1 Linux Magic System Request Key Hacks 1 Linux Magic System Request Key Hacks
2 ==================================== 2 ====================================
3 3
4 Documentation for sysrq.c 4 Documentation for sysrq.c
5 5
6 What is the magic SysRq key? 6 What is the magic SysRq key?
7 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 7 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
8 8
9 It is a 'magical' key combo you can hit which 9 It is a 'magical' key combo you can hit which the kernel will respond to
10 regardless of whatever else it is doing, unles 10 regardless of whatever else it is doing, unless it is completely locked up.
11 11
12 How do I enable the magic SysRq key? 12 How do I enable the magic SysRq key?
13 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 13 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
14 14
15 You need to say "yes" to 'Magic SysRq key (CON 15 You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when
16 configuring the kernel. When running a kernel 16 configuring the kernel. When running a kernel with SysRq compiled in,
17 /proc/sys/kernel/sysrq controls the functions 17 /proc/sys/kernel/sysrq controls the functions allowed to be invoked via
18 the SysRq key. The default value in this file 18 the SysRq key. The default value in this file is set by the
19 CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE config symbo 19 CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE config symbol, which itself defaults
20 to 1. Here is the list of possible values in / 20 to 1. Here is the list of possible values in /proc/sys/kernel/sysrq:
21 21
22 - 0 - disable sysrq completely 22 - 0 - disable sysrq completely
23 - 1 - enable all functions of sysrq 23 - 1 - enable all functions of sysrq
24 - >1 - bitmask of allowed sysrq functions ( 24 - >1 - bitmask of allowed sysrq functions (see below for detailed function
25 description):: 25 description)::
26 26
27 2 = 0x2 - enable control of consol 27 2 = 0x2 - enable control of console logging level
28 4 = 0x4 - enable control of keyboa 28 4 = 0x4 - enable control of keyboard (SAK, unraw)
29 8 = 0x8 - enable debugging dumps o 29 8 = 0x8 - enable debugging dumps of processes etc.
30 16 = 0x10 - enable sync command 30 16 = 0x10 - enable sync command
31 32 = 0x20 - enable remount read-only 31 32 = 0x20 - enable remount read-only
32 64 = 0x40 - enable signalling of pro 32 64 = 0x40 - enable signalling of processes (term, kill, oom-kill)
33 128 = 0x80 - allow reboot/poweroff 33 128 = 0x80 - allow reboot/poweroff
34 256 = 0x100 - allow nicing of all RT t 34 256 = 0x100 - allow nicing of all RT tasks
35 35
36 You can set the value in the file by the follo 36 You can set the value in the file by the following command::
37 37
38 echo "number" >/proc/sys/kernel/sysrq 38 echo "number" >/proc/sys/kernel/sysrq
39 39
40 The number may be written here either as decim 40 The number may be written here either as decimal or as hexadecimal
41 with the 0x prefix. CONFIG_MAGIC_SYSRQ_DEFAULT 41 with the 0x prefix. CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE must always be
42 written in hexadecimal. 42 written in hexadecimal.
43 43
44 Note that the value of ``/proc/sys/kernel/sysr 44 Note that the value of ``/proc/sys/kernel/sysrq`` influences only the invocation
45 via a keyboard. Invocation of any operation vi 45 via a keyboard. Invocation of any operation via ``/proc/sysrq-trigger`` is
46 always allowed (by a user with admin privilege 46 always allowed (by a user with admin privileges).
47 47
48 How do I use the magic SysRq key? 48 How do I use the magic SysRq key?
49 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 49 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
50 50
51 On x86 51 On x86
52 You press the key combo :kbd:`ALT-SysR 52 You press the key combo :kbd:`ALT-SysRq-<command key>`.
53 53
54 .. note:: 54 .. note::
55 Some 55 Some
56 keyboards may not have a key labele 56 keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is
57 also known as the 'Print Screen' ke 57 also known as the 'Print Screen' key. Also some keyboards cannot
58 handle so many keys being pressed a 58 handle so many keys being pressed at the same time, so you might
59 have better luck with press :kbd:`A 59 have better luck with press :kbd:`Alt`, press :kbd:`SysRq`,
60 release :kbd:`SysRq`, press :kbd:`< 60 release :kbd:`SysRq`, press :kbd:`<command key>`, release everything.
