+++ /dev/null
-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
-
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