1 Linux Magic System Request Key Hacks
2 ====================================
3
4 Documentation for sysrq.c
5
6 What is the magic SysRq key?
7 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
8
9 It is a 'magical' key combo you can hit which
10 regardless of whatever else it is doing, unles
11
12 How do I enable the magic SysRq key?
13 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
14
15 You need to say "yes" to 'Magic SysRq key (CON
16 configuring the kernel. When running a kernel
17 /proc/sys/kernel/sysrq controls the functions
18 the SysRq key. The default value in this file
19 CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE config symbo
20 to 1. Here is the list of possible values in /
21
22 - 0 - disable sysrq completely
23 - 1 - enable all functions of sysrq
24 - >1 - bitmask of allowed sysrq functions (
25 description)::
26
27 2 = 0x2 - enable control of consol
28 4 = 0x4 - enable control of keyboa
29 8 = 0x8 - enable debugging dumps o
30 16 = 0x10 - enable sync command
31 32 = 0x20 - enable remount read-only
32 64 = 0x40 - enable signalling of pro
33 128 = 0x80 - allow reboot/poweroff
34 256 = 0x100 - allow nicing of all RT t
35
36 You can set the value in the file by the follo
37
38 echo "number" >/proc/sys/kernel/sysrq
39
40 The number may be written here either as decim
41 with the 0x prefix. CONFIG_MAGIC_SYSRQ_DEFAULT
42 written in hexadecimal.
43
44 Note that the value of ``/proc/sys/kernel/sysr
45 via a keyboard. Invocation of any operation vi
46 always allowed (by a user with admin privilege
47
48 How do I use the magic SysRq key?
49 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
50
51 On x86
52 You press the key combo :kbd:`ALT-SysR
53
54 .. note::
55 Some
56 keyboards may not have a key labele
57 also known as the 'Print Screen' ke
58 handle so many keys being pressed a
59 have better luck with press :kbd:`A
60 release :kbd:`SysRq`, press :kbd:`<
61
62 On SPARC
63 You press :kbd:`ALT-STOP-<command key>
64
65 On the serial console (PC style standard seria
66 You send a ``BREAK``, then within 5 se
67 ``BREAK`` twice is interpreted as a no
68
69 On PowerPC
70 Press :kbd:`ALT - Print Screen` (or :k
71 :kbd:`Print Screen` (or :kbd:`F13`) -
72
73 On other
74 If you know of the key combos for othe
75 submit a patch to be included in this
76
77 On all
78 Write a single character to /proc/sysr
79 Only the first character is processed,
80 ignored. However, it is not recommende
81 as the behavior is undefined and might
82 E.g.::
83
84 echo t > /proc/sysrq-trigger
85
86 Alternatively, write multiple characte
87 This way, all characters will be proce
88
89 echo _reisub > /proc/sysrq-tri
90
91 The :kbd:`<command key>` is case sensitive.
92
93 What are the 'command' keys?
94 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
95
96 =========== ==================================
97 Command Function
98 =========== ==================================
99 ``b`` Will immediately reboot the system
100 your disks.
101
102 ``c`` Will perform a system crash and a
103 if configured.
104
105 ``d`` Shows all locks that are held.
106
107 ``e`` Send a SIGTERM to all processes, e
108
109 ``f`` Will call the oom killer to kill a
110 panic if nothing can be killed.
111
112 ``g`` Used by kgdb (kernel debugger)
113
114 ``h`` Will display help (actually any ot
115 here will display help. but ``h``
116
117 ``i`` Send a SIGKILL to all processes, e
118
119 ``j`` Forcibly "Just thaw it" - filesyst
120
121 ``k`` Secure Access Key (SAK) Kills all
122 console. NOTE: See important comme
123
124 ``l`` Shows a stack backtrace for all ac
125
126 ``m`` Will dump current memory info to y
127
128 ``n`` Used to make RT tasks nice-able
129
130 ``o`` Will shut your system off (if conf
131
132 ``p`` Will dump the current registers an
133
134 ``q`` Will dump per CPU lists of all arm
135 timer_list timers) and detailed in
136 clockevent devices.
137
138 ``r`` Turns off keyboard raw mode and se
139
140 ``s`` Will attempt to sync all mounted f
141
142 ``t`` Will dump a list of current tasks
143 console.
144
145 ``u`` Will attempt to remount all mounte
146
147 ``v`` Forcefully restores framebuffer co
148 ``v`` Causes ETM buffer dump [ARM-specif
149
150 ``w`` Dumps tasks that are in uninterrup
151
152 ``x`` Used by xmon interface on ppc/powe
153 Show global PMU Registers on sparc
154 Dump all TLB entries on MIPS.
155
156 ``y`` Show global CPU Registers [SPARC-6
157
158 ``z`` Dump the ftrace buffer
159
160 ``0``-``9`` Sets the console log level, contro
161 will be printed to your console. (
162 it so that only emergency messages
163 make it to your console.)
