Check if device is already mounted before mounting it again

probonopd commented 3 years ago

On the helloSystem Live ISO which is heavily based on automount, some volumes (e.g., MONKEYPATCH in the picture) need to be mounted very early in the boot process, before automount is started. This results to these being mounted twice (see MONKEYPATCH-2).

It would be good to check if a geom is already mounted and if so, exit without trying to mount it again.

probonopd commented 3 years ago

Note that MONKEYPATCH can be addressed as /dev/da1 and /dev/msdosfs/MONKEYPATCH.

In this case, /dev/msdosfs/MONKEYPATCH was already mounted before automount was started, and then was mounted again from /dev/da1.

% mount
/dev/iso9660/LIVE on / (cd9660, local, read-only)
(...)
/dev/msdosfs/MONKEYPATCH on /media/MONKEYPATCH (msdosfs, local)
(...)
/dev/da1 on /media/MONKEYPATCH-2 (msdosfs, local, noatime, read-only)

vermaden commented 3 years ago

Thats the 'price' for alternative approach with 'read only' mount.

How would you know if a device is mounted? Check output of mount(8) command.

After /dev/msdosfs/MONKEYPATCH was mounted ... does /dev/da1 appear in the mounted list? No. So its not mounted to the automount. Then automount tries to mount it read write and it fails because its already mounted so it tries to mount it read only and that works.

Can you do not use /dev/msdosfs path for mounting and just use 'plain' devices as da0/da1/...?

probonopd commented 3 years ago

We can't trust that user would always do this, so we need to find a solution.

So I propose, when e.g., /usr/local/sbin/automount da1 attach is invoked, to check using something like

dmesg | grep "Label for provider da1 is" | tail -n 1 | cut -d " " -f 7 | rev | cut -c 2- | rev

in __check_already_mounted(). And then check whether it is still mounted (because it could have been unmounted since the message appeared in dmesg).

Seems to work for me:

% dmesg | grep "Label for provider da1 is" | tail -n 1 | cut -d " " -f 7 | rev | cut -c 2- | rev
msdosfs/MONKEYPATCH

So from there the next check would be:

  mount | grep -e "^/dev/msdosfs/MONKEYPATCH on"

and if that returns nothing, then assume that it was not mounted.

vermaden commented 2 years ago

That would also require additional checks for UFS labels, GPT labels, GLABEL labels, ext2/3/4 labels, and so on. It will make supporting automount(8) a nightmare ...