systemd/FAQ

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>
   There is also a udev presentation given at OLS 2003 available at:
     <http://www.kroah.com/linux/talks/ols_2003_udev_talk/>

Q: How is udev related to devfs?
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 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.

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: 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
   devices present on the system should generate hotplug events, loading
   the appropriate driver, and udev will notice and create the
   appropriate device node.  If you don't want to keep all drivers for your
   hardware in memory, then use something else to manage your modules
   (scripts, modules.conf, etc.)  This is not a task for udev.

Q: But I love that feature of devfs, please?
A: The devfs approach caused a lot of spurious modprobe attempts as
   programs probed to see if devices were present or not.  Every probe
   attempt created a process to run modprobe, almost all of which were
   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. 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 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: Does udev support symlinks?
A: Yes, multiple symlinks per device node are supported.

Q: How will udev handle the /dev filesystem?
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 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. 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 /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
   point to different hardware over time. If a user has permission to access a
   specific device node directly and is able to create hard links to this node,
   he or she can do so to create a copy of the device node. When the device is
   unplugged and udev removes the device node, the user's copy remains.
   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 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@vger.kernel.org
   Information on joining can be found at:
      http://vger.kernel.org/vger-lists.html