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 the kernel will respond to 10 regardless of whatever else it is doing, unless it is completely locked up. 11 12 How do I enable the magic SysRq key? 13 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 14 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 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: 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 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 35 36 You can set the value in the file by the following command:: 37 38 echo "number" >/proc/sys/kernel/sysrq 39 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. 43 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). 47 48 How do I use the magic SysRq key? 49 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 50 51 On x86 52 You press the key combo :kbd:`ALT-SysRq-<command key>`. 53 54 .. note:: 55 Some 56 keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is 57 also known as the 'Print Screen' key. Also some keyboards cannot 58 handle so many keys being pressed at the same time, so you might 59 have better luck with press :kbd:`Alt`, press :kbd:`SysRq`, 60 release :kbd:`SysRq`, press :kbd:`<command key>`, release everything. 61 62 On SPARC 63 You press :kbd:`ALT-STOP-<command key>`, I believe. 64 65 On the serial console (PC style standard serial ports only) 66 You send a ``BREAK``, then within 5 seconds a command key. Sending 67 ``BREAK`` twice is interpreted as a normal BREAK. 68 69 On PowerPC 70 Press :kbd:`ALT - Print Screen` (or :kbd:`F13`) - :kbd:`<command key>`. 71 :kbd:`Print Screen` (or :kbd:`F13`) - :kbd:`<command key>` may suffice. 72 73 On other 74 If you know of the key combos for other architectures, please 75 submit a patch to be included in this section. 76 77 On all 78 Write a single character to /proc/sysrq-trigger. 79 Only the first character is processed, the rest of the string is 80 ignored. However, it is not recommended to write any extra characters 81 as the behavior is undefined and might change in the future versions. 82 E.g.:: 83 84 echo t > /proc/sysrq-trigger 85 86 Alternatively, write multiple characters prepended by underscore. 87 This way, all characters will be processed. E.g.:: 88 89 echo _reisub > /proc/sysrq-trigger 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 without syncing or unmounting 100 your disks. 101 102 ``c`` Will perform a system crash and a crashdump will be taken 103 if configured. 104 105 ``d`` Shows all locks that are held. 106 107 ``e`` Send a SIGTERM to all processes, except for init. 108 109 ``f`` Will call the oom killer to kill a memory hog process, but do not 110 panic if nothing can be killed. 111 112 ``g`` Used by kgdb (kernel debugger) 113 114 ``h`` Will display help (actually any other key than those listed 115 here will display help. but ``h`` is easy to remember :-) 116 117 ``i`` Send a SIGKILL to all processes, except for init. 118 119 ``j`` Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl. 120 121 ``k`` Secure Access Key (SAK) Kills all programs on the current virtual 122 console. NOTE: See important comments below in SAK section. 123 124 ``l`` Shows a stack backtrace for all active CPUs. 125 126 ``m`` Will dump current memory info to your console. 127 128 ``n`` Used to make RT tasks nice-able 129 130 ``o`` Will shut your system off (if configured and supported). 131 132 ``p`` Will dump the current registers and flags to your console. 133 134 ``q`` Will dump per CPU lists of all armed hrtimers (but NOT regular 135 timer_list timers) and detailed information about all 136 clockevent devices. 137 138 ``r`` Turns off keyboard raw mode and sets it to XLATE. 139 140 ``s`` Will attempt to sync all mounted filesystems. 141 142 ``t`` Will dump a list of current tasks and their information to your 143 console. 144 145 ``u`` Will attempt to remount all mounted filesystems read-only. 146 147 ``v`` Forcefully restores framebuffer console 148 ``v`` Causes ETM buffer dump [ARM-specific] 149 150 ``w`` Dumps tasks that are in uninterruptible (blocked) state. 151 152 ``x`` Used by xmon interface on ppc/powerpc platforms. 153 Show global PMU Registers on sparc64. 154 Dump all TLB entries on MIPS. 155 156 ``y`` Show global CPU Registers [SPARC-64 specific] 157 158 ``z`` Dump the ftrace buffer 159 160 ``0``-``9`` Sets the console log level, controlling which kernel messages 161 will be printed to your console. (``0``, for example would make 162 it so that only emergency messages like PANICs or OOPSes would 163 make it to your console.) 164 165 ``R`` Replay the kernel log messages on consoles. 166 =========== =================================================================== 167 168 Okay, so what can I use them for? 169 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 170 171 Well, unraw(r) is very handy when your X server or a svgalib program crashes. 172 173 sak(k) (Secure Access Key) is useful when you want to be sure there is no 174 trojan program running at console which could grab your password 175 when you would try to login. It will kill all programs on given console, 176 thus letting you make sure that the login prompt you see is actually 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 the one in a 182 c2 compliant system, and it should not be mistaken as 183 such. 184 185 It seems others find it useful as (System Attention Key) which is 186 useful when you want to exit a program that will not let you switch consoles. 187 (For example, X or a svgalib program.) 188 189 ``reboot(b)`` is good when you're unable to shut down, it is an equivalent 190 of pressing the "reset" button. 