Udev device Management and usb devices

References

localrules

Some local udev rules must be placed before defaults because the first matched rule is applied, Some other rules can be placed after the main rules and will only modify a yet created device as when you change permissions, add a symlink or a RUN action.

We also need to check that each rule matches on all keys, and the rule match exactly one device.

You can put your rules in /etc/udev/rules.d/, and as the rules are used in lexicographical order you give a proper priority such they are used before or after a corresponding rule in /lib/udev/rules.d/.

If you want to replace a provided rule in /lib/udev/rules.d/, you can just write a rule with the same name in /etc/udev/rules.d/ as they take precedence over the /lib rules.

finding usb devices

  • To have a list of usb devices just do:

    $ lsusb

  • A slightly more informative list is got by:

    $ lsusb -v \| grep -E '<(Bus\|iProduct\|bDeviceClass\|bDeviceProtocol)' 2>/dev/null

  • To get detailled information you can

    • Use the bus:devnum reported by lsusb as in:

      lsusb -v -s 003:001

    • Use the vendor:product that you find with udevadm info like:

      $ lsusb -v -d 04b8:082b

Many of the keys in lsusb are attributes of udevadm info and can be used in udev configuration.

Using /sys keys

We obtain the keys by looking for dev /sys info, with udevadm info:

  1. find the key by either

    • Report with udevadm info providing the device name

      $ udevadm info --query=property --name /dev/usb/lp0

  • use the path that you also get from device by

    $ udevadm info --query=path --name /dev/usb/lp0
/class/usb/lp0

    and get dev /sys info by

    $ udevadm info --query=property --path /class/usb/lp0
  1. We can then find appropriate keys to identify uniquely our device:
SYSFS{manufacturer}=="Hewlett-Packard"
SYSFS{product}=="DeskJet 840C"
  1. We then add our rule in /etc/udev/rules.d/10-local.rules
BUS=="usb", SYSFS{product}=="DeskJet 840C", NAME="%k", KERNEL=="lp[0-9]*", NAME="usb/%k", GROUP="lp", SYMLINK="deskjet"

  1. Reload udev conf by:

    $ udevadm control --reload

  2. Test the config with:

    $ udevadm test $(udevadm info -q path -n /dev/usb/lp0)

    or:

    $ udevadm test /class/usb/lp0 usb
main: looking at device '/class/usb/lp0' from subsystem 'usb'
main: opened class_dev->name='lp0'
udev_rules_get_name: reset symlink list
udev_rules_get_name: add symlink 'deskjet'
udev_rules_get_name: rule applied, 'lp0' becomes 'usb/lp0'
create_node: creating device node '/dev/usb/lp0', major = '180', minor     = '0', mode = '0660', uid = '0', gid = '7'
create_node: creating symlink '/dev/deskjet' to 'usb/lp0'
    • note that it is only a udev simulation, not the true udev creating devices, I have experimented cases where udevtest was working, but udev did not. In some case it seems it was caused by multiple devices matching the same key.
    • If the device was yet present reloading the rules or restarting udev, is not sufficient to have the new device, you have to unplug the device, it can be a hot plugging when available, otherwise you need to restart the computer.

  3. We must now have:

    $ ls -l /dev/usb/lp0
crw-rw----  1 root lp 180, 0 Mar 14 21:42 /dev/usb/lp0
$ ls -l /dev/deskjet
lrwxrwxrwx  1 root root 7 Apr  7 18:23 /dev/deskjet -> usb/lp0

In the same way we can link a specific mass-storage to a special /dev entry by looking at the keys by:

$ udevadm info -a -p $(udevadm info -q path -n /dev/uba1)

then add in /etc/udev/rules.d/10-local.rules

BUS="usb", SYSFS{serial}="0402170100000020EB5D00000000000", KERNEL="ub?1", NAME="%k", SYMLINK="usbfoo"

But usually it it is better to use the provided symlinks which yet allow persistent naming.

If we want to automount some removable storage we can create a rule in /etc/udev/rules.d/ and use systemd-mount to mount it. As the recent man page explain in the section The udev database a udev rule like the following automatically mount all USB storage plugged in:

ACTION=="add", SUBSYSTEMS=="usb", SUBSYSTEM=="block", ENV{ID_FS_USAGE}=="filesystem", \
RUN{program}+="/usr/bin/systemd-mount --no-block --automount=yes --collect $devnode"

In this case systemd-mount honors the of additional udev properties SYSTEMD_MOUNT_OPTIONS= to give additional mount options, SYSTEMD_MOUNT_WHERE= The file system path to place the mount point at, instead of /run/media/system/.

Here to mount a specific usb key we create a rule by using some keys to identify the file system using keys from the top sysfs entry and possibly also from a parent, like this:

ACTION=="add"
SUBSYSTEMS=="usb"
SUBSYSTEM=="block"
ATTRS{serial}=="0709289778d6a5"
ATTR{partition}=="1"
RUN{program}+="/usr/bin/systemd-mount --no-block --automount=yes --discover $devnode"

Here the serial identify the usb key so I have to add a partition number to properly identify the partition. The mounted device in /run/media/system directory

For disk devices you can use any persistent naming for identifying them. The same information used by symlinks in /dev/disk/ tree and ref:reported by ldblk <uuid_with_lsblk>, is also reported by:

$ udevadm info --query=property -n /dev/sdc

as ID_SERIAL, ID_SERIAL_SHORT, ID_WWN for a disk and also for a partition filesystem ID_FS_UUID, ID_FS_LABEL, ID_PART_ENTRY_UUID (instead of PARTUUID for lbslk). These variables are available in udev rules as ENV{``*variable*}``.

If I add a label on the partition of my usb key, I can now use:

ACTION=="add"
SUBSYSTEMS=="usb"
SUBSYSTEM=="block"
ENV{ID_FS_LABEL}=="VFAT_SHARE"
ENV{SYSTEMD_MOUNT_OPTIONS}="gid=john,uid=john"
ENV{SYSTEMD_MOUNT_WHERE}="/run/media/john/$env{ID_FS_LABEL}"
RUN{program}+="/usr/bin/systemd-mount --no-block --automount=yes $devnode"

and I find the mounted device at /run/media/john/VFAT_SHARE with the john user and group.

debugging udev

To debug udev we can:

  1. use udevadm test

  2. log udevd by issuing:

    log="yes"

    in /etc/udev.conf and change the level of debugging with:

    $ udevadm control log_priority=level

the priority is a numerical or symbolic level from systlog err, info and debug

  1. udevadm monitor reports to the console the udevd activity