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1 <?xml version='1.0' encoding="UTF-8"?>
2 <!-- $Header: /home/cvsroot/gentoo/xml/htdocs/doc/en/devfs-guide.xml,v 1.5 2003/12/11 16:13:37 dertobi123 Exp $ -->
3
4 <!DOCTYPE guide SYSTEM "/dtd/guide.dtd">
5
6 <guide link="/doc/en/devfs-guide.xml">
7 <title>Device File System Guide</title>
8 <author title="Author">
9 <mail link="swift@gentoo.org">Sven Vermeulen</mail>
10 </author>
11 <author title="Reviewer">
12 <mail link="seemant@gentoo.org">Seemant Kulleen</mail>
13 </author>
14
15 <abstract>
16 In this document you'll find information on what devfs is really about
17 and how to work with it.
18 </abstract>
19
20 <license/>
21
22 <version>0.1</version>
23 <date>September 11, 2003</date>
24
25 <chapter>
26 <title>What is devfs?</title>
27 <section>
28 <title>The (good?) old days</title>
29 <body>
30
31 <p>
32 Traditional Linux implementations provide their users with an
33 abstract device path, called <path>/dev</path>. Inside this path the
34 user finds <e>device nodes</e>, special files that represent devices
35 inside their system. For instance, <path>/dev/hda</path> represents the
36 first IDE device in their system. By providing device files to the
37 users, they can create programs that interact with hardware as if the
38 hardware was a regular file instead of using special APIs.
39 </p>
40
41 <p>
42 The device files are split in two groups, called <e>character</e>
43 devices and <e>block</e> devices. The first group consists of hardware
44 of which read/writes are not buffered. The second group naturally
45 consists of hardware of which read/writes are buffered. Both devices can
46 be read one character at a time, or in blocks. Therefore, the naming
47 might sound confusing and in fact is wrong.
48 </p>
49
50 <p>
51 If you take a look at a certain device file, you might find something
52 like this:
53 </p>
54
55 <pre caption = "Checking the information of a device file">
56 # <i>ls -l /dev/hda</i>
57 brw-rw---- 1 root disk 3, 0 Jul 5 2000 /dev/hda
58 </pre>
59
60 <p>
61 In the previous example we see that <path>/dev/hda</path> is a block
62 device. However, more importantly, it has two special numbers assigned
63 to it: <path>3, 0</path>. This pair is called the <e>major-minor</e>
64 pair. It is used by the kernel to map a device file to a real device.
65 The major corresponds with a certain device, the minor with a subdevice.
66 Seems confusing? It isn't.
67 </p>
68
69 <p>
70 Two examples are <path>/dev/hda4</path> and <path>/dev/tty5</path>. The
71 first device file corresponds with the fourth partition on the first IDE
72 device. Its major-minor pair is <path>3, 4</path>. In other words, the
73 minor corresponds with the partition where the major corresponds with
74 the device. The second example has <path>4, 5</path> as major-minor
75 pair. In this case, the major corresponds with the terminal driver,
76 while the minor corresponds with the terminal number (in this case, the
77 fifth terminal).
78 </p>
79
80 </body>
81 </section>
82 <section>
83 <title>The problems</title>
84 <body>
85
86 <p>
87 If you do a quick check in such a <path>/dev</path>, you'll find out
88 that not only all your devices are listed, but <e>all</e> possible
89 devices that you can imagine. In other words, you have device files for
90 devices you don't have. Managing such a device group is cumbersome to
91 say the least. Imagine having to change the permissions of all device
92 files that have a corresponding device in your system, and leaving the
93 rest of the device files as they are.
94 </p>
95
96 <p>
97 When you add new hardware to your system, and this hardware didn't have
98 a device file previously, you would have to create one. Advanced users know
99 that this task can be accomplished with <c>./MAKEDEV</c> inside the
100 <path>/dev</path> tree, but do you immediately know what device you have
101 to create?
102 </p>
103
104 <p>
105 When you have programs interacting with hardware using the device files,
106 you can't have the root partition mounted read only, while there is no
107 further need to have it mounted read-write. And you can't have
108 <path>/dev</path> on a seperate partition, since <c>mount</c> needs
109 <path>/dev</path> to mount partitions.
110 </p>
111
112 </body>
113 </section>
114 <section>
115 <title>The solutions</title>
116 <body>
117
118 <p>
119 As you can imagine, the kernel hackers have found quite a number of
120 solutions to the aforementioned problems. However, many of them had
121 other flaws as described in
122 <uri>http://www.atnf.csiro.au/people/rgooch/linux/docs/devfs.html#faq-why</uri>.
