

Right.

Still, those two (propagation and flags) are properties of the mount.
No fundamental difference in how to handle them, that I see.  Okay, we
have MS_REC handling in the propagation and not in the flags, but
that's something that might make sense for flags as well.

What's more interesting is how MS_PRIVATE + MS_REC semantics are
complete failure in the real world: the logical thing would be to mark
a mount private on the supplied mount AND propagate an umount event to
everywhere else.

This is utter nonsense.  Most of the time it's "Fedora, in its infinite
bogo^Wwisdom has made everything shared; I don't fucking need that
idiocy, so please unshare this, this and that".  You really don't want
(or have permissions for) unmounting e.g. /mnt in namespace of init
when you do that.

Sure, we get tons of bug reports.  Due to idiotic Fedora setup, with
everything shared.  The same setup that would go up in flames on the
semantics change you propose.

I wouldn't propose to change existing --make-private, as this would
not be backward compatible. The new semantics would mean a new op,
obviously.

Documenting  --make-private thing properly would also help.  To me the
wording "make private" strongly implies "I want to make submounts
private to this instance".  See for example rhbz#1432211.

If anything, "private bind on itself" would be a useful operation.
Turning given location into a mountpoint, and having everything
under it looking as it used to, but with no propagation at all.
Without bothering anybody else, even if location currently happens
to be on a shared/master mount.

I can slap that together for mount(2), but I'm not sure what a sane
combination of flags for that would look like ;-)  For fsmount
I think it would be very useful thing to have.

Yes, I think such an operation would be pretty useful.   Not sure if
it's the whole story, though.

Thanks,
Miklos