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1da177e4 | 1 | Linux Magic System Request Key Hacks |
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2 | ==================================== |
3 | ||
d346cce3 | 4 | Documentation for sysrq.c |
1da177e4 | 5 | |
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6 | What is the magic SysRq key? |
7 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
8 | ||
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9 | It is a 'magical' key combo you can hit which the kernel will respond to |
10 | regardless of whatever else it is doing, unless it is completely locked up. | |
11 | ||
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12 | How do I enable the magic SysRq key? |
13 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
14 | ||
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15 | You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when |
16 | configuring the kernel. When running a kernel with SysRq compiled in, | |
17 | /proc/sys/kernel/sysrq controls the functions allowed to be invoked via | |
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18 | the SysRq key. The default value in this file is set by the |
19 | CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE config symbol, which itself defaults | |
20 | to 1. Here is the list of possible values in /proc/sys/kernel/sysrq: | |
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21 | |
22 | - 0 - disable sysrq completely | |
23 | - 1 - enable all functions of sysrq | |
24 | - >1 - bitmask of allowed sysrq functions (see below for detailed function | |
25 | description):: | |
26 | ||
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27 | 2 = 0x2 - enable control of console logging level |
28 | 4 = 0x4 - enable control of keyboard (SAK, unraw) | |
29 | 8 = 0x8 - enable debugging dumps of processes etc. | |
30 | 16 = 0x10 - enable sync command | |
31 | 32 = 0x20 - enable remount read-only | |
32 | 64 = 0x40 - enable signalling of processes (term, kill, oom-kill) | |
33 | 128 = 0x80 - allow reboot/poweroff | |
34 | 256 = 0x100 - allow nicing of all RT tasks | |
1da177e4 | 35 | |
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36 | You can set the value in the file by the following command:: |
37 | ||
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38 | echo "number" >/proc/sys/kernel/sysrq |
39 | ||
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40 | The number may be written here either as decimal or as hexadecimal |
41 | with the 0x prefix. CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE must always be | |
42 | written in hexadecimal. | |
e8b5cbb0 | 43 | |
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44 | Note that the value of ``/proc/sys/kernel/sysrq`` influences only the invocation |
45 | via a keyboard. Invocation of any operation via ``/proc/sysrq-trigger`` is | |
46 | always allowed (by a user with admin privileges). | |
1da177e4 | 47 | |
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48 | How do I use the magic SysRq key? |
49 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
50 | ||
51 | On x86 - You press the key combo :kbd:`ALT-SysRq-<command key>`. | |
52 | ||
53 | .. note:: | |
54 | Some | |
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55 | keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is |
56 | also known as the 'Print Screen' key. Also some keyboards cannot | |
57 | handle so many keys being pressed at the same time, so you might | |
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58 | have better luck with press :kbd:`Alt`, press :kbd:`SysRq`, |
59 | release :kbd:`SysRq`, press :kbd:`<command key>`, release everything. | |
1da177e4 | 60 | |
c8956bb7 | 61 | On SPARC - You press :kbd:`ALT-STOP-<command key>`, I believe. |
1da177e4 | 62 | |
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63 | On the serial console (PC style standard serial ports only) |
64 | You send a ``BREAK``, then within 5 seconds a command key. Sending | |
65 | ``BREAK`` twice is interpreted as a normal BREAK. | |
1da177e4 | 66 | |
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67 | On PowerPC |
68 | Press :kbd:`ALT - Print Screen` (or :kbd:`F13`) - :kbd:`<command key>`, | |
69 | :kbd:`Print Screen` (or :kbd:`F13`) - :kbd:`<command key>` may suffice. | |
1da177e4 | 70 | |
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71 | On other |
72 | If you know of the key combos for other architectures, please | |
73 | let me know so I can add them to this section. | |
1da177e4 | 74 | |
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75 | On all |
76 | write a character to /proc/sysrq-trigger. e.g.:: | |