123 We are not going to talk about these implementations, but focus on the
124 one implementation that did make it to the official kernel sources:
125 devfs.
126 </p>
127
128 </body>
129 </section>
130 <section>
131 <title>devfs as all-round winner</title>
132 <body>
133
134 <p>
135 devfs tackles all listed problems. It only provides the user with
136 existing devices, adds new nodes when new devices are found, and makes
137 it possible to mount the root filesystem read only. And it tackles more
138 problems we haven't discussed previously because they are less
139 interesting for users...
140 </p>
141
142 <p>
143 For instance, with devfs, you don't have to worry about major/minor
144 pairs. It is still supported (for backwards compatibility), but isn't
145 needed. This makes it possible for Linux to support even more devices,
146 since there are no limits anymore (numbers always have boundaries :)
147 </p>
148
149 </body>
150 </section>
151 </chapter>
152
153 <chapter>
154 <title>Navigating through the device tree</title>
155 <section>
156 <title>Directories</title>
157 <body>
158
159 <p>
160 One of the first things you might notice is that devfs uses directories
161 to group devices together. This improves readability, as now all related
162 devices are inside a common directory.
163 </p>
164
165 <p>
166 For instance, all IDE-related devices are inside the
167 <path>/dev/ide/</path> device directory, and SCSI-related devices are inside
168 <path>/dev/scsi/</path>. SCSI and IDE disks are seen in the same way,
169 meaning they both have the same subdirectory structure.
170 </p>
171
172 <p>
173 IDE and SCSI disks are controlled by an adapter (on-board or a seperate
174 card), called the <e>host</e>. Every adapter can have several channels.
175 A channel is called a <e>bus</e>. On each channel, you can have several
176 IDs. Such an ID identifies a disk. This ID is called the <e>target</e>.
177 Some SCSI devices can have multiple luns (<e>Logical Unit Numbers</e>),
178 for instance devices that handle multiple media simultaneously (hi-end
179 tapedrives). You mostly have only a single lun, <path>lun0/</path>.
180 </p>
181
182 <p>
183 So, whereas <path>/dev/hda4</path> was used previously, we now have
184 <path>/dev/ide/host0/bus0/target0/lun0/part4</path>. This is far more
185 easy... no, don't argue with me... it <e>is</e> easier... ah whatever!
186 :)
187 </p>
188
189 <note>
190 You can also use more Unix-like device file naming for hard disks, such as
191 <path>c0b0t0u0p2</path>. They can be found in <path>/dev/ide/hd</path>,
192 <path>/dev/scsi/hd</path> etc.
193 </note>
194
195 <p>
196 To give you an idea on the directories, this is a listing of the
197 directories which I have on my laptop:
198 </p>
199
200 <pre caption = "Directories in /dev">
201 cdroms/ cpu/ discs/ floppy/
202 ide/ input/ loop/ misc/
203 netlink/ printers/ pts/ pty/
204 scsi/ sg/ shm/ sound/
205 sr/ usb/ vc/ vcc/
206 </pre>
207
208 </body>
209 </section>
210 <section>
211 <title>Backwards compatibility using devfsd</title>
212 <body>
213
214 <p>
215 Using this new scheme sounds fun, but several tools and programs make
216 use of the previous, old scheme. To make sure no system is broken,
217 <c>devfsd</c> is created. This daemon creates symlinks with the old
218 names, pointing to the new device files.
219 </p>
220
221 <pre caption = "Created symlinks">
222 $ <i>ls -l /dev/hda4</i>
223 lr-xr-xr-x 1 root root 33 Aug 25 12:08 /dev/hda4 -> ide/host0/bus0/target0/lun0/part4
224 </pre>
225
226 <p>
227 With <c>devfsd</c>, you can also set the permissions, create new device
228 files, define actions etc. All this is described in the next chapter.
229 </p>
230
231 </body>
232 </section>
233 </chapter>
234
235 <chapter>
236 <title>Administrating the device tree</title>
237 <section>
238 <title>Restarting devfsd</title>
239 <body>
240
241 <p>
242 When you alter the <path>/etc/devfsd.conf</path> file, and you want the
243 changes to be forced onto the system, you don't have to reboot.
244 Depending on what you want, you can use any of the two following
245 signals:
246 </p>
247
248 <p>
249 <b>SIGHUP</b> will have <c>devfsd</c> reread the configuration file,
250 reload the shared objects and generate the REGISTER events for each leaf
251 node in the device tree.
252 </p>
253
254 <p>
255 <b>SIGUSR1</b> will do the same, but won't generate REGISTER events.