61 61
62 On SPARC 62 On SPARC
63 You press :kbd:`ALT-STOP-<command key> 63 You press :kbd:`ALT-STOP-<command key>`, I believe.
64 64
65 On the serial console (PC style standard seria 65 On the serial console (PC style standard serial ports only)
66 You send a ``BREAK``, then within 5 se 66 You send a ``BREAK``, then within 5 seconds a command key. Sending
67 ``BREAK`` twice is interpreted as a no 67 ``BREAK`` twice is interpreted as a normal BREAK.
68 68
69 On PowerPC 69 On PowerPC
70 Press :kbd:`ALT - Print Screen` (or :k 70 Press :kbd:`ALT - Print Screen` (or :kbd:`F13`) - :kbd:`<command key>`.
71 :kbd:`Print Screen` (or :kbd:`F13`) - 71 :kbd:`Print Screen` (or :kbd:`F13`) - :kbd:`<command key>` may suffice.
72 72
73 On other 73 On other
74 If you know of the key combos for othe 74 If you know of the key combos for other architectures, please
75 submit a patch to be included in this 75 submit a patch to be included in this section.
76 76
77 On all 77 On all
78 Write a single character to /proc/sysr 78 Write a single character to /proc/sysrq-trigger.
79 Only the first character is processed, 79 Only the first character is processed, the rest of the string is
80 ignored. However, it is not recommende 80 ignored. However, it is not recommended to write any extra characters
81 as the behavior is undefined and might 81 as the behavior is undefined and might change in the future versions.
82 E.g.:: 82 E.g.::
83 83
84 echo t > /proc/sysrq-trigger 84 echo t > /proc/sysrq-trigger
85 85
86 Alternatively, write multiple characte 86 Alternatively, write multiple characters prepended by underscore.
87 This way, all characters will be proce 87 This way, all characters will be processed. E.g.::
88 88
89 echo _reisub > /proc/sysrq-tri 89 echo _reisub > /proc/sysrq-trigger
90 90
91 The :kbd:`<command key>` is case sensitive. 91 The :kbd:`<command key>` is case sensitive.
92 92
93 What are the 'command' keys? 93 What are the 'command' keys?
94 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 94 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
95 95
96 =========== ================================== 96 =========== ===================================================================
97 Command Function 97 Command Function
98 =========== ================================== 98 =========== ===================================================================
99 ``b`` Will immediately reboot the system 99 ``b`` Will immediately reboot the system without syncing or unmounting
100 your disks. 100 your disks.
101 101
102 ``c`` Will perform a system crash and a 102 ``c`` Will perform a system crash and a crashdump will be taken
103 if configured. 103 if configured.
104 104
105 ``d`` Shows all locks that are held. 105 ``d`` Shows all locks that are held.
106 106
107 ``e`` Send a SIGTERM to all processes, e 107 ``e`` Send a SIGTERM to all processes, except for init.
108 108
109 ``f`` Will call the oom killer to kill a 109 ``f`` Will call the oom killer to kill a memory hog process, but do not
110 panic if nothing can be killed. 110 panic if nothing can be killed.
111 111
112 ``g`` Used by kgdb (kernel debugger) 112 ``g`` Used by kgdb (kernel debugger)
113 113
114 ``h`` Will display help (actually any ot 114 ``h`` Will display help (actually any other key than those listed
115 here will display help. but ``h`` 115 here will display help. but ``h`` is easy to remember :-)
116 116
117 ``i`` Send a SIGKILL to all processes, e 117 ``i`` Send a SIGKILL to all processes, except for init.
118 118
119 ``j`` Forcibly "Just thaw it" - filesyst 119 ``j`` Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.
120 120
121 ``k`` Secure Access Key (SAK) Kills all 121 ``k`` Secure Access Key (SAK) Kills all programs on the current virtual
122 console. NOTE: See important comme 122 console. NOTE: See important comments below in SAK section.
123 123
124 ``l`` Shows a stack backtrace for all ac 124 ``l`` Shows a stack backtrace for all active CPUs.
125 125
126 ``m`` Will dump current memory info to y 126 ``m`` Will dump current memory info to your console.
127 127
128 ``n`` Used to make RT tasks nice-able 128 ``n`` Used to make RT tasks nice-able
129 129
130 ``o`` Will shut your system off (if conf 130 ``o`` Will shut your system off (if configured and supported).
131 131
132 ``p`` Will dump the current registers an 132 ``p`` Will dump the current registers and flags to your console.
133 133
134 ``q`` Will dump per CPU lists of all arm 134 ``q`` Will dump per CPU lists of all armed hrtimers (but NOT regular
135 timer_list timers) and detailed in 135 timer_list timers) and detailed information about all
136 clockevent devices. 136 clockevent devices.
137 137
138 ``r`` Turns off keyboard raw mode and se 138 ``r`` Turns off keyboard raw mode and sets it to XLATE.
