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 - You press the key combo :kbd:`ALT-SysRq-<command key>`. 52 53.. note:: 54 Some 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 58 have better luck with press :kbd:`Alt`, press :kbd:`SysRq`, 59 release :kbd:`SysRq`, press :kbd:`<command key>`, release everything. 60 61On SPARC - You press :kbd:`ALT-STOP-<command key>`, I believe. 62 63On 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. 66 67On 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. 70 71On 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. 74 75On all 76 write a character to /proc/sysrq-trigger. e.g.:: 77 78 echo t > /proc/sysrq-trigger 79 80What are the 'command' keys? 81~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 82 83=========== =================================================================== 84Command Function 85=========== =================================================================== 86``b`` Will immediately reboot the system without syncing or unmounting 87 your disks. 88 89``c`` Will perform a system crash by a NULL pointer dereference. 90 A crashdump will be taken if configured. 91 92``d`` Shows all locks that are held. 93 94``e`` Send a SIGTERM to all processes, except for init. 95 96``f`` Will call the oom killer to kill a memory hog process, but do not 97 panic if nothing can be killed. 98 99``g`` Used by kgdb (kernel debugger) 100 101``h`` Will display help (actually any other key than those listed 102 here will display help. but ``h`` is easy to remember :-) 103 104``i`` Send a SIGKILL to all processes, except for init. 105 106``j`` Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl. 107 108``k`` Secure Access Key (SAK) Kills all programs on the current virtual 109 console. NOTE: See important comments below in SAK section. 110 111``l`` Shows a stack backtrace for all active CPUs. 112 113``m`` Will dump current memory info to your console. 114 115``n`` Used to make RT tasks nice-able 116 117``o`` Will shut your system off (if configured and supported). 118 119``p`` Will dump the current registers and flags to your console. 120 121``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. 124 125``r`` Turns off keyboard raw mode and sets it to XLATE. 126 127``s`` Will attempt to sync all mounted filesystems. 128 129``t`` Will dump a list of current tasks and their information to your 130 console. 131 132``u`` Will attempt to remount all mounted filesystems read-only. 133 134``v`` Forcefully restores framebuffer console 135``v`` Causes ETM buffer dump [ARM-specific] 136 137``w`` Dumps tasks that are in uninterruptable (blocked) state. 138 139``x`` Used by xmon interface on ppc/powerpc platforms. 140 Show global PMU Registers on sparc64. 141 Dump all TLB entries on MIPS. 142 143``y`` Show global CPU Registers [SPARC-64 specific] 144 145``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 153Okay, so what can I use them for? 154~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 155 156Well, unraw(r) is very handy when your X server or a svgalib program crashes. 157 158sak(k) (Secure Access Key) is useful when you want to be sure there is no 159trojan program running at console which could grab your password 160when you would try to login. It will kill all programs on given console, 161thus letting you make sure that the login prompt you see is actually 162the one from init, not some trojan program. 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 170It seems others find it useful as (System Attention Key) which is 171useful when you want to exit a program that will not let you switch consoles. 172(For example, X or a svgalib program.) 173 174``reboot(b)`` is good when you're unable to shut down, it is an equivalent 175of pressing the "reset" button. 176 177``crash(c)`` can be used to manually trigger a crashdump when the system is hung. 178Note that this just triggers a crash if there is no dump mechanism available. 179 180``sync(s)`` is handy before yanking removable medium or after using a rescue 181shell that provides no graceful shutdown -- it will ensure your data is 182safely written to the disk. Note that the sync hasn't taken place until you see 183the "OK" and "Done" appear on the screen. 184 185``umount(u)`` can be used to mark filesystems as properly unmounted. From the 186running system's point of view, they will be remounted read-only. The remount 187isn't complete until you see the "OK" and "Done" message appear on the screen. 188 189The loglevels ``0``-``9`` are useful when your console is being flooded with 190kernel messages you do not want to see. Selecting ``0`` will prevent all but 191the most urgent kernel messages from reaching your console. (They will 192still be logged if syslogd/klogd are alive, though.) 193 194``term(e)`` and ``kill(i)`` are useful if you have some sort of runaway process 195you are unable to kill any other way, especially if it's spawning other 196processes. 