256 </p>
257
258 <p>
259 To send a signal, simply use <c>kill</c> or <c>killall</c>:
260 </p>
261
262 <pre caption = "Sending the SIGHUP signal to devfsd">
263 # <i>kill -s SIGHUP `pidof devfsd`</i>
264 <comment>or</comment>
265 # <i>killall -s SIGHUP devfsd</i>
266 </pre>
267
268 </body>
269 </section>
270 <section>
271 <title>Removing compatibility symlinks</title>
272 <body>
273
274 <warn>
275 Currently, Gentoo cannot live without the compatibility symlinks.
276 </warn>
277
278 <p>
279 If you want the compatibility symlinks that clutter up <path>/dev</path>
280 removed from your Gentoo system (Gentoo activates it per default), edit
281 <path>/etc/devfsd.conf</path> and remove the following two lines:
282 </p>
283
284 <pre caption = "/etc/devfsd.conf for backwards compatibility">
285 <comment># Comment the following two lines out to remove the symlinks</comment>
286 REGISTER .* MKOLDCOMPAT
287 UNREGISTER .* RMOLDCOMPAT
288 </pre>
289
290 <p>
291 You need to reboot your system for the changes to take affect.
292 </p>
293
294 </body>
295 </section>
296 <section>
297 <title>Removing autoload functionality</title>
298 <body>
299
300 <p>
301 When you load a module, devfs will automatically create the device
302 files. If you don't want this behaviour, remove the following line from
303 <path>/etc/devfsd.conf</path>:
304 </p>
305
306 <pre caption = "/etc/devfsd.conf, autoload functionality">
307 LOOKUP .* MODLOAD
308 </pre>
309
310 </body>
311 </section>
312 </chapter>
313
314 <chapter>
315 <title>Permission Related Items</title>
316 <section>
317 <title>Set/change permissions using PAM</title>
318 <body>
319
320 <p>
321 Although you can set permissions in <path>/etc/devfsd.conf</path>, you
322 are advised to use PAM (<e>Pluggable Authentification Modules</e>). This
323 is because PAM has the final say on permissions, possibly ignoring the
324 changes you make in <path>/etc/devfsd.conf</path>.
325 </p>
326
327 <p>
328 PAM uses the <path>/etc/security/console.perms</path> file for the
329 permissions. The file consists of two parts: the first one describes the
330 groups, and the second one the permissions.
331 </p>
332
333 <p>
334 Let's first take a look at the groups part. As an example we view the
335 sound-group:
336 </p>
337
338 <pre caption = "Sound group in /etc/security/console.perms">
339 &lt;sound&gt;=/dev/dsp* /dev/audio* /dev/midi* \
340 /dev/mixer* /dev/sequencer* \
341 /dev/sound/* /dev/snd/* /dev/beep \
342 /dev/admm* \
343 /dev/adsp* /dev/aload* /dev/amidi* /dev/dmfm* \
344 /dev/dmmidi* /dev/sndstat
345 </pre>
346
347 <p>
348 The syntax is quite easy: you start with a group-name, and end with a
349 list of devices that belong to that group.
350 </p>
351
352 <p>
353 Now, groups aren't very useful if you can't do anything with them. So
354 the next part describes how permissions are handled.
355 </p>
356
357 <pre caption = "Permissions for sound group in /etc/security/console.perms">
358 &lt;console&gt; 0600 &lt;sound&gt; 0600 root.audio
359 </pre>
360
361 <p>
362 The first field is the terminal check. On most systems, this is the
363 console-group. PAM will check this field for every login. If the login
364 happens on a device contained in the console-group, PAM will check and
365 possibly change the permissions on some device files.
366 </p>
367
368 <p>
369 The second field contains the permissions to which a device file is set
370 upon succesfull login. When a person logs into the system, and the device
371 files are owned by a default owner/group, PAM wil change the ownership
372 to the logged on user, and set the permissions to those in this second
373 field. In this case, 0600 is used (user has read/write access,
374 all others don't).
375 </p>
376
377 <p>
378 The third field contains the device-group whose permissions will be
379 changed. In this case, the sound-group (all device files related to
380 sound) will be changed.
381 </p>
382
383 <p>
384 The fourth field defines the permissions to which the device file is set after
385 returning to the default state. In other words, if the person who owns
386 all the device files logs out, PAM will set the permissions back to a
387 default state, described by this fourth field.
388 </p>
389
390 <p>
391 The fifth field defines the ownership (with group if you want) to which
392 the device attributes are set after returning to the default state. In
393 other words, if the person who owns all the device files logs out, PAM
394 will set the ownership back to a default state, described by this fifth
395 field.
