add option to make mountpoints shared

[martinetd]

Hello,

I'd be interested in making all mountpoints shared (for example by allowing to run mount --make-rshared / in init.d/root) in alpine.
The implementation itself isn't difficult, busybox mount does support --make-rshared despite not being listed in the help so it's just a matter of adding a setting (EDIT: original idea was to look for 'shared' in fstabinfo -o /) and doing it -- I'll be happy to open a PR if there is some approval first.

I was told to ask here in the alpine ticket: https://gitlab.alpinelinux.org/alpine/tsc/-/issues/42

(rationale below copied from that link verbatim)
-=-
The rationale for this is that I selfishly need to have some mountpoints mounted as shared for some container usages (when alpine is the host OS), in order to backpropagate mounts made by a privileged container in a volume marked as 'shared'.
In my case the volumes shouldn't be made directly off /, so just marking the FS where volumes should be as shared is probably going to be enough but it's not something I have direct control of so I'd be more comfortable with / marked shared as systemd has been doing (not a great argument admittedly... But it's what users are used to)

FWIW, it looks like podman 4 in rootless mode also seems to depend on it: containers/podman#12721 and aports#13541 (closed) (the hang might not be related but it illustrates the warning message being shown), so we might get other requests at this point.
-=-

Thank you!

[ncopa]

It would be nice to have this configurable, one way or the other, and I think /etc/init.d/root would be the proper place.

[williamh]

I read the Alpine issue, and I'm open to this, but I have a question.
It looks like this can be done by adding rshared to the mount options for / in fstab.
What are the advantages or disadvantages of modifying init.d/root vs fstab?
Thanks a lot for explaining this.

[martinetd]

Can it be done with mount options in fstab? I couldn't get it to work. mount options shared or private work for new mounts, but as far as I can see:

• mount -o remount,(r)shared / doesn't work, we have to use mount --make-(r)shared / specific options
• fstab options are not read by the kernel when mounting /, initrd might? but I'm not using initrd on the systems I'm targetting here, I don't think we should rely on it
• (rootflags= kernel option would probably work, though; but conceptually I'd prefer to use fstab)

My intention was to add handling in init.d/root that would just call fstabinfo -o / and do the mount --make-rshared / there, like we do for mount -o remount,rw if ro isn't present.

Is there any preference to which way the default should go? all mount implementations might not support make-rshared (I'm not sure it'd make sense on BSDs, not sure how mount propagation work there this all looks very linux-y), so default of not doing anything and adding mount --make-rshared if shared is present is probably the best?

[vampywiz17]

@vapier

It will merge it to the near future? I use Alpine linux to a Docker host and this settings that would be good to docker volumes...