Udev rule to mount disk does not work

Question

I have the following content in in /etc/udev/rules.d/81-external-disk.rules:

ENV{ID_FS_UUID}=="6826692e-79f4-4423-8467-cef4d5e840c5", RUN{program}+="/bin/mount -o nofail,x-systemd.device-timeout=1 -t ext4 -U 6826692e-79f4-4423-8467-cef4d5e840c5 /backup/external"

After running:

udevadm control --reload ; udevadm trigger /dev/sdb1

It does nothing at all. However if II change the mount command for something such as /bin/touch /tmp/xyz it works.

Versions:

[root@helsinki rules.d]# rpm -qa | grep udev
libgudev1-219-19.el7_2.12.x86_64
python-pyudev-0.15-7.el7_2.1.noarch
[root@helsinki rules.d]# rpm -qa | grep systemd
systemd-libs-219-19.el7_2.12.x86_64
systemd-219-19.el7_2.12.x86_64
systemd-sysv-219-19.el7_2.12.x86_64
[root@helsinki rules.d]# cat /etc/redhat-release 
CentOS Linux release 7.2.1511 (Core) 

Answer 1

This same problem occurs in Ubuntu 18.04 with the same underlying cause. To fix it we create an override file for systemd-udevd:

sudo systemctl edit systemd-udevd

And insert into it:

[Service]
MountFlags=shared

(on Ubuntu 20.04, replace MountFlags=shared with PrivateMounts=no.)

Save the file and then execute:

sudo systemctl daemon-reload
sudo service systemd-udevd --full-restart

Answer 2

This is a systemd feature. The original udev command has been replaced by systemd-udevd (see its man page). One of the differences is that it creates its own filesystem namespace, so your mount is done, but it is not visible in the principal namespace. (You can check this by doing systemctl status systemd-udevd to get the Main PID of the service, then looking through the contents of /proc/<pid>/mountinfo for your filesystem).

If you want to go back to having a shared instead of private filesystem namespace, then create a file /etc/systemd/system/systemd-udevd.service with contents

.include /usr/lib/systemd/system/systemd-udevd.service
[Service]
MountFlags=shared 

or a new directory and file /etc/systemd/system/systemd-udevd.service.d/myoverride.conf with just the last 2 lines, i.e.

[Service]
MountFlags=shared

and restart the systemd-udevd service. I haven't found the implications of doing this.

Answer 3

While there are are solutions that currently work based setting the MountFlags as described in earlier answers, my concern is that given that the developers appear to have change the default setting in the past, they may just remove it outright in the future so as to get yet more control in their continuing drive for absolute power.

So, here's my alternative solution: In /usr/local/bin place the following into the file uysdm

#!/bin/bash
umask 077
rp=`mktemp -d /tmp/uysd_XXXXX`
if [ $? != 0 ]; then
        exit 1
fi
mkfifo $rp/fifo

echo $rp/fifo "$*" >/var/uysd/fifo &
X=$!
res=`cat $rp/fifo`
stat=$?
rm -rf "$rp"
wait $X
if [ $? != 0 ]; then
        exit $?
fi
exit "$res"

Then put into file uysdmd

#!/bin/bash
# Up yours systemd mount daemon

if [ ! -e /var/uysd/fifo ]; then
        umask 0077
        mkdir -p /var/uysd
        mkfifo /var/uysd/fifo
fi

(
        while true; do (
                IFS=" "
                read rp mt </var/uysd/fifo
                if [ "$mt" = "" ]; then
                        exit 1
                fi
                echo "$mt" | xargs mount >&2
                echo $? >"$rp"
        ) done
) &

And make it executable.

Put into file /lib/systemd/system/uysdmd.service

[Unit]
Description=Up Yours Systemd udev mount daemon

[Service]
ExecStart=/usr/local/bin/uysdmd
Type=forking

[Install]
WantedBy=systemd-udevd.service

Then do sudo systemctl enable uysdmd.service Finally do sudo systemctl restart systemd-udevd

Now the uysdm program can be used like mount from the udev hook.

This creates a daemon that is listening for requests to perform mounts that it receives from the uysdm command.

It does rather depend on the udev events being handled serially, rather than concurrently, which I believe is the case.