164
165 ``R`` Replay the kernel log messages on
166 =========== ==================================
167
168 Okay, so what can I use them for?
169 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
170
171 Well, unraw(r) is very handy when your X serve
172
173 sak(k) (Secure Access Key) is useful when you
174 trojan program running at console which could
175 when you would try to login. It will kill all
176 thus letting you make sure that the login prom
177 the one from init, not some trojan program.
178
179 .. important::
180
181 In its true form it is not a true SAK like
182 c2 compliant system, and it should not be m
183 such.
184
185 It seems others find it useful as (System Atte
186 useful when you want to exit a program that wi
187 (For example, X or a svgalib program.)
188
189 ``reboot(b)`` is good when you're unable to sh
190 of pressing the "reset" button.
191
192 ``crash(c)`` can be used to manually trigger a
193 Note that this just triggers a crash if there
194
195 ``sync(s)`` is handy before yanking removable
196 shell that provides no graceful shutdown -- it
197 safely written to the disk. Note that the sync
198 the "OK" and "Done" appear on the screen.
199
200 ``umount(u)`` can be used to mark filesystems
201 running system's point of view, they will be r
202 isn't complete until you see the "OK" and "Don
203
204 The loglevels ``0``-``9`` are useful when your
205 kernel messages you do not want to see. Select
206 the most urgent kernel messages from reaching
207 still be logged if syslogd/klogd are alive, th
208
209 ``term(e)`` and ``kill(i)`` are useful if you
210 you are unable to kill any other way, especial
211 processes.
212
213 "just thaw ``it(j)``" is useful if your system
214 frozen (probably root) filesystem via the FIFR
215
216 ``Replay logs(R)`` is useful to view the kerne
217 or you are not able to use dmesg command to vi
218 User may have to press the key combination mul
219 busy. If it is completely locked up, then mess
220 messages depend on current console loglevel, w
221 sysrq[0-9] (see above).
222
223 Sometimes SysRq seems to get 'stuck' after usi
224 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
225
226 When this happens, try tapping shift, alt and
227 keyboard, and hitting an invalid sysrq sequenc
228 :kbd:`alt-sysrq-z`).
229
230 Switching to another virtual console (:kbd:`AL
231 should also help.
232
233 I hit SysRq, but nothing seems to happen, what
234 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
235
236 There are some keyboards that produce a differ
237 pre-defined value of 99
238 (see ``KEY_SYSRQ`` in ``include/uapi/linux/inp
239 which don't have a SysRq key at all. In these
240 an appropriate scancode sequence, and use ``se
241 this sequence to the usual SysRq code (e.g., `
242 probably best to put this command in a boot sc
243 exit ``showkey`` by not typing anything for te
244
245 I want to add SysRQ key events to a module, ho
246 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
247
248 In order to register a basic function with the
249 the header ``include/linux/sysrq.h``, this wil
250 Next, you must create a ``sysrq_key_op`` struc
251 handler function you will use, B) a help_msg s
252 prints help, and C) an action_msg string, that
253 handler is called. Your handler must conform t
254
255 After the ``sysrq_key_op`` is created, you can
256 ``register_sysrq_key(int key, const struct sys
257 register the operation pointed to by ``op_p``
258 if that slot in the table is blank. At module
259 the function ``unregister_sysrq_key(int key, c
260 which will remove the key op pointed to by 'op
261 only if it is currently registered in that slo
262 been overwritten since you registered it.
263
264 The Magic SysRQ system works by registering ke
265 lookup table, which is defined in 'drivers/tty
266 a number of operations registered into it at c
267 and 2 functions are exported for interface to
268
269 register_sysrq_key and unregister_sysr
270
271 Of course, never ever leave an invalid pointer
272 your module that called register_sysrq_key() e
273 unregister_sysrq_key() to clean up the sysrq k
274 Null pointers in the table are always safe. :)
275
276 If for some reason you feel the need to call t
277 within a function called by handle_sysrq, you
278 a lock (you are also in an interrupt handler,
279 you must call ``__handle_sysrq_nolock`` instea
280
281 When I hit a SysRq key combination only the he
282 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
283
284 Sysrq output is subject to the same console lo
285 other console output. This means that if the
286 as is common on distro kernels the output may
287 console, even though it will appear in the dme
288 via the dmesg command and to the consumers of
289 exception the header line from the sysrq comma
290 consumers as if the current loglevel was maxim
291 is emitted it is almost certain that the kerne
292 Should you require the output on the console c
293 to temporarily up the console loglevel using :
294
295 echo 8 > /proc/sysrq-trigger
296
297 Remember to return the loglevel to normal afte
298 command you are interested in.
299
300 I have more questions, who can I ask?
301 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
302
303 Just ask them on the linux-kernel mailing list
304 linux-kernel@vger.kernel.org
305
306 Credits
307 ~~~~~~~
308
309 - Written by Mydraal <vulpyne@vulpyne.net>
310 - Updated by Adam Sulmicki <adam@cfar.umd.edu>
311 - Updated by Jeremy M. Dolan <jmd@turbogeek.org
312 - Added to by Crutcher Dunnavant <crutcher+kern
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