Based on kernel version 2.6.26. Page generated on 2008-07-16 21:13 EST.
1 Linux Magic System Request Key Hacks 2 Documentation for sysrq.c 3 Last update: 2007-AUG-04 4 5 * What is the magic SysRq key? 6 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 7 It is a 'magical' key combo you can hit which the kernel will respond to 8 regardless of whatever else it is doing, unless it is completely locked up. 9 10 * How do I enable the magic SysRq key? 11 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 12 You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when 13 configuring the kernel. When running a kernel with SysRq compiled in, 14 /proc/sys/kernel/sysrq controls the functions allowed to be invoked via 15 the SysRq key. By default the file contains 1 which means that every 16 possible SysRq request is allowed (in older versions SysRq was disabled 17 by default, and you were required to specifically enable it at run-time 18 but this is not the case any more). Here is the list of possible values 19 in /proc/sys/kernel/sysrq: 20 0 - disable sysrq completely 21 1 - enable all functions of sysrq 22 >1 - bitmask of allowed sysrq functions (see below for detailed function 23 description): 24 2 - enable control of console logging level 25 4 - enable control of keyboard (SAK, unraw) 26 8 - enable debugging dumps of processes etc. 27 16 - enable sync command 28 32 - enable remount read-only 29 64 - enable signalling of processes (term, kill, oom-kill) 30 128 - allow reboot/poweroff 31 256 - allow nicing of all RT tasks 32 33 You can set the value in the file by the following command: 34 echo "number" >/proc/sys/kernel/sysrq 35 36 Note that the value of /proc/sys/kernel/sysrq influences only the invocation 37 via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always 38 allowed (by a user with admin privileges). 39 40 * How do I use the magic SysRq key? 41 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 42 On x86 - You press the key combo 'ALT-SysRq-<command key>'. Note - Some 43 keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is 44 also known as the 'Print Screen' key. Also some keyboards cannot 45 handle so many keys being pressed at the same time, so you might 46 have better luck with "press Alt", "press SysRq", "release SysRq", 47 "press <command key>", release everything. 48 49 On SPARC - You press 'ALT-STOP-<command key>', I believe. 50 51 On the serial console (PC style standard serial ports only) - 52 You send a BREAK, then within 5 seconds a command key. Sending 53 BREAK twice is interpreted as a normal BREAK. 54 55 On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>, 56 Print Screen (or F13) - <command key> may suffice. 57 58 On other - If you know of the key combos for other architectures, please 59 let me know so I can add them to this section. 60 61 On all - write a character to /proc/sysrq-trigger. e.g.: 62 63 echo t > /proc/sysrq-trigger 64 65 * What are the 'command' keys? 66 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 67 'b' - Will immediately reboot the system without syncing or unmounting 68 your disks. 69 70 'c' - Will perform a kexec reboot in order to take a crashdump. 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 on ppc and sh platforms. 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 'k' - Secure Access Key (SAK) Kills all programs on the current virtual 86 console. NOTE: See important comments below in SAK section. 87 88 'm' - Will dump current memory info to your console. 89 90 'n' - Used to make RT tasks nice-able 91 92 'o' - Will shut your system off (if configured and supported). 93 94 'p' - Will dump the current registers and flags to your console. 95 96 'q' - Will dump a list of all running timers. 97 98 'r' - Turns off keyboard raw mode and sets it to XLATE. 99 100 's' - Will attempt to sync all mounted filesystems. 101 102 't' - Will dump a list of current tasks and their information to your 103 console. 104 105 'u' - Will attempt to remount all mounted filesystems read-only. 106 107 'v' - Dumps Voyager SMP processor info to your console. 108 109 'w' - Dumps tasks that are in uninterruptable (blocked) state. 110 111 'x' - Used by xmon interface on ppc/powerpc platforms. 112 113 '0'-'9' - Sets the console log level, controlling which kernel messages 114 will be printed to your console. ('0', for example would make 115 it so that only emergency messages like PANICs or OOPSes would 116 make it to your console.) 117 118 * Okay, so what can I use them for? 119 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 120 Well, un'R'aw is very handy when your X server or a svgalib program crashes. 121 122 sa'K' (Secure Access Key) is useful when you want to be sure there is no 123 trojan program running at console which could grab your password 124 when you would try to login. It will kill all programs on given console, 125 thus letting you make sure that the login prompt you see is actually 126 the one from init, not some trojan program. 127 IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT 128 IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT 129 IMPORTANT: such. :IMPORTANT 130 It seems others find it useful as (System Attention Key) which is 131 useful when you want to exit a program that will not let you switch consoles. 