Frequently Asked Questions about udev
-
Q: What's this udev thing, and what is it trying to do?
A: Read the OLS 2003 paper about udev, available in the docs/ directory,
and at:
- <http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf>
+ <http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf>
There is also a udev presentation given at OLS 2003 available at:
- <http://www.kroah.com/linux/talks/ols_2003_udev_talk/>
+ <http://www.kroah.com/linux/talks/ols_2003_udev_talk/>
Q: How is udev related to devfs?
-A: udev works entirely in userspace, using /sbin/hotplug calls that the
- kernel makes whenever a device is added or removed from the kernel. All
- naming policy, and permission control is done in userspace. devfs
- operated from within the kernel.
+A: udev works entirely in userspace, using hotplug events the kernel sends
+ whenever a device is added or removed from the kernel. Details about
+ the devices are exported by the kernel to the sysfs filesystem at /sys
+ All device naming policy permission control and event handling is done in
+ userspace. devfs is operated from within the kernel.
-Q: Why was devfs marked OBSOLETE if udev is not finished yet?
+Q: Why was devfs removed if udev can't do everthing devfs did?
A: To quote Al Viro (Linux VFS kernel maintainer):
- - it was determined that the same thing could be done in userspace
- - devfs had been shoved into the tree in hope that its quality will
- catch up
- - devfs was found to have fixable and unfixable bugs
- - the former had stayed around for many months with maintainer
- claiming that everything works fine
- - the latter had stayed, period.
- - the devfs maintainer/author disappeared and stopped maintaining
- the code.
+ - it was determined that the same thing could be done in userspace
+ - devfs had been shoved into the tree in hope that its quality will
+ catch up
+ - devfs was found to have fixable and unfixable bugs
+ - the former had stayed around for many months with maintainer
+ claiming that everything works fine
+ - the latter had stayed, period.
+ - the devfs maintainer/author disappeared and stopped maintaining
+ the code.
Q: But udev will not automatically load a driver if a /dev node is opened
when it is not present like devfs will do.
-A: If you really require this functionality, then use devfs. It is still
- present in the kernel.
-
-Q: But wait, I really want udev to automatically load drivers when they
- are not present but the device node is opened. It's the only reason I
- like using devfs. Please make udev do this.
-A: No. udev is for managing /dev, not loading kernel drivers.
+A: Right, but Linux is supposed to load a module when a device is discovered
+ not to load a module when it's accessed.
Q: Oh come on, pretty please. It can't be that hard to do.
A: Such a functionality isn't needed on a properly configured system. All
spurious.
Q: I really like the devfs naming scheme, will udev do that?
-A: Yes, udev can create /dev nodes using the devfs naming policy. A
- configuration file needs to be created to map the kernel default names
- to the devfs names. See the initial udev.rules.devfs file in the udev
- release. It is the start of such a configuration file. If there are
- any things missing, please let the udev authors know.
+A: Yes, udev can create /dev nodes using the devfs naming policy. But you
+ will need a custom configuration and scripts that enumerate your devices
+ sequentially while events run in parallel, without a predictable order.
+ The devfs scheme is not recommended or supported because it is a stupid
+ idea to simply enumerate devices in a world where devices can come and go
+ at any time. These numbers give you nothing but problems, and are not
+ useful to identify a device. Have a look at the persistent rules for
+ examples how to create persistent device names in userspace without any
+ device enumeration depending on the device probing order.
Q: What kinds of devices does udev create nodes for?
-A: All devices that are shown in sysfs will work with udev. If more
- support is added for devices to the kernel, udev will automatically
- start working for them. All block devices are currently supported, and
- almost all major char devices are supported. Kernel developers are
- working on adding support for all char devices at this time. See the
- linux-kernel mailing list for patches and status of these patches.
+A: All devices that are shown in the kernel's sysfs tree will work with udev.
Q: Will udev remove the limit on the number of anonymous devices?
A: udev is entirely in userspace. If the kernel supports a greater number
of anonymous devices, udev will support it.
-Q: Will udev support symlinks?
-A: Yes, It now does. Multiple symlinks per device node too.
+Q: Does udev support symlinks?
+A: Yes, multiple symlinks per device node are supported.
Q: How will udev handle the /dev filesystem?
-A: /dev can be a ramfs, or a backing filesystem. udev does not care what
- kind of filesystem it runs on.
+A: /dev is recomended to be a tmpfs filesystem that is recreated on every reboot.
+ Although, udev does not care what kind of filesystem it runs on.
Q: How will udev handle devices found before init runs?
-A: udev will be placed in initramfs and run for every device that is found.
- Work to get this implemented is still underway.
+A: udev can be placed in initramfs and run for every device that is found.
+ udev can also populate an initial /dev directory from the content of /sys
+ after the real root is mounted.
Q: Can I use udev to automount a USB device when I connect it?
-A: Technically, yes, but udev is not intended for this. Projects that do
- automount hotplugged storage devices are:
- * Usb-mount http://users.actrix.co.nz/michael/usbmount.html
- * devlabel http://linux.dell.com/projects.shtml#devlabel
-
- Alternatively, it is easy to add the following to fstab:
- /udev/pendrive /pendrive vfat user,noauto 0 0
+A: Technically, yes, but udev is not intended for this. All major distributions
+ use HAL (http://freedesktop.org/wiki/Software_2fhal) for this, which also
+ watches devices with removable media and integrates the Desktop environment.
+
+ Alternatively, it is easy to add the following to fstab:
+ /dev/disk/by-label/PENDRIVE /media/PENDRIVE vfat user,noauto 0 0
This means that users can access the device with:
- $ mount /pendrive
- And don't have to be root but will get full permissions on /pendrive.
- This works even without udev if /udev/pendrive is replaced by /dev/sda1
+ $mount /media/PENDRIVE
+ and doen't have to be root, but will get full permissions on the device.
+ Using the persistent disk links (label, uuid) will always catch the
+ same device regardless of the actual kernel name.
Q: Are there any security issues that I should be aware of?
A: When using dynamic device numbers, a given pair of major/minor numbers may
If the device node is later recreated with different permissions the hard
link can still be used to access the device using the old permissions.
(The same problem exists when using PAM to change permissions on login.)
-
+
The simplest solution is to prevent the creation of hard links by putting
- /dev in a separate filesystem (tmpfs, ramfs, ...).
-
+ /dev on a separate filesystem like tmpfs.
+
Q: I have other questions about udev, where do I ask them?
A: The linux-hotplug-devel mailing list is the proper place for it. The
- address for it is linux-hotplug-devel@lists.sourceforge.net
- Information on joining can be found at
- <https://lists.sourceforge.net/lists/listinfo/linux-hotplug-devel>
- Archives of the mailing list can be found at:
- <http://marc.theaimsgroup.com/?l=linux-hotplug-devel>
+ address for it is:
+ linux-hotplug@vger.kernel.org
+ Information on joining can be found at:
+ http://vger.kernel.org/vger-lists.html
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