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