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 character to /proc/sysrq-trigger. e.g.::
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 79
84 echo t > /proc/sysrq-trigger 80 echo t > /proc/sysrq-trigger
85 81
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. 82 The :kbd:`<command key>` is case sensitive.
92 83
93 What are the 'command' keys? 84 What are the 'command' keys?
94 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 85 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
95 86
96 =========== ================================== 87 =========== ===================================================================
97 Command Function 88 Command Function
98 =========== ================================== 89 =========== ===================================================================
99 ``b`` Will immediately reboot the system 90 ``b`` Will immediately reboot the system without syncing or unmounting
100 your disks. 91 your disks.
101 92
102 ``c`` Will perform a system crash and a 93 ``c`` Will perform a system crash and a crashdump will be taken
103 if configured. 94 if configured.
104 95
105 ``d`` Shows all locks that are held. 96 ``d`` Shows all locks that are held.
106 97
107 ``e`` Send a SIGTERM to all processes, e 98 ``e`` Send a SIGTERM to all processes, except for init.
108 99
109 ``f`` Will call the oom killer to kill a 100 ``f`` Will call the oom killer to kill a memory hog process, but do not
110 panic if nothing can be killed. 101 panic if nothing can be killed.
111 102
112 ``g`` Used by kgdb (kernel debugger) 103 ``g`` Used by kgdb (kernel debugger)
113 104
114 ``h`` Will display help (actually any ot 105 ``h`` Will display help (actually any other key than those listed
115 here will display help. but ``h`` 106 here will display help. but ``h`` is easy to remember :-)
116 107
117 ``i`` Send a SIGKILL to all processes, e 108 ``i`` Send a SIGKILL to all processes, except for init.
118 109
119 ``j`` Forcibly "Just thaw it" - filesyst 110 ``j`` Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.
120 111
121 ``k`` Secure Access Key (SAK) Kills all 112 ``k`` Secure Access Key (SAK) Kills all programs on the current virtual
122 console. NOTE: See important comme 113 console. NOTE: See important comments below in SAK section.
123 114
124 ``l`` Shows a stack backtrace for all ac 115 ``l`` Shows a stack backtrace for all active CPUs.
125 116
126 ``m`` Will dump current memory info to y 117 ``m`` Will dump current memory info to your console.
127 118
128 ``n`` Used to make RT tasks nice-able 119 ``n`` Used to make RT tasks nice-able
129 120
130 ``o`` Will shut your system off (if conf 121 ``o`` Will shut your system off (if configured and supported).
131 122
132 ``p`` Will dump the current registers an 123 ``p`` Will dump the current registers and flags to your console.
133 124
134 ``q`` Will dump per CPU lists of all arm 125 ``q`` Will dump per CPU lists of all armed hrtimers (but NOT regular
135 timer_list timers) and detailed in 126 timer_list timers) and detailed information about all
136 clockevent devices. 127 clockevent devices.
137 128
138 ``r`` Turns off keyboard raw mode and se 129 ``r`` Turns off keyboard raw mode and sets it to XLATE.
139 130
140 ``s`` Will attempt to sync all mounted f 131 ``s`` Will attempt to sync all mounted filesystems.
141 132
142 ``t`` Will dump a list of current tasks 133 ``t`` Will dump a list of current tasks and their information to your
143 console. 134 console.
144 135
145 ``u`` Will attempt to remount all mounte 136 ``u`` Will attempt to remount all mounted filesystems read-only.
146 137
147 ``v`` Forcefully restores framebuffer co 138 ``v`` Forcefully restores framebuffer console
148 ``v`` Causes ETM buffer dump [ARM-specif 139 ``v`` Causes ETM buffer dump [ARM-specific]
149 140
150 ``w`` Dumps tasks that are in uninterrup !! 141 ``w`` Dumps tasks that are in uninterruptable (blocked) state.
151 142
152 ``x`` Used by xmon interface on ppc/powe 143 ``x`` Used by xmon interface on ppc/powerpc platforms.
153 Show global PMU Registers on sparc 144 Show global PMU Registers on sparc64.
154 Dump all TLB entries on MIPS. 145 Dump all TLB entries on MIPS.
155 146
156 ``y`` Show global CPU Registers [SPARC-6 147 ``y`` Show global CPU Registers [SPARC-64 specific]
157 148
158 ``z`` Dump the ftrace buffer 149 ``z`` Dump the ftrace buffer
159 150
160 ``0``-``9`` Sets the console log level, contro 151 ``0``-``9`` Sets the console log level, controlling which kernel messages
161 will be printed to your console. ( 152 will be printed to your console. (``0``, for example would make
162 it so that only emergency messages 153 it so that only emergency messages like PANICs or OOPSes would
163 make it to your console.) 154 make it to your console.)