197 198"just thaw ``it(j)``" is useful if your system becomes unresponsive due to a 199frozen (probably root) filesystem via the FIFREEZE ioctl. 200 201Sometimes SysRq seems to get 'stuck' after using it, what can I do? 202~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 203 204That happens to me, also. I've found that tapping shift, alt, and control 205on both sides of the keyboard, and hitting an invalid sysrq sequence again 206will fix the problem. (i.e., something like :kbd:`alt-sysrq-z`). Switching to 207another virtual console (:kbd:`ALT+Fn`) and then back again should also help. 208 209I hit SysRq, but nothing seems to happen, what's wrong? 210~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 211 212There are some keyboards that produce a different keycode for SysRq than the 213pre-defined value of 99 214(see ``KEY_SYSRQ`` in ``include/uapi/linux/input-event-codes.h``), or 215which don't have a SysRq key at all. In these cases, run ``showkey -s`` to find 216an appropriate scancode sequence, and use ``setkeycodes <sequence> 99`` to map 217this sequence to the usual SysRq code (e.g., ``setkeycodes e05b 99``). It's 218probably best to put this command in a boot script. Oh, and by the way, you 219exit ``showkey`` by not typing anything for ten seconds. 220 221I want to add SysRQ key events to a module, how does it work? 222~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 223 224In order to register a basic function with the table, you must first include 225the header ``include/linux/sysrq.h``, this will define everything else you need. 226Next, you must create a ``sysrq_key_op`` struct, and populate it with A) the key 227handler function you will use, B) a help_msg string, that will print when SysRQ 228prints help, and C) an action_msg string, that will print right before your 229handler is called. Your handler must conform to the prototype in 'sysrq.h'. 230 231After the ``sysrq_key_op`` is created, you can call the kernel function 232``register_sysrq_key(int key, struct sysrq_key_op *op_p);`` this will 233register the operation pointed to by ``op_p`` at table key 'key', 234if that slot in the table is blank. At module unload time, you must call 235the function ``unregister_sysrq_key(int key, struct sysrq_key_op *op_p)``, which 236will remove the key op pointed to by 'op_p' from the key 'key', if and only if 237it is currently registered in that slot. This is in case the slot has been 238overwritten since you registered it. 239 240The Magic SysRQ system works by registering key operations against a key op 241lookup table, which is defined in 'drivers/tty/sysrq.c'. This key table has 242a number of operations registered into it at compile time, but is mutable, 243and 2 functions are exported for interface to it:: 244 245 register_sysrq_key and unregister_sysrq_key. 246 247Of course, never ever leave an invalid pointer in the table. I.e., when 248your module that called register_sysrq_key() exits, it must call 249unregister_sysrq_key() to clean up the sysrq key table entry that it used. 250Null pointers in the table are always safe. :) 251 252If for some reason you feel the need to call the handle_sysrq function from 253within a function called by handle_sysrq, you must be aware that you are in 254a lock (you are also in an interrupt handler, which means don't sleep!), so 255you must call ``__handle_sysrq_nolock`` instead. 256 257When I hit a SysRq key combination only the header appears on the console? 258~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 259 260Sysrq output is subject to the same console loglevel control as all 261other console output. This means that if the kernel was booted 'quiet' 262as is common on distro kernels the output may not appear on the actual 263console, even though it will appear in the dmesg buffer, and be accessible 264via the dmesg command and to the consumers of ``/proc/kmsg``. As a specific 265exception the header line from the sysrq command is passed to all console 266consumers as if the current loglevel was maximum. If only the header 267is emitted it is almost certain that the kernel loglevel is too low. 268Should you require the output on the console channel then you will need 269to temporarily up the console loglevel using :kbd:`alt-sysrq-8` or:: 270 271 echo 8 > /proc/sysrq-trigger 272 273Remember to return the loglevel to normal after triggering the sysrq 274command you are interested in. 275 276I have more questions, who can I ask? 277~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 278 279Just ask them on the linux-kernel mailing list: 280 linux-kernel@vger.kernel.org 281 282Credits 283~~~~~~~ 284 285Written by Mydraal <vulpyne@vulpyne.net> 286Updated by Adam Sulmicki <adam@cfar.umd.edu> 287Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59 288Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com> 289