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