164 <<
165 ``R`` Replay the kernel log messages on <<
166 =========== ================================== 155 =========== ===================================================================
167 156
168 Okay, so what can I use them for? 157 Okay, so what can I use them for?
169 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 158 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
170 159
171 Well, unraw(r) is very handy when your X serve 160 Well, unraw(r) is very handy when your X server or a svgalib program crashes.
172 161
173 sak(k) (Secure Access Key) is useful when you 162 sak(k) (Secure Access Key) is useful when you want to be sure there is no
174 trojan program running at console which could 163 trojan program running at console which could grab your password
175 when you would try to login. It will kill all 164 when you would try to login. It will kill all programs on given console,
176 thus letting you make sure that the login prom 165 thus letting you make sure that the login prompt you see is actually
177 the one from init, not some trojan program. 166 the one from init, not some trojan program.
178 167
179 .. important:: 168 .. important::
180 169
181 In its true form it is not a true SAK like 170 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 171 c2 compliant system, and it should not be mistaken as
183 such. 172 such.
184 173
185 It seems others find it useful as (System Atte 174 It seems others find it useful as (System Attention Key) which is
186 useful when you want to exit a program that wi 175 useful when you want to exit a program that will not let you switch consoles.
187 (For example, X or a svgalib program.) 176 (For example, X or a svgalib program.)
188 177
189 ``reboot(b)`` is good when you're unable to sh 178 ``reboot(b)`` is good when you're unable to shut down, it is an equivalent
190 of pressing the "reset" button. 179 of pressing the "reset" button.
191 180
192 ``crash(c)`` can be used to manually trigger a 181 ``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 182 Note that this just triggers a crash if there is no dump mechanism available.
194 183
195 ``sync(s)`` is handy before yanking removable 184 ``sync(s)`` is handy before yanking removable medium or after using a rescue
196 shell that provides no graceful shutdown -- it 185 shell that provides no graceful shutdown -- it will ensure your data is
197 safely written to the disk. Note that the sync 186 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. 187 the "OK" and "Done" appear on the screen.
199 188
200 ``umount(u)`` can be used to mark filesystems 189 ``umount(u)`` can be used to mark filesystems as properly unmounted. From the
201 running system's point of view, they will be r 190 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 191 isn't complete until you see the "OK" and "Done" message appear on the screen.
203 192
204 The loglevels ``0``-``9`` are useful when your 193 The loglevels ``0``-``9`` are useful when your console is being flooded with
205 kernel messages you do not want to see. Select 194 kernel messages you do not want to see. Selecting ``0`` will prevent all but
206 the most urgent kernel messages from reaching 195 the most urgent kernel messages from reaching your console. (They will
207 still be logged if syslogd/klogd are alive, th 196 still be logged if syslogd/klogd are alive, though.)
208 197
209 ``term(e)`` and ``kill(i)`` are useful if you 198 ``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 199 you are unable to kill any other way, especially if it's spawning other
211 processes. 200 processes.
212 201
213 "just thaw ``it(j)``" is useful if your system 202 "just thaw ``it(j)``" is useful if your system becomes unresponsive due to a
214 frozen (probably root) filesystem via the FIFR 203 frozen (probably root) filesystem via the FIFREEZE ioctl.
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). <<
222 204
223 Sometimes SysRq seems to get 'stuck' after usi 205 Sometimes SysRq seems to get 'stuck' after using it, what can I do?
224 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 206 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
225 207
226 When this happens, try tapping shift, alt and 208 When this happens, try tapping shift, alt and control on both sides of the
227 keyboard, and hitting an invalid sysrq sequenc 209 keyboard, and hitting an invalid sysrq sequence again. (i.e., something like
228 :kbd:`alt-sysrq-z`). 210 :kbd:`alt-sysrq-z`).
229 211
230 Switching to another virtual console (:kbd:`AL 212 Switching to another virtual console (:kbd:`ALT+Fn`) and then back again
231 should also help. 213 should also help.
232 214
233 I hit SysRq, but nothing seems to happen, what 215 I hit SysRq, but nothing seems to happen, what's wrong?
234 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 216 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
235 217
236 There are some keyboards that produce a differ 218 There are some keyboards that produce a different keycode for SysRq than the
237 pre-defined value of 99 219 pre-defined value of 99
238 (see ``KEY_SYSRQ`` in ``include/uapi/linux/inp 220 (see ``KEY_SYSRQ`` in ``include/uapi/linux/input-event-codes.h``), or
239 which don't have a SysRq key at all. In these 221 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 222 an appropriate scancode sequence, and use ``setkeycodes <sequence> 99`` to map
241 this sequence to the usual SysRq code (e.g., ` 223 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 224 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 225 exit ``showkey`` by not typing anything for ten seconds.
244 226
245 I want to add SysRQ key events to a module, ho 227 I want to add SysRQ key events to a module, how does it work?
246 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 228 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
247 229
248 In order to register a basic function with the 230 In order to register a basic function with the table, you must first include
249 the header ``include/linux/sysrq.h``, this wil 231 the header ``include/linux/sysrq.h``, this will define everything else you need.