396 </p>
397
398 </body>
399 </section>
400 <section>
401 <title>Set/change permissions with devfsd</title>
402 <body>
403
404 <p>
405 If you really want to set permissions using
406 <path>/etc/devfsd.conf</path>, then use the syntax used in the following
407 example:
408 </p>
409
410 <pre caption = "Permissions in /etc/devfsd.conf">
411 REGISTER ^cdroms/.* PERMISSIONS root.cdrom 0660
412 </pre>
413
414 <p>
415 The second field is the device group, starting from <path>/dev</path>.
416 It is a regular expression, meaning you can select several device files
417 in one rule.
418 </p>
419
420 <p>
421 The fourth field is the ownership of the device file. Unlike with PAM
422 this isn't changed (unless it is mentioned in <path>console.perms</path>
423 since PAM always wins).
424 </p>
425
426 <p>
427 The fifth field contains the permissions of the device file.
428 </p>
429
430 </body>
431 </section>
432 <section>
433 <title>Manually set permissions and have devfsd save it</title>
434 <body>
435
436 <p>
437 This is the default behaviour for Gentoo: if you <c>chown</c> (CHange
438 OWNer) and <c>chmod</c> (CHange MODe) some device files, <c>devfsd</c>
439 will save the information when you are shutting down the system. This is
440 because the <path>/etc/devfsd.conf</path> file contains the following
441 lines:
442 </p>
443
444 <pre caption = "/etc/devfsd.conf for saving permissions">
445 REGISTER ^pt[sy]/.* IGNORE
446 CHANGE ^pt[sy]/.* IGNORE
447 CREATE ^pt[sy]/.* IGNORE
448 DELETE ^pt[sy] IGNORE
449 REGISTER ^log IGNORE
450 CHANGE ^log IGNORE
451 CREATE ^log IGNORE
452 DELETE ^log IGNORE
453 REGISTER .* COPY /lib/dev-state/$devname $devpath
454 CHANGE .* COPY $devpath /lib/dev-state/$devname
455 CREATE .* COPY $devpath /lib/dev-state/$devname
456 DELETE .* CFUNCTION GLOBAL unlink
457 /lib/dev-state/$devname
458 RESTORE /lib/dev-state
459 </pre>
460
461 <p>
462 In other words, changed device files are copied over to
463 <path>/lib/dev-state</path> when shutting down the system, and are
464 copied over to <path>/dev</path> when booting the system.
465 </p>
466
467 <p>
468 Another possibility is to mount <path>/lib/dev-state</path> on
469 <path>/dev</path> at boot-time. To do this, you must make sure that
470 devfs is not mounted automatically (meaning you'll have to recompile
471 your kernel) and that <path>/dev/console</path> exists. Then, somewhere
472 at the beginning of the bootscripts of your system, you place:
473 </p>
474
475 <pre caption = "Mounting /lib/dev-state on top of /dev">
476 mount --bind /dev /lib/dev-state
477 mount -t devfs none /dev
478 devfsd /dev
479 </pre>
480
481 </body>
482 </section>
483 </chapter>
484
485 <chapter>
486 <title>Resources</title>
487 <section>
488 <body>
489
490 <p>
491 For more information on devfs, check out the following resources.
492 </p>
493
494 <p>
495 The devfsd.conf manpage explains the syntax of the
496 <path>/etc/devfsd.conf</path> file. To view it, type <c>man
497 devfsd.conf</c>.
498 </p>
499
500 <p>
501 The <uri
502 link="http://www.atnf.csiro.au/people/rgooch/linux/docs/devfs.html">devfs
503 FAQ</uri> explains everything about devfs. It also contains information
504 about the internal devfs structure and how drivers can support devfs.
505 </p>
506
507 <p>
508 On <uri link="http://www.linuxjournal.com">LinuxJournal</uri> there is
509 an interesting article on <uri
510 link="http://www.linuxjournal.com/article.php?sid=6035">devfs for
511 Management and Administration</uri>.
512 </p>
513
514 <p>
515 Daniel Robbins has written a set of articles for IBM's DeveloperWorks
516 about Advanced filesystems. Three of them are about devfs:
517 </p>
518
519 <ul>
520 <li>
521 <uri link="http://www-106.ibm.com/developerworks/linux/library/l-fs4/">
522 Introduction to devfs</uri>
523 </li>
524 <li>
525 <uri link="http://www-106.ibm.com/developerworks/linux/library/l-fs5/">
526 Setting up devfs</uri>
527 </li>
528 <li>
529 <uri link="http://www-106.ibm.com/developerworks/linux/library/l-fs6/">
530 Implementing devfs</uri>
531 </li>
532 </ul>
533
534 </body>
535 </section>
536 </chapter>
537 </guide>

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