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1 | Frequently Asked Questions about udev |
2 | ||
3 | ||
4 | Q: What's this udev thing, and what is it trying to do? | |
5 | A: Read the OLS 2003 paper about udev, available in the docs/ directory, | |
6 | and at: | |
7 | <http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf> | |
8 | There is also a udev presentation given at OLS 2003 available at: | |
9 | <http://www.kroah.com/linux/talks/ols_2003_udev_talk/> | |
10 | ||
11 | Q: How is udev related to devfs? | |
12 | A: udev works entirely in userspace, using /sbin/hotplug calls that the | |
13 | kernel makes whenever a device is added or removed from the kernel. All | |
14 | naming policy, and permission control is done in userspace. devfs | |
15 | operated from within the kernel. | |
16 | ||
17 | Q: Why was devfs marked OBSOLETE if udev is not finished yet? | |
18 | A: To quote Al Viro (Linux VFS kernel maintainer): | |
19 | - it was determined that the same thing could be done in userspace | |
20 | - devfs had been shoved into the tree in hope that its quality will | |
21 | catch up | |
22 | - devfs was found to have fixable and unfixable bugs | |
23 | - the former had stayed around for many months with maintainer | |
24 | claiming that everything works fine | |
25 | - the latter had stayed, period. | |
4723bf98 | 26 | - the devfs maintainer/author disappeared and stopped maintaining |
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27 | the code. |
28 | ||
29 | Q: But udev will not automatically load a driver if a /dev node is opened | |
30 | when it is not present like devfs will do. | |
31 | A: If you really require this functionality, then use devfs. It is still | |
32 | present in the kernel. | |
33 | ||
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34 | Q: But wait, I really want udev to automatically load drivers when they |
35 | are not present but the device node is opened. It's the only reason I | |
36 | like using devfs. Please make udev do this. | |
37 | A: No. udev is for managing /dev, not loading kernel drivers. | |
38 | ||
39 | Q: Oh come on, pretty please. It can't be that hard to do. | |
40 | A: Such a functionality isn't needed on a properly configured system. All | |
41 | devices present on the system should generate hotplug events, loading | |
42 | the appropriate driver, and udev will notice and create the | |
43 | appropriate device node. If you don't want to keep all drivers for your | |
44 | hardware in memory, then use something else to manage your modules | |
45 | (scripts, modules.conf, etc.) This is not a task for udev. | |
46 | ||
470e365d GKH |
47 | Q: But I love that feature of devfs, please? |
48 | A: The devfs approach caused a lot of spurious modprobe attempts as | |
49 | programs probed to see if devices were present or not. Every probe | |
50 | attempt created a process to run modprobe, almost all of which were | |
51 | spurious. | |
52 | ||
10a479f5 | 53 | Q: I really like the devfs naming scheme, will udev do that? |
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54 | A: Yes, udev can create /dev nodes using the devfs naming policy. A |
55 | configuration file needs to be created to map the kernel default names | |
786f9231 | 56 | to the devfs names. See the initial udev.rules.devfs file in the udev |
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57 | release. It is the start of such a configuration file. If there are |
58 | any things missing, please let the udev authors know. | |
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59 | |
60 | Q: What kinds of devices does udev create nodes for? | |
61 | A: All devices that are shown in sysfs will work with udev. If more | |
62 | support is added for devices to the kernel, udev will automatically | |
63 | start working for them. All block devices are currently supported, and | |
64 | almost all major char devices are supported. Kernel developers are | |
65 | working on adding support for all char devices at this time. See the | |
66 | linux-kernel mailing list for patches and status of these patches. | |
67 | ||
68 | Q: Will udev remove the limit on the number of anonymous devices? | |
69 | A: udev is entirely in userspace. If the kernel supports a greater number | |
70 | of anonymous devices, udev will support it. | |
71 | ||
72 | Q: Will udev support symlinks? | |
10a479f5 | 73 | A: Yes, It now does. Multiple symlinks per device node too. |
b1830e79 | 74 | |
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75 | Q: How will udev handle the /dev filesystem? |
76 | A: /dev can be a ramfs, or a backing filesystem. udev does not care what | |
77 | kind of filesystem it runs on. | |
78 | ||
79 | Q: How will udev handle devices found before init runs? | |
80 | A: udev will be placed in initramfs and run for every device that is found. | |
81 | Work to get this implemented is still underway. | |
82 | ||
166612d7 GKH |
83 | Q: Can I use udev to automount a USB device when I connect it? |
84 | A: Technically, yes, but udev is not intended for this. Projects that do | |
85 | automount hotplugged storage devices are: | |
86 | * Usb-mount http://users.actrix.co.nz/michael/usbmount.html | |
87 | * devlabel http://linux.dell.com/projects.shtml#devlabel | |
88 | ||
89 | Alternatively, it is easy to add the following to fstab: | |
90 | /udev/pendrive /pendrive vfat user,noauto 0 0 | |
91 | ||
92 | This means that users can access the device with: | |
93 | $ mount /pendrive | |
94 | And don't have to be root but will get full permissions on /pendrive. | |
95 | This works even without udev if /udev/pendrive is replaced by /dev/sda1 | |
96 | ||
4723bf98 BM |
97 | Q: Are there any security issues that I should be aware of? |
98 | A: When using dynamic device numbers, a given pair of major/minor numbers may | |
99 | point to different hardware over time. If a user has permission to access a | |
100 | specific device node directly and is able to create hard links to this node, | |
101 | he or she can do so to create a copy of the device node. When the device is | |
102 | unplugged and udev removes the device node, the user's copy remains. | |
103 | If the device node is later recreated with different permissions the hard | |
104 | link can still be used to access the device using the old permissions. | |
105 | (The same problem exists when using PAM to change permissions on login.) | |
106 | ||
107 | The simplest solution is to prevent the creation of hard links by putting | |
108 | /dev in a separate filesystem (tmpfs, ramfs, ...). | |
109 | ||
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110 | Q: I have other questions about udev, where do I ask them? |
111 | A: The linux-hotplug-devel mailing list is the proper place for it. The | |
112 | address for it is linux-hotplug-devel@lists.sourceforge.net | |
113 | Information on joining can be found at | |
114 | <https://lists.sourceforge.net/lists/listinfo/linux-hotplug-devel> | |
115 | Archives of the mailing list can be found at: | |
116 | <http://marc.theaimsgroup.com/?l=linux-hotplug-devel> | |
117 |