250 Next, you must create a ``sysrq_key_op`` struc 232 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 233 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 234 prints help, and C) an action_msg string, that will print right before your
253 handler is called. Your handler must conform t 235 handler is called. Your handler must conform to the prototype in 'sysrq.h'.
254 236
255 After the ``sysrq_key_op`` is created, you can 237 After the ``sysrq_key_op`` is created, you can call the kernel function
256 ``register_sysrq_key(int key, const struct sys 238 ``register_sysrq_key(int key, const struct sysrq_key_op *op_p);`` this will
257 register the operation pointed to by ``op_p`` 239 register the operation pointed to by ``op_p`` at table key 'key',
258 if that slot in the table is blank. At module 240 if that slot in the table is blank. At module unload time, you must call
259 the function ``unregister_sysrq_key(int key, c 241 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 242 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 243 only if it is currently registered in that slot. This is in case the slot has
262 been overwritten since you registered it. 244 been overwritten since you registered it.
263 245
264 The Magic SysRQ system works by registering ke 246 The Magic SysRQ system works by registering key operations against a key op
265 lookup table, which is defined in 'drivers/tty 247 lookup table, which is defined in 'drivers/tty/sysrq.c'. This key table has
266 a number of operations registered into it at c 248 a number of operations registered into it at compile time, but is mutable,
267 and 2 functions are exported for interface to 249 and 2 functions are exported for interface to it::
268 250
269 register_sysrq_key and unregister_sysr 251 register_sysrq_key and unregister_sysrq_key.
270 252
271 Of course, never ever leave an invalid pointer 253 Of course, never ever leave an invalid pointer in the table. I.e., when
272 your module that called register_sysrq_key() e 254 your module that called register_sysrq_key() exits, it must call
273 unregister_sysrq_key() to clean up the sysrq k 255 unregister_sysrq_key() to clean up the sysrq key table entry that it used.
274 Null pointers in the table are always safe. :) 256 Null pointers in the table are always safe. :)
275 257
276 If for some reason you feel the need to call t 258 If for some reason you feel the need to call the handle_sysrq function from
277 within a function called by handle_sysrq, you 259 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, 260 a lock (you are also in an interrupt handler, which means don't sleep!), so
279 you must call ``__handle_sysrq_nolock`` instea 261 you must call ``__handle_sysrq_nolock`` instead.
280 262
281 When I hit a SysRq key combination only the he 263 When I hit a SysRq key combination only the header appears on the console?
282 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 264 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
283 265
284 Sysrq output is subject to the same console lo 266 Sysrq output is subject to the same console loglevel control as all
285 other console output. This means that if the 267 other console output. This means that if the kernel was booted 'quiet'
286 as is common on distro kernels the output may 268 as is common on distro kernels the output may not appear on the actual
287 console, even though it will appear in the dme 269 console, even though it will appear in the dmesg buffer, and be accessible
288 via the dmesg command and to the consumers of 270 via the dmesg command and to the consumers of ``/proc/kmsg``. As a specific
289 exception the header line from the sysrq comma 271 exception the header line from the sysrq command is passed to all console
290 consumers as if the current loglevel was maxim 272 consumers as if the current loglevel was maximum. If only the header
291 is emitted it is almost certain that the kerne 273 is emitted it is almost certain that the kernel loglevel is too low.
292 Should you require the output on the console c 274 Should you require the output on the console channel then you will need
293 to temporarily up the console loglevel using : 275 to temporarily up the console loglevel using :kbd:`alt-sysrq-8` or::
294 276
295 echo 8 > /proc/sysrq-trigger 277 echo 8 > /proc/sysrq-trigger
296 278
297 Remember to return the loglevel to normal afte 279 Remember to return the loglevel to normal after triggering the sysrq
298 command you are interested in. 280 command you are interested in.
299 281
300 I have more questions, who can I ask? 282 I have more questions, who can I ask?
301 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 283 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
302 284
303 Just ask them on the linux-kernel mailing list 285 Just ask them on the linux-kernel mailing list:
304 linux-kernel@vger.kernel.org 286 linux-kernel@vger.kernel.org
305 287
306 Credits 288 Credits
307 ~~~~~~~ 289 ~~~~~~~
308 290
309 - Written by Mydraal <vulpyne@vulpyne.net> 291 - Written by Mydraal <vulpyne@vulpyne.net>
310 - Updated by Adam Sulmicki <adam@cfar.umd.edu> 292 - Updated by Adam Sulmicki <adam@cfar.umd.edu>
311 - Updated by Jeremy M. Dolan <jmd@turbogeek.org 293 - Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
312 - Added to by Crutcher Dunnavant <crutcher+kern 294 - Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>
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