139 139
140 ``s`` Will attempt to sync all mounted f 140 ``s`` Will attempt to sync all mounted filesystems.
141 141
142 ``t`` Will dump a list of current tasks 142 ``t`` Will dump a list of current tasks and their information to your
143 console. 143 console.
144 144
145 ``u`` Will attempt to remount all mounte 145 ``u`` Will attempt to remount all mounted filesystems read-only.
146 146
147 ``v`` Forcefully restores framebuffer co 147 ``v`` Forcefully restores framebuffer console
148 ``v`` Causes ETM buffer dump [ARM-specif 148 ``v`` Causes ETM buffer dump [ARM-specific]
149 149
150 ``w`` Dumps tasks that are in uninterrup 150 ``w`` Dumps tasks that are in uninterruptible (blocked) state.
151 151
152 ``x`` Used by xmon interface on ppc/powe 152 ``x`` Used by xmon interface on ppc/powerpc platforms.
153 Show global PMU Registers on sparc 153 Show global PMU Registers on sparc64.
154 Dump all TLB entries on MIPS. 154 Dump all TLB entries on MIPS.
155 155
156 ``y`` Show global CPU Registers [SPARC-6 156 ``y`` Show global CPU Registers [SPARC-64 specific]
157 157
158 ``z`` Dump the ftrace buffer 158 ``z`` Dump the ftrace buffer
159 159
160 ``0``-``9`` Sets the console log level, contro 160 ``0``-``9`` Sets the console log level, controlling which kernel messages
161 will be printed to your console. ( 161 will be printed to your console. (``0``, for example would make
162 it so that only emergency messages 162 it so that only emergency messages like PANICs or OOPSes would
163 make it to your console.) 163 make it to your console.)
164 164
165 ``R`` Replay the kernel log messages on 165 ``R`` Replay the kernel log messages on consoles.
166 =========== ================================== 166 =========== ===================================================================
167 167
168 Okay, so what can I use them for? 168 Okay, so what can I use them for?
169 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 169 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
170 170
171 Well, unraw(r) is very handy when your X serve 171 Well, unraw(r) is very handy when your X server or a svgalib program crashes.
172 172
173 sak(k) (Secure Access Key) is useful when you 173 sak(k) (Secure Access Key) is useful when you want to be sure there is no
174 trojan program running at console which could 174 trojan program running at console which could grab your password
175 when you would try to login. It will kill all 175 when you would try to login. It will kill all programs on given console,
176 thus letting you make sure that the login prom 176 thus letting you make sure that the login prompt you see is actually
177 the one from init, not some trojan program. 177 the one from init, not some trojan program.
178 178
179 .. important:: 179 .. important::
180 180
181 In its true form it is not a true SAK like 181 In its true form it is not a true SAK like the one in a
182 c2 compliant system, and it should not be m 182 c2 compliant system, and it should not be mistaken as
183 such. 183 such.
184 184
185 It seems others find it useful as (System Atte 185 It seems others find it useful as (System Attention Key) which is
186 useful when you want to exit a program that wi 186 useful when you want to exit a program that will not let you switch consoles.
187 (For example, X or a svgalib program.) 187 (For example, X or a svgalib program.)
188 188
189 ``reboot(b)`` is good when you're unable to sh 189 ``reboot(b)`` is good when you're unable to shut down, it is an equivalent
190 of pressing the "reset" button. 190 of pressing the "reset" button.
191 191
192 ``crash(c)`` can be used to manually trigger a 192 ``crash(c)`` can be used to manually trigger a crashdump when the system is hung.
193 Note that this just triggers a crash if there 193 Note that this just triggers a crash if there is no dump mechanism available.
194 194
195 ``sync(s)`` is handy before yanking removable 195 ``sync(s)`` is handy before yanking removable medium or after using a rescue
196 shell that provides no graceful shutdown -- it 196 shell that provides no graceful shutdown -- it will ensure your data is
197 safely written to the disk. Note that the sync 197 safely written to the disk. Note that the sync hasn't taken place until you see
198 the "OK" and "Done" appear on the screen. 198 the "OK" and "Done" appear on the screen.
199 199
200 ``umount(u)`` can be used to mark filesystems 200 ``umount(u)`` can be used to mark filesystems as properly unmounted. From the
201 running system's point of view, they will be r 201 running system's point of view, they will be remounted read-only. The remount
202 isn't complete until you see the "OK" and "Don 202 isn't complete until you see the "OK" and "Done" message appear on the screen.
