Based on kernel version 2.6.30. Page generated on 2009-06-11 10:13 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 kexec reboot in order to take a crashdump. 70 71 'd' - Shows all locks that are held. 72 73 'e' - Send a SIGTERM to all processes, except for init. 74 75 'f' - Will call oom_kill to kill a memory hog process. 76 77 'g' - Used by kgdb on ppc and sh platforms. 78 79 'h' - Will display help (actually any other key than those listed 80 here will display help. but 'h' is easy to remember :-) 81 82 'i' - Send a SIGKILL to all processes, except for init. 83 84 'j' - Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl. 85 86 'k' - Secure Access Key (SAK) Kills all programs on the current virtual 87 console. NOTE: See important comments below in SAK section. 88 89 'l' - Shows a stack backtrace for all active CPUs. 90 91 'm' - Will dump current memory info to your console. 92 93 'n' - Used to make RT tasks nice-able 94 95 'o' - Will shut your system off (if configured and supported). 96 97 'p' - Will dump the current registers and flags to your console. 98 99 'q' - Will dump per CPU lists of all armed hrtimers (but NOT regular 100 timer_list timers) and detailed information about all 101 clockevent devices. 102 103 'r' - Turns off keyboard raw mode and sets it to XLATE. 104 105 's' - Will attempt to sync all mounted filesystems. 106 107 't' - Will dump a list of current tasks and their information to your 108 console. 109 110 'u' - Will attempt to remount all mounted filesystems read-only. 111 112 'v' - Dumps Voyager SMP processor info to your console. 113 114 'w' - Dumps tasks that are in uninterruptable (blocked) state. 115 116 'x' - Used by xmon interface on ppc/powerpc platforms. 117 118 'z' - Dump the ftrace buffer 119 120 '0'-'9' - Sets the console log level, controlling which kernel messages 121 will be printed to your console. ('0', for example would make 122 it so that only emergency messages like PANICs or OOPSes would 123 make it to your console.) 124 125 * Okay, so what can I use them for? 126 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 127 Well, un'R'aw is very handy when your X server or a svgalib program crashes. 128 129 sa'K' (Secure Access Key) is useful when you want to be sure there is no 130 trojan program running at console which could grab your password 131 when you would try to login. It will kill all programs on given console, 132 thus letting you make sure that the login prompt you see is actually 133 the one from init, not some trojan program. 134 IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT 135 IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT 136 IMPORTANT: such. :IMPORTANT 137 It seems others find it useful as (System Attention Key) which is 138 useful when you want to exit a program that will not let you switch consoles. 139 (For example, X or a svgalib program.) 140 141 re'B'oot is good when you're unable to shut down. But you should also 'S'ync 142 and 'U'mount first. 143 144 'C'rashdump can be used to manually trigger a crashdump when the system is hung. 145 The kernel needs to have been built with CONFIG_KEXEC enabled. 146 147 'S'ync is great when your system is locked up, it allows you to sync your 148 disks and will certainly lessen the chance of data loss and fscking. Note 149 that the sync hasn't taken place until you see the "OK" and "Done" appear 150 on the screen. (If the kernel is really in strife, you may not ever get the 151 OK or Done message...) 152 153 'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync, 154 'U'mount, then re'B'oot when my system locks. It's saved me many a fsck. 155 Again, the unmount (remount read-only) hasn't taken place until you see the 156 "OK" and "Done" message appear on the screen. 157 158 The loglevels '0'-'9' are useful when your console is being flooded with 159 kernel messages you do not want to see. Selecting '0' will prevent all but 160 the most urgent kernel messages from reaching your console. (They will 161 still be logged if syslogd/klogd are alive, though.) 162 163 t'E'rm and k'I'll are useful if you have some sort of runaway process you 164 are unable to kill any other way, especially if it's spawning other 165 processes. 166 167 "'J'ust thaw it" is useful if your system becomes unresponsive due to a frozen 168 (probably root) filesystem via the FIFREEZE ioctl. 