mount is not executed when called by udev

Question

I tried to create some udev rules to mount and unmount my USB flash drives; the rules for the moment are very simple:

ACTION=="add",KERNEL=="sd[b-z]",RUN+="/root/scripts/plug_flash_drive.sh %k"
ACTION=="remove",KERNEL=="sd[b-z]",RUN+="/root/scripts/unplug_flash_drive.sh %k"

plug_flash_drive.sh is also very simple:

device_name=$1
mount_options="umask=000,utf8"
if [ ! -e "/media/$device_name" ]; then
    mkdir "/media/$device_name"
fi
sleep 1
/usr/bin/mount "/dev/$device_name" "/media/$device_name" -o "$mount_options"

unplug_flash_drive.sh:

device_name=$1

umount "/dev/$device_name"
rmdir "/media/$device_name"

I have done some tests so I can ascertain that:

• When plugged in, my flash drive is detected; a file is created in /dev
• plug_flash_drive.sh is called by udev
• the mkdir part of the script works
• however, it seems that the "mount" part of the script is not executed, so my drive is not mounted
• when I call my scripts on the command line, they perfectly work

Does anybody know why mount is not executed when called by udev?

EDIT 28/08/14: I added "grep -q /proc/mounts && echo success || echo failure" at the end of my script to check in my debug log if the device is actually mounted before the script ends. It appears that the device is mounted at that point even when the script is called by udev. So the real problem is now "my block device is seemingly unmounted after the mount script end when called through udev" :s

Answer 1

systemd-udevd runs in its own file system namespace and by default mounts done within udev .rules do not propagate to the host. To make your old scripts work you can set MountFlags=shared in /usr/lib/systemd/system/systemd-udevd.service or (better) creating and editing its copy at /etc/systemd/system/

See man 5 systemd.exec for more information, MountFlags option.

Answer 2

As of this writing, the other answers are incorrect (or out of date).

You should not run mount from a Systemd service. Even after commenting the MountFlags and PrivateMounts lines in systemd-udevd.service, your rule will not work for FUSE filesystems like NTFS or exFAT, because the FUSE process will get helpfully killed by Systemd.

See this ArchWiki page which lists several better options. My preference is a tiny project on GitHub called udev-media-automount, which simply restarts a Systemd service from the Udev rule. This is a convenient way to get around Udev's various cumbersome restrictions on namespaces and child processes.

See also this anwser, which shows how to use the SYSTEMD_WANTS Udev variable to start a Systemd unit.

Answer 3

You can use systemd-mount to mount block devices with a udev rule. Example of auto-mounting the Raspberry Pi Pico derived from the ArchWiki page mentioned by @Metamorphic (note that you need to make /mnt/pico ahead of time:

ACTION=="add", SUBSYSTEMS=="usb", SUBSYSTEM=="block", ENV{ID_FS_USAGE}=="filesystem", ATTRS{idVendor}=="2e8a", ATTRS{idProduct}=="0003", RUN+="/usr/bin/logger --tag rpi-pico-mount Mounting what seems to be a Raspberry Pi Pico", RUN+="/usr/bin/systemd-mount --no-block --collect -o gid=users,fmask=113,dmask=002 $devnode /mnt/pico"

The post mentions adding the flag --automount=yes, but this caused remount problems for me.

Make sure to reload the udev rules so your changes take effect:

sudo udevadm control --reload

Adding the logger message to the rule helped with debugging. You can see it get printed by watching journalctl -f. It's useful to start with a generic match pattern, then build up to something more specific.

Answer 4

You could try to use := instead of += in the rules RUN assignments.

The := operator sets the list value and disallows further changes.