Because FLOSS is handy, isn’t it?

Beware of new UDEV rules!

Posted by isilanes on January 6, 2007

As some of you might know, udev is a nice program that gives the user the possibility of giving persistent names to hotplugged items (e.g. USB devices), in GNU/Linux systems.

When a USB device is plugged, the kernel “finds” it, and gives it a device name. This “name” is a special file (located at the /dev directory), with which the communication with the USB device is done. For example, this is the name that has to be used for mounting the device:

# mount /dev/devicename /mnt/mountpoint

Now, the old devfs (superceded by udev) gave subsequently plugged USB devices sequential names (e.g., the first one sda, the second one sdb…). So, the device name would not correspond to the physical device you were plugging: an external HD and a portable music player would be given devices sda and sdb respectively, or the opposite, depending on the plugging order!

To fix this, udev allows for creating rules, so that a device matching this rules will always be given the same device name. Each USB device passes some info to the kernel at plugtime, so udev can use that info to identify the device. For example, an excerpt of dmesg in my Debian box, when I connect my external HD:

scsi6 : SCSI emulation for USB Mass Storage devices
usb-storage: device found at 9
usb-storage: waiting for device to settle before scanning
Vendor: FUJITSU Model: MHT2080AT Rev: 0811
Type: Direct-Access ANSI SCSI revision: 00
SCSI device sda: 156301488 512-byte hdwr sectors (80026 MB)

The relevant points are that the HD identifies itself as a FUJITSU product, with the model name MHT2080AT. I can now tell udev to create a /dev/woxter device node, each time I plug in a device made by “FUJITSU”, and by the model name of “MHT2080AT”. To do so, I can create a file /etc/udev/rules.d/myrules.rules, with content:

# My Woxter disk:
BUS=="scsi", SYSFS{vendor}=="FUJITSU", SYSFS{model}=="MHT2080AT", NAME="woxter"

Now, the original reason to write this post was that… do you see the '=='? behind 'BUS' and 'SYSFS'? Well, they have the usual meaning of ‘BUS==”scsi”‘ means ‘if BUS equals “scsi”‘, whereas ‘NAME=”woxter”‘ means ‘assign the value “woxter” to “name”‘.

However, in previous versions of udev (I don’t know when they changed), all equal signs in the udev rules were single ‘=’s, and that is the way I had them.

Now, all of a sudden, I update udev, and my USB devices do not get the name they should, according to my rules, because my rules are wrong! Man, they should give a warning or something! Something like:

Warning: file X, line Y. Use of '=' where '==' is expected!

Oh, well. In the end I found out by myself.


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