Re: Read-only `slaves` with shared subtrees?

From: Ram Pai
Date: Mon Sep 18 2017 - 16:47:21 EST

On Fri, Sep 15, 2017 at 10:57:30AM -0700, Dawid Ciezarkiewicz wrote:
> Hi,
> (Please keep me in CC me when responding.)
> I have an use-case for shared subtrees that is not covered by:
> and I wasn't able to figure out any working solution - it might not be possible
> ATM.
> Long story short:
> I'd like the `slave` mount (service in a container) to mount propagated events
> as RO, no matter how did `master` (host) mount them. Host might need that data
> RW, but slave must have it RO only.
> I'm using Linux containers to isolate processes. I need the container
> to follow part of the host system mount tree, but not have a write-access to it
> (for security reasons). It's a trivial setup as long
> as everything is static, but as soon as a part of what the container needs
> to access is mounted/unmounted at runtime (and thus shared subtrees
> are involved),
> there seems to be no way to control the flags of the propagated mount events.

It is possible to make a slave mount readonly, by remounting it with
'ro' flags.

something like

mount -o bind,remount,ro <slave-mount-dir>

Any mount-propagation events reaching a read-only-slave does
inherit the slave attribute. However it does not inherit the
read-only attribute.

Should it inherit? or should it not? -- that has not been thought
off AFAICT. it think we should let it inherit.