203 203
204 The loglevels ``0``-``9`` are useful when your 204 The loglevels ``0``-``9`` are useful when your console is being flooded with
205 kernel messages you do not want to see. Select 205 kernel messages you do not want to see. Selecting ``0`` will prevent all but
206 the most urgent kernel messages from reaching 206 the most urgent kernel messages from reaching your console. (They will
207 still be logged if syslogd/klogd are alive, th 207 still be logged if syslogd/klogd are alive, though.)
208 208
209 ``term(e)`` and ``kill(i)`` are useful if you 209 ``term(e)`` and ``kill(i)`` are useful if you have some sort of runaway process
210 you are unable to kill any other way, especial 210 you are unable to kill any other way, especially if it's spawning other
211 processes. 211 processes.
212 212
213 "just thaw ``it(j)``" is useful if your system 213 "just thaw ``it(j)``" is useful if your system becomes unresponsive due to a
214 frozen (probably root) filesystem via the FIFR 214 frozen (probably root) filesystem via the FIFREEZE ioctl.
215 215
216 ``Replay logs(R)`` is useful to view the kerne 216 ``Replay logs(R)`` is useful to view the kernel log messages when system is hung
217 or you are not able to use dmesg command to vi 217 or you are not able to use dmesg command to view the messages in printk buffer.
218 User may have to press the key combination mul 218 User may have to press the key combination multiple times if console system is
219 busy. If it is completely locked up, then mess 219 busy. If it is completely locked up, then messages won't be printed. Output
220 messages depend on current console loglevel, w 220 messages depend on current console loglevel, which can be modified using
221 sysrq[0-9] (see above). 221 sysrq[0-9] (see above).
222 222
223 Sometimes SysRq seems to get 'stuck' after usi 223 Sometimes SysRq seems to get 'stuck' after using it, what can I do?
224 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 224 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
225 225
226 When this happens, try tapping shift, alt and 226 When this happens, try tapping shift, alt and control on both sides of the
227 keyboard, and hitting an invalid sysrq sequenc 227 keyboard, and hitting an invalid sysrq sequence again. (i.e., something like
228 :kbd:`alt-sysrq-z`). 228 :kbd:`alt-sysrq-z`).
229 229
230 Switching to another virtual console (:kbd:`AL 230 Switching to another virtual console (:kbd:`ALT+Fn`) and then back again
231 should also help. 231 should also help.
232 232
233 I hit SysRq, but nothing seems to happen, what 233 I hit SysRq, but nothing seems to happen, what's wrong?
234 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 234 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
235 235
236 There are some keyboards that produce a differ 236 There are some keyboards that produce a different keycode for SysRq than the
237 pre-defined value of 99 237 pre-defined value of 99
238 (see ``KEY_SYSRQ`` in ``include/uapi/linux/inp 238 (see ``KEY_SYSRQ`` in ``include/uapi/linux/input-event-codes.h``), or
239 which don't have a SysRq key at all. In these 239 which don't have a SysRq key at all. In these cases, run ``showkey -s`` to find
240 an appropriate scancode sequence, and use ``se 240 an appropriate scancode sequence, and use ``setkeycodes <sequence> 99`` to map
241 this sequence to the usual SysRq code (e.g., ` 241 this sequence to the usual SysRq code (e.g., ``setkeycodes e05b 99``). It's
242 probably best to put this command in a boot sc 242 probably best to put this command in a boot script. Oh, and by the way, you
243 exit ``showkey`` by not typing anything for te 243 exit ``showkey`` by not typing anything for ten seconds.
244 244
245 I want to add SysRQ key events to a module, ho 245 I want to add SysRQ key events to a module, how does it work?
246 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 246 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
247 247
248 In order to register a basic function with the 248 In order to register a basic function with the table, you must first include
249 the header ``include/linux/sysrq.h``, this wil 249 the header ``include/linux/sysrq.h``, this will define everything else you need.
250 Next, you must create a ``sysrq_key_op`` struc 250 Next, you must create a ``sysrq_key_op`` struct, and populate it with A) the key
251 handler function you will use, B) a help_msg s 251 handler function you will use, B) a help_msg string, that will print when SysRQ
252 prints help, and C) an action_msg string, that 252 prints help, and C) an action_msg string, that will print right before your
253 handler is called. Your handler must conform t 253 handler is called. Your handler must conform to the prototype in 'sysrq.h'.