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77 | |
78 | echo t > /proc/sysrq-trigger | |
79 | ||
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80 | What are the 'command' keys? |
81 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
1da177e4 | 82 | |
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83 | =========== =================================================================== |
84 | Command Function | |
85 | =========== =================================================================== | |
86 | ``b`` Will immediately reboot the system without syncing or unmounting | |
87 | your disks. | |
86b1ae38 | 88 | |
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89 | ``c`` Will perform a system crash by a NULL pointer dereference. |
90 | A crashdump will be taken if configured. | |
d346cce3 | 91 | |
c8956bb7 | 92 | ``d`` Shows all locks that are held. |
1da177e4 | 93 | |
c8956bb7 | 94 | ``e`` Send a SIGTERM to all processes, except for init. |
1da177e4 | 95 | |
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96 | ``f`` Will call the oom killer to kill a memory hog process, but do not |
97 | panic if nothing can be killed. | |
1da177e4 | 98 | |
c8956bb7 | 99 | ``g`` Used by kgdb (kernel debugger) |
1da177e4 | 100 | |
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101 | ``h`` Will display help (actually any other key than those listed |
102 | here will display help. but ``h`` is easy to remember :-) | |
78831ba6 | 103 | |
c8956bb7 | 104 | ``i`` Send a SIGKILL to all processes, except for init. |
c2d75438 | 105 | |
c8956bb7 | 106 | ``j`` Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl. |
1da177e4 | 107 | |
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108 | ``k`` Secure Access Key (SAK) Kills all programs on the current virtual |
109 | console. NOTE: See important comments below in SAK section. | |
5045bcae | 110 | |
c8956bb7 | 111 | ``l`` Shows a stack backtrace for all active CPUs. |
1da177e4 | 112 | |
c8956bb7 | 113 | ``m`` Will dump current memory info to your console. |
d346cce3 | 114 | |
c8956bb7 | 115 | ``n`` Used to make RT tasks nice-able |
78831ba6 | 116 | |
c8956bb7 | 117 | ``o`` Will shut your system off (if configured and supported). |
78831ba6 | 118 | |
c8956bb7 | 119 | ``p`` Will dump the current registers and flags to your console. |
acf11fae | 120 | |
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121 | ``q`` Will dump per CPU lists of all armed hrtimers (but NOT regular |
122 | timer_list timers) and detailed information about all | |
123 | clockevent devices. | |
78831ba6 | 124 | |
c8956bb7 | 125 | ``r`` Turns off keyboard raw mode and sets it to XLATE. |
78831ba6 | 126 | |
c8956bb7 | 127 | ``s`` Will attempt to sync all mounted filesystems. |
78831ba6 | 128 | |
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129 | ``t`` Will dump a list of current tasks and their information to your |
130 | console. | |
78831ba6 | 131 | |
c8956bb7 | 132 | ``u`` Will attempt to remount all mounted filesystems read-only. |
1da177e4 | 133 | |
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134 | ``v`` Forcefully restores framebuffer console |
135 | ``v`` Causes ETM buffer dump [ARM-specific] | |
d346cce3 | 136 | |
c8956bb7 | 137 | ``w`` Dumps tasks that are in uninterruptable (blocked) state. |
d346cce3 | 138 | |
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139 | ``x`` Used by xmon interface on ppc/powerpc platforms. |
140 | Show global PMU Registers on sparc64. | |
141 | Dump all TLB entries on MIPS. | |
003bb8ab | 142 | |
c8956bb7 | 143 | ``y`` Show global CPU Registers [SPARC-64 specific] |
3871f2ff | 144 | |
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145 | ``z`` Dump the ftrace buffer |
146 | ||
147 | ``0``-``9`` Sets the console log level, controlling which kernel messages | |
148 | will be printed to your console. (``0``, for example would make | |
149 | it so that only emergency messages like PANICs or OOPSes would | |
150 | make it to your console.) | |
151 | =========== =================================================================== | |
152 | ||
153 | Okay, so what can I use them for? | |
154 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
1da177e4 | 155 | |
e2a8b0a7 | 156 | Well, unraw(r) is very handy when your X server or a svgalib program crashes. |
1da177e4 | 157 | |
e2a8b0a7 | 158 | sak(k) (Secure Access Key) is useful when you want to be sure there is no |
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159 | trojan program running at console which could grab your password |
160 | when you would try to login. It will kill all programs on given console, | |