169 170 * Sometimes SysRq seems to get 'stuck' after using it, what can I do? 171 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 172 That happens to me, also. I've found that tapping shift, alt, and control 173 on both sides of the keyboard, and hitting an invalid sysrq sequence again 174 will fix the problem. (i.e., something like alt-sysrq-z). Switching to another 175 virtual console (ALT+Fn) and then back again should also help. 176 177 * I hit SysRq, but nothing seems to happen, what's wrong? 178 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 179 There are some keyboards that send different scancodes for SysRq than the 180 pre-defined 0x54. So if SysRq doesn't work out of the box for a certain 181 keyboard, run 'showkey -s' to find out the proper scancode sequence. Then 182 use 'setkeycodes <sequence> 84' to define this sequence to the usual SysRq 183 code (84 is decimal for 0x54). It's probably best to put this command in a 184 boot script. Oh, and by the way, you exit 'showkey' by not typing anything 185 for ten seconds. 186 187 * I want to add SysRQ key events to a module, how does it work? 188 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 189 In order to register a basic function with the table, you must first include 190 the header 'include/linux/sysrq.h', this will define everything else you need. 191 Next, you must create a sysrq_key_op struct, and populate it with A) the key 192 handler function you will use, B) a help_msg string, that will print when SysRQ 193 prints help, and C) an action_msg string, that will print right before your 194 handler is called. Your handler must conform to the prototype in 'sysrq.h'. 195 196 After the sysrq_key_op is created, you can call the kernel function 197 register_sysrq_key(int key, struct sysrq_key_op *op_p); this will 198 register the operation pointed to by 'op_p' at table key 'key', 199 if that slot in the table is blank. At module unload time, you must call 200 the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which 201 will remove the key op pointed to by 'op_p' from the key 'key', if and only if 202 it is currently registered in that slot. This is in case the slot has been 203 overwritten since you registered it. 204 205 The Magic SysRQ system works by registering key operations against a key op 206 lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has 207 a number of operations registered into it at compile time, but is mutable, 208 and 2 functions are exported for interface to it: 209 register_sysrq_key and unregister_sysrq_key. 210 Of course, never ever leave an invalid pointer in the table. I.e., when 211 your module that called register_sysrq_key() exits, it must call 212 unregister_sysrq_key() to clean up the sysrq key table entry that it used. 213 Null pointers in the table are always safe. :) 214 215 If for some reason you feel the need to call the handle_sysrq function from 216 within a function called by handle_sysrq, you must be aware that you are in 217 a lock (you are also in an interrupt handler, which means don't sleep!), so 218 you must call __handle_sysrq_nolock instead. 219 220 * When I hit a SysRq key combination only the header appears on the console? 221 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 222 Sysrq output is subject to the same console loglevel control as all 223 other console output. This means that if the kernel was booted 'quiet' 224 as is common on distro kernels the output may not appear on the actual 225 console, even though it will appear in the dmesg buffer, and be accessible 226 via the dmesg command and to the consumers of /proc/kmsg. As a specific 227 exception the header line from the sysrq command is passed to all console 228 consumers as if the current loglevel was maximum. If only the header 229 is emitted it is almost certain that the kernel loglevel is too low. 230 Should you require the output on the console channel then you will need 231 to temporarily up the console loglevel using alt-sysrq-8 or: 232 233 echo 8 > /proc/sysrq-trigger 234 235 Remember to return the loglevel to normal after triggering the sysrq 236 command you are interested in. 237 238 * I have more questions, who can I ask? 239 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 240 And I'll answer any questions about the registration system you got, also 241 responding as soon as possible. 242 -Crutcher 243 244 * Credits 245 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 246 Written by Mydraal <vulpyne[AT]vulpyne[DOT]net> 247 Updated by Adam Sulmicki <adam[AT]cfar.umd[DOT]edu> 248 Updated by Jeremy M. Dolan <jmd[AT]turbogeek[DOT]org> 2001/01/28 10:15:59 249 Added to by Crutcher Dunnavant <crutcher+kernel[AT]datastacks[DOT]com>