254 254
255 After the ``sysrq_key_op`` is created, you can 255 After the ``sysrq_key_op`` is created, you can call the kernel function
256 ``register_sysrq_key(int key, const struct sys 256 ``register_sysrq_key(int key, const struct sysrq_key_op *op_p);`` this will
257 register the operation pointed to by ``op_p`` 257 register the operation pointed to by ``op_p`` at table key 'key',
258 if that slot in the table is blank. At module 258 if that slot in the table is blank. At module unload time, you must call
259 the function ``unregister_sysrq_key(int key, c 259 the function ``unregister_sysrq_key(int key, const struct sysrq_key_op *op_p)``,
260 which will remove the key op pointed to by 'op 260 which will remove the key op pointed to by 'op_p' from the key 'key', if and
261 only if it is currently registered in that slo 261 only if it is currently registered in that slot. This is in case the slot has
262 been overwritten since you registered it. 262 been overwritten since you registered it.
263 263
264 The Magic SysRQ system works by registering ke 264 The Magic SysRQ system works by registering key operations against a key op
265 lookup table, which is defined in 'drivers/tty 265 lookup table, which is defined in 'drivers/tty/sysrq.c'. This key table has
266 a number of operations registered into it at c 266 a number of operations registered into it at compile time, but is mutable,
267 and 2 functions are exported for interface to 267 and 2 functions are exported for interface to it::
268 268
269 register_sysrq_key and unregister_sysr 269 register_sysrq_key and unregister_sysrq_key.
270 270
271 Of course, never ever leave an invalid pointer 271 Of course, never ever leave an invalid pointer in the table. I.e., when
272 your module that called register_sysrq_key() e 272 your module that called register_sysrq_key() exits, it must call
273 unregister_sysrq_key() to clean up the sysrq k 273 unregister_sysrq_key() to clean up the sysrq key table entry that it used.
274 Null pointers in the table are always safe. :) 274 Null pointers in the table are always safe. :)
275 275
276 If for some reason you feel the need to call t 276 If for some reason you feel the need to call the handle_sysrq function from
277 within a function called by handle_sysrq, you 277 within a function called by handle_sysrq, you must be aware that you are in
278 a lock (you are also in an interrupt handler, 278 a lock (you are also in an interrupt handler, which means don't sleep!), so
279 you must call ``__handle_sysrq_nolock`` instea 279 you must call ``__handle_sysrq_nolock`` instead.
280 280
281 When I hit a SysRq key combination only the he 281 When I hit a SysRq key combination only the header appears on the console?
282 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 282 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
283 283
284 Sysrq output is subject to the same console lo 284 Sysrq output is subject to the same console loglevel control as all
285 other console output. This means that if the 285 other console output. This means that if the kernel was booted 'quiet'
286 as is common on distro kernels the output may 286 as is common on distro kernels the output may not appear on the actual
287 console, even though it will appear in the dme 287 console, even though it will appear in the dmesg buffer, and be accessible
288 via the dmesg command and to the consumers of 288 via the dmesg command and to the consumers of ``/proc/kmsg``. As a specific
289 exception the header line from the sysrq comma 289 exception the header line from the sysrq command is passed to all console
290 consumers as if the current loglevel was maxim 290 consumers as if the current loglevel was maximum. If only the header
291 is emitted it is almost certain that the kerne 291 is emitted it is almost certain that the kernel loglevel is too low.
292 Should you require the output on the console c 292 Should you require the output on the console channel then you will need
293 to temporarily up the console loglevel using : 293 to temporarily up the console loglevel using :kbd:`alt-sysrq-8` or::
294 294
295 echo 8 > /proc/sysrq-trigger 295 echo 8 > /proc/sysrq-trigger
296 296
297 Remember to return the loglevel to normal afte 297 Remember to return the loglevel to normal after triggering the sysrq
298 command you are interested in. 298 command you are interested in.
299 299
300 I have more questions, who can I ask? 300 I have more questions, who can I ask?
301 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 301 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
302 302
303 Just ask them on the linux-kernel mailing list 303 Just ask them on the linux-kernel mailing list:
304 linux-kernel@vger.kernel.org 304 linux-kernel@vger.kernel.org
305 305
306 Credits 306 Credits
307 ~~~~~~~ 307 ~~~~~~~
308 308
309 - Written by Mydraal <vulpyne@vulpyne.net> 309 - Written by Mydraal <vulpyne@vulpyne.net>
310 - Updated by Adam Sulmicki <adam@cfar.umd.edu> 310 - Updated by Adam Sulmicki <adam@cfar.umd.edu>
311 - Updated by Jeremy M. Dolan <jmd@turbogeek.org 311 - Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
312 - Added to by Crutcher Dunnavant <crutcher+kern 312 - Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>
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