161 | thus letting you make sure that the login prompt you see is actually | |
1da177e4 | 162 | the one from init, not some trojan program. |
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163 | |
164 | .. important:: | |
165 | ||
166 | In its true form it is not a true SAK like the one in a | |
167 | c2 compliant system, and it should not be mistaken as | |
168 | such. | |
169 | ||
170 | It seems others find it useful as (System Attention Key) which is | |
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171 | useful when you want to exit a program that will not let you switch consoles. |
172 | (For example, X or a svgalib program.) | |
173 | ||
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174 | ``reboot(b)`` is good when you're unable to shut down. But you should also |
175 | ``sync(s)`` and ``umount(u)`` first. | |
1da177e4 | 176 | |
c8956bb7 | 177 | ``crash(c)`` can be used to manually trigger a crashdump when the system is hung. |
cab8bd34 | 178 | Note that this just triggers a crash if there is no dump mechanism available. |
86b1ae38 | 179 | |
c8956bb7 | 180 | ``sync(s)`` is great when your system is locked up, it allows you to sync your |
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181 | disks and will certainly lessen the chance of data loss and fscking. Note |
182 | that the sync hasn't taken place until you see the "OK" and "Done" appear | |
183 | on the screen. (If the kernel is really in strife, you may not ever get the | |
184 | OK or Done message...) | |
185 | ||
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186 | ``umount(u)`` is basically useful in the same ways as ``sync(s)``. I generally |
187 | ``sync(s)``, ``umount(u)``, then ``reboot(b)`` when my system locks. It's saved | |
188 | me many a fsck. Again, the unmount (remount read-only) hasn't taken place until | |
189 | you see the "OK" and "Done" message appear on the screen. | |
1da177e4 | 190 | |
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191 | The loglevels ``0``-``9`` are useful when your console is being flooded with |
192 | kernel messages you do not want to see. Selecting ``0`` will prevent all but | |
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193 | the most urgent kernel messages from reaching your console. (They will |
194 | still be logged if syslogd/klogd are alive, though.) | |
195 | ||
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196 | ``term(e)`` and ``kill(i)`` are useful if you have some sort of runaway process |
197 | you are unable to kill any other way, especially if it's spawning other | |
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198 | processes. |
199 | ||
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200 | "just thaw ``it(j)``" is useful if your system becomes unresponsive due to a |
201 | frozen (probably root) filesystem via the FIFREEZE ioctl. | |
202 | ||
203 | Sometimes SysRq seems to get 'stuck' after using it, what can I do? | |
204 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
c2d75438 | 205 | |
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206 | That happens to me, also. I've found that tapping shift, alt, and control |
207 | on both sides of the keyboard, and hitting an invalid sysrq sequence again | |
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208 | will fix the problem. (i.e., something like :kbd:`alt-sysrq-z`). Switching to |
209 | another virtual console (:kbd:`ALT+Fn`) and then back again should also help. | |
210 | ||
211 | I hit SysRq, but nothing seems to happen, what's wrong? | |
212 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
1da177e4 | 213 | |
a2056ffd | 214 | There are some keyboards that produce a different keycode for SysRq than the |
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215 | pre-defined value of 99 |
216 | (see ``KEY_SYSRQ`` in ``include/uapi/linux/input-event-codes.h``), or | |
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217 | which don't have a SysRq key at all. In these cases, run ``showkey -s`` to find |
218 | an appropriate scancode sequence, and use ``setkeycodes <sequence> 99`` to map | |
219 | this sequence to the usual SysRq code (e.g., ``setkeycodes e05b 99``). It's | |
a2056ffd | 220 | probably best to put this command in a boot script. Oh, and by the way, you |
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221 | exit ``showkey`` by not typing anything for ten seconds. |
222 | ||
223 | I want to add SysRQ key events to a module, how does it work? | |
224 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
1da177e4 | 225 | |
1da177e4 | 226 | In order to register a basic function with the table, you must first include |
