weinimo.github.io

How to Write udev Rules for USB Devices

Udev-tux If you use a custom USB device for which there isn't a suitable udev rule installed on your system yet you might notice that only the root user has read and write access for it. In order to make it usable for normal users you need to write a new udev rule. Here is an easy way how to do this.

First we have to collect some information about that device. As we have an USB device in this example, we get some basic information from lsusb. Its output must look something like this:

Bus 001 Device 003: ID 1111:2222 FooDev

Now we know where in the Linux filesystem that device has been mounted:

/dev/bus/usb/001/003

We can use this path now to ask udev about how it sees the device:

udevadm info -a -p $(udevadm info -q path -n /dev/bus/usb/001/003)

Maybe you have to use udevinfo instead of udevadm info. The output should look like this:

looking at device '/devices/pci0000:00/0000:00:06.0/usb1/1-2':
    KERNEL=="1-2"
    SUBSYSTEM=="usb"
    DRIVER=="usb"
    ATTR{bDeviceSubClass}=="00"
    ATTR{bDeviceProtocol}=="00"
    ATTR{devpath}=="2"
    ATTR{idVendor}=="1111"
    ATTR{speed}=="12"
    ATTR{bNumInterfaces}==" 1"
    ATTR{bConfigurationValue}=="1"
    ATTR{bMaxPacketSize0}=="64"
    ATTR{busnum}=="1"
    ATTR{devnum}=="3"
    ATTR{configuration}==""
    ATTR{bMaxPower}=="500mA"
    ATTR{authorized}=="1"
    ATTR{bmAttributes}=="80"
    ATTR{bNumConfigurations}=="1"
    ATTR{maxchild}=="0"
    ATTR{bcdDevice}=="0106"
    ATTR{avoid_reset_quirk}=="0"
    ATTR{quirks}=="0x0"
    ATTR{serial}==" 10T7371 9028039"
    ATTR{version}==" 1.10"
    ATTR{urbnum}=="10"
    ATTR{ltm_capable}=="no"
    ATTR{manufacturer}=="Foo Inc"
    ATTR{removable}=="unknown"
    ATTR{idProduct}=="2222"
    ATTR{bDeviceClass}=="00"
    ATTR{product}=="Some FooDev"

With this information we can finally start writing the new udev rule. We do this by creating a new file:

vim /etc/udev/rules.d/10-local.rules

For instance if we would like to give everyone in the group "users" read and write access to the device, we put following line in that file:

SUBSYSTEMS=="usb", ATTRS{product}=="Some FooDev", GROUP="users"

Please note that it's ATTRS in the rules file, not ATTR.

Now we can test the new rule. For this we need the /devices/... path from the udevadm info output from before.

udevadm test /devices/pci0000:00/0000:00:06.0/usb1/1-2

Like before, you might have to use udevtest instead of udevadm test. The output should also contain information about the permissions for this device:

[...] handling device node '/dev/bus/usb/001/003', devnum=c189:2, mode=0664, uid=0, gid=100 set permissions /dev/bus/usb/001/003, 020664, uid=0, gid=100 [...]

gid=100 is the group id of the "users" group. So this is the proof that it works.

If your rule was not applied although it seems to be correct, chances are good that it has been overridden by another rule. In that case use the ":=" operator instead of "=". This tells udev not to override this property.

There are comments.

social