libmtp udev rule removes symlink from custom udev rule

I use Ubuntu 20.04 LTS and have wrote a custom udev rule in /etc/udev/rules.d/, which creates a symlink, called "mybackup" and wrotes a short message to syslog/journalctl, if a specific external USB hard drive is connected. My udev rule with the name 99-datensicherung.rules:

ENV{DEVTYPE}=="usb_device", \
SUBSYSTEM=="usb", \
ATTRS{idVendor}=="174c", \
ATTRS{idProduct}=="55aa", \
ATTRS{bcdDevice}=="0100", \
SYMLINK+="mybackup", \
ACTION=="add", \
RUN+="/usr/bin/logger MY DEVICE CONNECTED."

If I look into the syslog, I found the message of logger and see, that the symlink is created:

Okt 15 11:22:46 SWPITSLNB systemd-udevd[185107]: 2-4: /etc/udev/rules.d/99-datensicherung.rules:9 LINK 'mybackup'
Okt 15 11:22:46 SWPITSLNB systemd-udevd[185107]: 2-4: /etc/udev/rules.d/99-datensicherung.rules:9 RUN '/usr/bin/logger MY DEVICE CONNECTED.'
Okt 15 11:22:46 SWPITSLNB systemd-udevd[185107]: 2-4: Handling device node '/dev/bus/usb/002/055', devnum=c189:182
Okt 15 11:22:46 SWPITSLNB systemd-udevd[185107]: 2-4: Setting permissions /dev/bus/usb/002/055, uid=0, gid=0, mode=0664
Okt 15 11:22:46 SWPITSLNB systemd-udevd[185107]: 2-4: Creating symlink '/dev/char/189:182' to '../bus/usb/002/055'
Okt 15 11:22:46 SWPITSLNB systemd-udevd[185107]: 2-4: Creating symlink '/dev/mybackup' to 'bus/usb/002/055'

But a very short time later, the syslog tell me:

Okt 15 11:22:46 SWPITSLNB systemd-udevd[185107]: 2-4: /usr/lib/udev/rules.d/69-libmtp.rules:2685 Running PROGRAM 'mtp-probe /sys/devices/pci0000:00/0000:00:14.0/usb2/2-4 2 55'
Okt 15 11:22:46 SWPITSLNB systemd-udevd[185107]: 2-4: Starting 'mtp-probe /sys/devices/pci0000:00/0000:00:14.0/usb2/2-4 2 55'
Okt 15 11:22:46 SWPITSLNB systemd-udevd[185107]: Successfully forked off '(spawn)' as PID 185132.
Okt 15 11:22:46 SWPITSLNB mtp-probe[185132]: checking bus 2, device 55: "/sys/devices/pci0000:00/0000:00:14.0/usb2/2-4"
Okt 15 11:22:46 SWPITSLNB mtp-probe[185132]: bus: 2, device: 55 was not an MTP device
Okt 15 11:22:46 SWPITSLNB systemd-udevd[185107]: 2-4: 'mtp-probe /sys/devices/pci0000:00/0000:00:14.0/usb2/2-4 2 55'(out) '0'
Okt 15 11:22:46 SWPITSLNB systemd-udevd[185107]: 2-4: Process 'mtp-probe /sys/devices/pci0000:00/0000:00:14.0/usb2/2-4 2 55' succeeded.
Okt 15 11:22:46 SWPITSLNB systemd-udevd[185107]: 2-4: Updating old name, '/dev/mybackup' no longer belonging to '/devices/pci0000:00/0000:00:14.0/usb2/2-4'
Okt 15 11:22:46 SWPITSLNB systemd-udevd[185107]: 2-4: No reference left, removing '/dev/mybackup'

How can I prevent the deletion of my symlink, from the other udev rule? Thank you very much!

Full syslog: https://pastebin.com/k5q1uuB0

Solution 1:

I'm unable to test this at the moment, but my suspicion is that you should be using ACTION!="remove" – certain things set via udev rules are not sticky across events so instead of only matching "add" you should also match "bind", "change", and so on.

(Do not just replace "add" with "bind", as you have no guarantee that there will always be exactly these two events in exactly that order.)

However, the device you're trying to match will not be very useful if the goal is to mount it like a disk. The 'usb_device' devtype is the "low-level" device that deals with raw USB packets – on top of which the OS will layer a usb_interface, then an scsi_host, then probably something like SCSI target and LUN, and only then you get 'sda' as an abstract "block device" which can be mounted or partitioned.

To actually get something useful, you need to match SUBSYSTEM=="block" (which covers both 'sda' and 'sdaX'), and optionally either devtype "disk" or "partition" to narrow things down.

Also, these block devices actually already have various symlinks predefined under /dev/disk/by-xxx – some of them involving filesystem IDs, partition IDs, and hardware serial numbers as well.