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227 | the header ``include/linux/sysrq.h``, this will define everything else you need. |
228 | Next, you must create a ``sysrq_key_op`` struct, and populate it with A) the key | |
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229 | handler function you will use, B) a help_msg string, that will print when SysRQ |
230 | prints help, and C) an action_msg string, that will print right before your | |
338cec32 | 231 | handler is called. Your handler must conform to the prototype in 'sysrq.h'. |
1da177e4 | 232 | |
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233 | After the ``sysrq_key_op`` is created, you can call the kernel function |
234 | ``register_sysrq_key(int key, struct sysrq_key_op *op_p);`` this will | |
235 | register the operation pointed to by ``op_p`` at table key 'key', | |
d346cce3 | 236 | if that slot in the table is blank. At module unload time, you must call |
c8956bb7 | 237 | the function ``unregister_sysrq_key(int key, struct sysrq_key_op *op_p)``, which |
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238 | will remove the key op pointed to by 'op_p' from the key 'key', if and only if |
239 | it is currently registered in that slot. This is in case the slot has been | |
240 | overwritten since you registered it. | |
241 | ||
242 | The Magic SysRQ system works by registering key operations against a key op | |
2fd872bd | 243 | lookup table, which is defined in 'drivers/tty/sysrq.c'. This key table has |
1da177e4 | 244 | a number of operations registered into it at compile time, but is mutable, |
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245 | and 2 functions are exported for interface to it:: |
246 | ||
d346cce3 | 247 | register_sysrq_key and unregister_sysrq_key. |
c8956bb7 | 248 | |
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249 | Of course, never ever leave an invalid pointer in the table. I.e., when |
250 | your module that called register_sysrq_key() exits, it must call | |
251 | unregister_sysrq_key() to clean up the sysrq key table entry that it used. | |
252 | Null pointers in the table are always safe. :) | |
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253 | |
254 | If for some reason you feel the need to call the handle_sysrq function from | |
255 | within a function called by handle_sysrq, you must be aware that you are in | |
256 | a lock (you are also in an interrupt handler, which means don't sleep!), so | |
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257 | you must call ``__handle_sysrq_nolock`` instead. |
258 | ||
259 | When I hit a SysRq key combination only the header appears on the console? | |
260 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
1da177e4 | 261 | |
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262 | Sysrq output is subject to the same console loglevel control as all |
263 | other console output. This means that if the kernel was booted 'quiet' | |
264 | as is common on distro kernels the output may not appear on the actual | |
265 | console, even though it will appear in the dmesg buffer, and be accessible | |
c8956bb7 | 266 | via the dmesg command and to the consumers of ``/proc/kmsg``. As a specific |
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267 | exception the header line from the sysrq command is passed to all console |
268 | consumers as if the current loglevel was maximum. If only the header | |
269 | is emitted it is almost certain that the kernel loglevel is too low. | |
270 | Should you require the output on the console channel then you will need | |
c8956bb7 | 271 | to temporarily up the console loglevel using :kbd:`alt-sysrq-8` or:: |
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272 | |
273 | echo 8 > /proc/sysrq-trigger | |
274 | ||
275 | Remember to return the loglevel to normal after triggering the sysrq | |
276 | command you are interested in. | |
277 | ||
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278 | I have more questions, who can I ask? |
279 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
280 | ||
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281 | Just ask them on the linux-kernel mailing list: |
282 | linux-kernel@vger.kernel.org | |
1da177e4 | 283 | |
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284 | Credits |
285 | ~~~~~~~ | |
286 | ||
5e03e2c4 | 287 | Written by Mydraal <vulpyne@vulpyne.net> |
1da177e4 LT |
288 | Updated by Adam Sulmicki <adam@cfar.umd.edu> |
289 | Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59 | |
290 | Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com> |