1Linux Magic System Request Key Hacks 2==================================== 3 4Documentation for sysrq.c 5 6What is the magic SysRq key? 7~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 8 9It is a 'magical' key combo you can hit which the kernel will respond to 10regardless of whatever else it is doing, unless it is completely locked up. 11 12How do I enable the magic SysRq key? 13~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 14 15You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when 16configuring the kernel. When running a kernel with SysRq compiled in, 17/proc/sys/kernel/sysrq controls the functions allowed to be invoked via 18the SysRq key. The default value in this file is set by the 19CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE config symbol, which itself defaults 20to 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 36You can set the value in the file by the following command:: 37 38 echo "number" >/proc/sys/kernel/sysrq 39 40The number may be written here either as decimal or as hexadecimal 41with the 0x prefix. CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE must always be 42written in hexadecimal. 43 44Note that the value of ``/proc/sys/kernel/sysrq`` influences only the invocation 45via a keyboard. Invocation of any operation via ``/proc/sysrq-trigger`` is 46always allowed (by a user with admin privileges). 47 48How do I use the magic SysRq key? 49~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 50 51On 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 62On SPARC 63 You press :kbd:`ALT-STOP-<command key>`, I believe. 64 65On 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 69On 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 73On other 74 If you know of the key combos for other architectures, please 75 submit a patch to be included in this section. 76 77On 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 91The :kbd:`<command key>` is case sensitive. 92 93What are the 'command' keys? 94~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 95 96=========== =================================================================== 97Command 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 166Okay, so what can I use them for? 167~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 168 169Well, unraw(r) is very handy when your X server or a svgalib program crashes. 170 171sak(k) (Secure Access Key) is useful when you want to be sure there is no 172trojan program running at console which could grab your password 173when you would try to login. It will kill all programs on given console, 174thus letting you make sure that the login prompt you see is actually 175the one from init, not some trojan program. 176 177.. important:: 178 179 In its true form it is not a true SAK like the one in a 180 c2 compliant system, and it should not be mistaken as 181 such. 182 183It seems others find it useful as (System Attention Key) which is 184useful when you want to exit a program that will not let you switch consoles. 185(For example, X or a svgalib program.) 186 187``reboot(b)`` is good when you're unable to shut down, it is an equivalent 188of pressing the "reset" button. 189 190``crash(c)`` can be used to manually trigger a crashdump when the system is hung. 191Note that this just triggers a crash if there is no dump mechanism available. 192 193``sync(s)`` is handy before yanking removable medium or after using a rescue 194shell that provides no graceful shutdown -- it will ensure your data is 195safely written to the disk. Note that the sync hasn't taken place until you see 196the "OK" and "Done" appear on the screen. 197 198``umount(u)`` can be used to mark filesystems as properly unmounted. From the 199running system's point of view, they will be remounted read-only. The remount 200isn't complete until you see the "OK" and "Done" message appear on the screen. 201 202The loglevels ``0``-``9`` are useful when your console is being flooded with 203kernel messages you do not want to see. Selecting ``0`` will prevent all but 204the most urgent kernel messages from reaching your console. (They will 205still be logged if syslogd/klogd are alive, though.) 206 207``term(e)`` and ``kill(i)`` are useful if you have some sort of runaway process 208you are unable to kill any other way, especially if it's spawning other 209processes. 210 211"just thaw ``it(j)``" is useful if your system becomes unresponsive due to a 212frozen (probably root) filesystem via the FIFREEZE ioctl. 213 214Sometimes SysRq seems to get 'stuck' after using it, what can I do? 215~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 216 217When this happens, try tapping shift, alt and control on both sides of the 218keyboard, and hitting an invalid sysrq sequence again. (i.e., something like 219:kbd:`alt-sysrq-z`). 220 221Switching to another virtual console (:kbd:`ALT+Fn`) and then back again 222should also help. 223 224I hit SysRq, but nothing seems to happen, what's wrong? 225~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 226 227There are some keyboards that produce a different keycode for SysRq than the 228pre-defined value of 99 229(see ``KEY_SYSRQ`` in ``include/uapi/linux/input-event-codes.h``), or 230which don't have a SysRq key at all. In these cases, run ``showkey -s`` to find 231an appropriate scancode sequence, and use ``setkeycodes <sequence> 99`` to map 232this sequence to the usual SysRq code (e.g., ``setkeycodes e05b 99``). It's 233probably best to put this command in a boot script. Oh, and by the way, you 234exit ``showkey`` by not typing anything for ten seconds. 235 236I want to add SysRQ key events to a module, how does it work? 237~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 238 239In order to register a basic function with the table, you must first include 240the header ``include/linux/sysrq.h``, this will define everything else you need. 241Next, you must create a ``sysrq_key_op`` struct, and populate it with A) the key 242handler function you will use, B) a help_msg string, that will print when SysRQ 243prints help, and C) an action_msg string, that will print right before your 244handler is called. Your handler must conform to the prototype in 'sysrq.h'. 245 246After the ``sysrq_key_op`` is created, you can call the kernel function 247``register_sysrq_key(int key, const struct sysrq_key_op *op_p);`` this will 248register the operation pointed to by ``op_p`` at table key 'key', 249if that slot in the table is blank. At module unload time, you must call 250the function ``unregister_sysrq_key(int key, const struct sysrq_key_op *op_p)``, 251which will remove the key op pointed to by 'op_p' from the key 'key', if and 252only if it is currently registered in that slot. This is in case the slot has 253been overwritten since you registered it. 254 255The Magic SysRQ system works by registering key operations against a key op 256lookup table, which is defined in 'drivers/tty/sysrq.c'. This key table has 257a number of operations registered into it at compile time, but is mutable, 258and 2 functions are exported for interface to it:: 259 260 register_sysrq_key and unregister_sysrq_key. 261 262Of course, never ever leave an invalid pointer in the table. I.e., when 263your module that called register_sysrq_key() exits, it must call 264unregister_sysrq_key() to clean up the sysrq key table entry that it used. 265Null pointers in the table are always safe. :) 266 267If for some reason you feel the need to call the handle_sysrq function from 268within a function called by handle_sysrq, you must be aware that you are in 269a lock (you are also in an interrupt handler, which means don't sleep!), so 270you must call ``__handle_sysrq_nolock`` instead. 271 272When I hit a SysRq key combination only the header appears on the console? 273~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 274 275Sysrq output is subject to the same console loglevel control as all 276other console output. This means that if the kernel was booted 'quiet' 277as is common on distro kernels the output may not appear on the actual 278console, even though it will appear in the dmesg buffer, and be accessible 279via the dmesg command and to the consumers of ``/proc/kmsg``. As a specific 280exception the header line from the sysrq command is passed to all console 281consumers as if the current loglevel was maximum. If only the header 282is emitted it is almost certain that the kernel loglevel is too low. 283Should you require the output on the console channel then you will need 284to temporarily up the console loglevel using :kbd:`alt-sysrq-8` or:: 285 286 echo 8 > /proc/sysrq-trigger 287 288Remember to return the loglevel to normal after triggering the sysrq 289command you are interested in. 290 291I have more questions, who can I ask? 292~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 293 294Just ask them on the linux-kernel mailing list: 295 linux-kernel@vger.kernel.org 296 297Credits 298~~~~~~~ 299 300- Written by Mydraal <vulpyne@vulpyne.net> 301- Updated by Adam Sulmicki <adam@cfar.umd.edu> 302- Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59 303- Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com> 304