191 192 ``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 is no dump mechanism available. 194 195 ``sync(s)`` is handy before yanking removable medium or after using a rescue 196 shell that provides no graceful shutdown -- it will ensure your data is 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. 199 200 ``umount(u)`` can be used to mark filesystems as properly unmounted. From the 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 "Done" message appear on the screen. 203 204 The loglevels ``0``-``9`` are useful when your console is being flooded with 205 kernel messages you do not want to see. Selecting ``0`` will prevent all but 206 the most urgent kernel messages from reaching your console. (They will 207 still be logged if syslogd/klogd are alive, though.) 208 209 ``term(e)`` and ``kill(i)`` are useful if you have some sort of runaway process 210 you are unable to kill any other way, especially if it's spawning other 211 processes. 212 213 "just thaw ``it(j)``" is useful if your system becomes unresponsive due to a 214 frozen (probably root) filesystem via the FIFREEZE ioctl. 215 216 ``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 view the messages in printk buffer. 218 User may have to press the key combination multiple times if console system is 219 busy. If it is completely locked up, then messages won't be printed. Output 220 messages depend on current console loglevel, which can be modified using 221 sysrq[0-9] (see above). 222 223 Sometimes SysRq seems to get 'stuck' after using it, what can I do? 224 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 225 226 When this happens, try tapping shift, alt and control on both sides of the 227 keyboard, and hitting an invalid sysrq sequence again. (i.e., something like 228 :kbd:`alt-sysrq-z`). 229 230 Switching to another virtual console (:kbd:`ALT+Fn`) and then back again 231 should also help. 232 233 I hit SysRq, but nothing seems to happen, what's wrong? 234 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 235 236 There are some keyboards that produce a different keycode for SysRq than the 237 pre-defined value of 99 238 (see ``KEY_SYSRQ`` in ``include/uapi/linux/input-event-codes.h``), or 239 which don't have a SysRq key at all. In these cases, run ``showkey -s`` to find 240 an appropriate scancode sequence, and use ``setkeycodes <sequence> 99`` to map 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 script. Oh, and by the way, you 243 exit ``showkey`` by not typing anything for ten seconds. 244 245 I want to add SysRQ key events to a module, how does it work? 246 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 247 248 In order to register a basic function with the table, you must first include 249 the header ``include/linux/sysrq.h``, this will define everything else you need. 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 string, that will print when SysRQ 252 prints help, and C) an action_msg string, that will print right before your 253 handler is called. Your handler must conform to the prototype in 'sysrq.h'. 254 255 After the ``sysrq_key_op`` is created, you can call the kernel function 256 ``register_sysrq_key(int key, const struct sysrq_key_op *op_p);`` this will 257 register the operation pointed to by ``op_p`` at table key 'key', 258 if that slot in the table is blank. At module unload time, you must call 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_p' from the key 'key', if and 261 only if it is currently registered in that slot. This is in case the slot has 262 been overwritten since you registered it. 263 264 The Magic SysRQ system works by registering key operations against a key op 265 lookup table, which is defined in 'drivers/tty/sysrq.c'. This key table has 266 a number of operations registered into it at compile time, but is mutable, 267 and 2 functions are exported for interface to it:: 268 269 register_sysrq_key and unregister_sysrq_key. 270 271 Of course, never ever leave an invalid pointer in the table. I.e., when 272 your module that called register_sysrq_key() exits, it must call 273 unregister_sysrq_key() to clean up the sysrq key table entry that it used. 274 Null pointers in the table are always safe. :) 275 276 If for some reason you feel the need to call the handle_sysrq function from 277 within a function called by handle_sysrq, you must be aware that you are in 278 a lock (you are also in an interrupt handler, which means don't sleep!), so 279 you must call ``__handle_sysrq_nolock`` instead. 280 281 When I hit a SysRq key combination only the header appears on the console? 282 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 283 284 Sysrq output is subject to the same console loglevel control as all 285 other console output. This means that if the kernel was booted 'quiet' 286 as is common on distro kernels the output may not appear on the actual 287 console, even though it will appear in the dmesg buffer, and be accessible 288 via the dmesg command and to the consumers of ``/proc/kmsg``. As a specific 289 exception the header line from the sysrq command is passed to all console 290 consumers as if the current loglevel was maximum. If only the header 291 is emitted it is almost certain that the kernel loglevel is too low. 292 Should you require the output on the console channel then you will need 293 to temporarily up the console loglevel using :kbd:`alt-sysrq-8` or:: 294 295 echo 8 > /proc/sysrq-trigger 296 297 Remember to return the loglevel to normal after triggering the sysrq 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> 2001/01/28 10:15:59 312 - Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>
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