Based on kernel version 3.12. Page generated on 2013-11-13 22:00 EST.
1 Linux Magic System Request Key Hacks 2 Documentation for sysrq.c 3 4 * What is the magic SysRq key? 5 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 6 It is a 'magical' key combo you can hit which the kernel will respond to 7 regardless of whatever else it is doing, unless it is completely locked up. 8 9 * How do I enable the magic SysRq key? 10 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 11 You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when 12 configuring the kernel. When running a kernel with SysRq compiled in, 13 /proc/sys/kernel/sysrq controls the functions allowed to be invoked via 14 the SysRq key. By default the file contains 1 which means that every 15 possible SysRq request is allowed (in older versions SysRq was disabled 16 by default, and you were required to specifically enable it at run-time 17 but this is not the case any more). Here is the list of possible values 18 in /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 32 You can set the value in the file by the following command: 33 echo "number" >/proc/sys/kernel/sysrq 34 35 Note that the value of /proc/sys/kernel/sysrq influences only the invocation 36 via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always 37 allowed (by a user with admin privileges). 38 39 * How do I use the magic SysRq key? 40 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 41 On 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 48 On SPARC - You press 'ALT-STOP-<command key>', I believe. 49 50 On 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 54 On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>, 55 Print Screen (or F13) - <command key> may suffice. 56 57 On 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 60 On 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 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 132 Well, unraw(r) is very handy when your X server or a svgalib program crashes. 133 134 sak(k) (Secure Access Key) is useful when you want to be sure there is no 135 trojan program running at console which could grab your password 136 when you would try to login. It will kill all programs on given console, 137 thus letting you make sure that the login prompt you see is actually 138 the one from init, not some trojan program. 139 IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT 140 IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT 141 IMPORTANT: such. :IMPORTANT 142 It seems others find it useful as (System Attention Key) which is 143 useful when you want to exit a program that will not let you switch consoles. 144 (For example, X or a svgalib program.) 145 146 reboot(b) is good when you're unable to shut down. But you should also 147 sync(s) and umount(u) first. 148 149 crash(c) can be used to manually trigger a crashdump when the system is hung. 150 Note that this just triggers a crash if there is no dump mechanism available. 151 152 sync(s) is great when your system is locked up, it allows you to sync your 153 disks and will certainly lessen the chance of data loss and fscking. Note 154 that the sync hasn't taken place until you see the "OK" and "Done" appear 155 on the screen. (If the kernel is really in strife, you may not ever get the 156 OK or Done message...) 157 158 umount(u) is basically useful in the same ways as sync(s). I generally sync(s), 159 umount(u), then reboot(b) when my system locks. It's saved me many a fsck. 160 Again, the unmount (remount read-only) hasn't taken place until you see the 161 "OK" and "Done" message appear on the screen. 162 163 The loglevels '0'-'9' are useful when your console is being flooded with 164 kernel messages you do not want to see. Selecting '0' will prevent all but 165 the most urgent kernel messages from reaching your console. (They will 166 still be logged if syslogd/klogd are alive, though.) 167 168 term(e) and kill(i) are useful if you have some sort of runaway process you 169 are unable to kill any other way, especially if it's spawning other 170 processes. 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 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 177 That happens to me, also. I've found that tapping shift, alt, and control 178 on both sides of the keyboard, and hitting an invalid sysrq sequence again 179 will fix the problem. (i.e., something like alt-sysrq-z). Switching to another 180 virtual console (ALT+Fn) and then back again should also help. 181 182 * I hit SysRq, but nothing seems to happen, what's wrong? 183 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 184 There are some keyboards that produce a different keycode for SysRq than the 185 pre-defined value of 99 (see KEY_SYSRQ in include/linux/input.h), or which 186 don't have a SysRq key at all. In these cases, run 'showkey -s' to find an 187 appropriate scancode sequence, and use 'setkeycodes <sequence> 99' to map 188 this sequence to the usual SysRq code (e.g., 'setkeycodes e05b 99'). It's 189 probably best to put this command in a boot script. Oh, and by the way, you 190 exit '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 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 194 In order to register a basic function with the table, you must first include 195 the header 'include/linux/sysrq.h', this will define everything else you need. 196 Next, you must create a sysrq_key_op struct, and populate it with A) the key 197 handler function you will use, B) a help_msg string, that will print when SysRQ 198 prints help, and C) an action_msg string, that will print right before your 199 handler is called. Your handler must conform to the prototype in 'sysrq.h'. 200 201 After the sysrq_key_op is created, you can call the kernel function 202 register_sysrq_key(int key, struct sysrq_key_op *op_p); this will 203 register the operation pointed to by 'op_p' at table key 'key', 204 if that slot in the table is blank. At module unload time, you must call 205 the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which 206 will remove the key op pointed to by 'op_p' from the key 'key', if and only if 207 it is currently registered in that slot. This is in case the slot has been 208 overwritten since you registered it. 209 210 The Magic SysRQ system works by registering key operations against a key op 211 lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has 212 a number of operations registered into it at compile time, but is mutable, 213 and 2 functions are exported for interface to it: 214 register_sysrq_key and unregister_sysrq_key. 215 Of course, never ever leave an invalid pointer in the table. I.e., when 216 your module that called register_sysrq_key() exits, it must call 217 unregister_sysrq_key() to clean up the sysrq key table entry that it used. 218 Null pointers in the table are always safe. :) 219 220 If for some reason you feel the need to call the handle_sysrq function from 221 within a function called by handle_sysrq, you must be aware that you are in 222 a lock (you are also in an interrupt handler, which means don't sleep!), so 223 you must call __handle_sysrq_nolock instead. 224 225 * When I hit a SysRq key combination only the header appears on the console? 226 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 227 Sysrq output is subject to the same console loglevel control as all 228 other console output. This means that if the kernel was booted 'quiet' 229 as is common on distro kernels the output may not appear on the actual 230 console, even though it will appear in the dmesg buffer, and be accessible 231 via the dmesg command and to the consumers of /proc/kmsg. As a specific 232 exception the header line from the sysrq command is passed to all console 233 consumers as if the current loglevel was maximum. If only the header 234 is emitted it is almost certain that the kernel loglevel is too low. 235 Should you require the output on the console channel then you will need 236 to temporarily up the console loglevel using alt-sysrq-8 or: 237 238 echo 8 > /proc/sysrq-trigger 239 240 Remember to return the loglevel to normal after triggering the sysrq 241 command you are interested in. 242 243 * I have more questions, who can I ask? 244 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 245 Just ask them on the linux-kernel mailing list: 246 firstname.lastname@example.org 247 248 * Credits 249 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 250 Written by Mydraal <email@example.com> 251 Updated by Adam Sulmicki <firstname.lastname@example.org> 252 Updated by Jeremy M. Dolan <email@example.com> 2001/01/28 10:15:59 253 Added to by Crutcher Dunnavant <firstname.lastname@example.org>