132 (For example, X or a svgalib program.) 133 134 re'B'oot is good when you're unable to shut down. But you should also 'S'ync 135 and 'U'mount first. 136 137 'C'rashdump can be used to manually trigger a crashdump when the system is hung. 138 The kernel needs to have been built with CONFIG_KEXEC enabled. 139 140 'S'ync is great when your system is locked up, it allows you to sync your 141 disks and will certainly lessen the chance of data loss and fscking. Note 142 that the sync hasn't taken place until you see the "OK" and "Done" appear 143 on the screen. (If the kernel is really in strife, you may not ever get the 144 OK or Done message...) 145 146 'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync, 147 'U'mount, then re'B'oot when my system locks. It's saved me many a fsck. 148 Again, the unmount (remount read-only) hasn't taken place until you see the 149 "OK" and "Done" message appear on the screen. 150 151 The loglevels '0'-'9' are useful when your console is being flooded with 152 kernel messages you do not want to see. Selecting '0' will prevent all but 153 the most urgent kernel messages from reaching your console. (They will 154 still be logged if syslogd/klogd are alive, though.) 155 156 t'E'rm and k'I'll are useful if you have some sort of runaway process you 157 are unable to kill any other way, especially if it's spawning other 158 processes. 159 160 * Sometimes SysRq seems to get 'stuck' after using it, what can I do? 161 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 162 That happens to me, also. I've found that tapping shift, alt, and control 163 on both sides of the keyboard, and hitting an invalid sysrq sequence again 164 will fix the problem. (i.e., something like alt-sysrq-z). Switching to another 165 virtual console (ALT+Fn) and then back again should also help. 166 167 * I hit SysRq, but nothing seems to happen, what's wrong? 168 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 169 There are some keyboards that send different scancodes for SysRq than the 170 pre-defined 0x54. So if SysRq doesn't work out of the box for a certain 171 keyboard, run 'showkey -s' to find out the proper scancode sequence. Then 172 use 'setkeycodes <sequence> 84' to define this sequence to the usual SysRq 173 code (84 is decimal for 0x54). It's probably best to put this command in a 174 boot script. Oh, and by the way, you exit 'showkey' by not typing anything 175 for ten seconds. 176 177 * I want to add SysRQ key events to a module, how does it work? 178 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 179 In order to register a basic function with the table, you must first include 180 the header 'include/linux/sysrq.h', this will define everything else you need. 181 Next, you must create a sysrq_key_op struct, and populate it with A) the key 182 handler function you will use, B) a help_msg string, that will print when SysRQ 183 prints help, and C) an action_msg string, that will print right before your 184 handler is called. Your handler must conform to the prototype in 'sysrq.h'. 185 186 After the sysrq_key_op is created, you can call the kernel function 187 register_sysrq_key(int key, struct sysrq_key_op *op_p); this will 188 register the operation pointed to by 'op_p' at table key 'key', 189 if that slot in the table is blank. At module unload time, you must call 190 the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which 191 will remove the key op pointed to by 'op_p' from the key 'key', if and only if 192 it is currently registered in that slot. This is in case the slot has been 193 overwritten since you registered it. 194 195 The Magic SysRQ system works by registering key operations against a key op 196 lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has 197 a number of operations registered into it at compile time, but is mutable, 198 and 2 functions are exported for interface to it: 199 register_sysrq_key and unregister_sysrq_key. 200 Of course, never ever leave an invalid pointer in the table. I.e., when 201 your module that called register_sysrq_key() exits, it must call 202 unregister_sysrq_key() to clean up the sysrq key table entry that it used. 203 Null pointers in the table are always safe. :) 204 205 If for some reason you feel the need to call the handle_sysrq function from 206 within a function called by handle_sysrq, you must be aware that you are in 207 a lock (you are also in an interrupt handler, which means don't sleep!), so 208 you must call __handle_sysrq_nolock instead. 209 210 * I have more questions, who can I ask? 211 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 212 And I'll answer any questions about the registration system you got, also 213 responding as soon as possible. 214 -Crutcher 215 216 * Credits 217 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 218 Written by Mydraal <vulpyne[AT]vulpyne[DOT]net> 219 Updated by Adam Sulmicki <adam[AT]cfar.umd[DOT]edu> 220 Updated by Jeremy M. Dolan <jmd[AT]turbogeek[DOT]org> 2001/01/28 10:15:59 221 Added to by Crutcher Dunnavant <crutcher+kernel[AT]datastacks[DOT]com>
