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. 119 120 'y' - Show global CPU Registers [SPARC-64 specific] 121 122 'z' - Dump the ftrace buffer 123 124 '0'-'9' - Sets the console log level, controlling which kernel messages 125 will be printed to your console. ('0', for example would make 126 it so that only emergency messages like PANICs or OOPSes would 127 make it to your console.) 128 129 * Okay, so what can I use them for? 130 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 131 Well, un'R'aw is very handy when your X server or a svgalib program crashes. 132 133 sa'K' (Secure Access Key) is useful when you want to be sure there is no 134 trojan program running at console which could grab your password 135 when you would try to login. It will kill all programs on given console, 136 thus letting you make sure that the login prompt you see is actually 137 the one from init, not some trojan program. 138 IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT 139 IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT 140 IMPORTANT: such. :IMPORTANT 141 It seems others find it useful as (System Attention Key) which is 142 useful when you want to exit a program that will not let you switch consoles. 143 (For example, X or a svgalib program.) 144 145 re'B'oot is good when you're unable to shut down. But you should also 'S'ync 146 and 'U'mount first. 147 148 'C'rash can be used to manually trigger a crashdump when the system is hung. 149 Note that this just triggers a crash if there is no dump mechanism available. 150 151 'S'ync is great when your system is locked up, it allows you to sync your 152 disks and will certainly lessen the chance of data loss and fscking. Note 153 that the sync hasn't taken place until you see the "OK" and "Done" appear 154 on the screen. (If the kernel is really in strife, you may not ever get the 155 OK or Done message...) 156 157 'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync, 158 'U'mount, then re'B'oot when my system locks. It's saved me many a fsck. 159 Again, the unmount (remount read-only) hasn't taken place until you see the 160 "OK" and "Done" message appear on the screen. 161 162 The loglevels '0'-'9' are useful when your console is being flooded with 163 kernel messages you do not want to see. Selecting '0' will prevent all but 164 the most urgent kernel messages from reaching your console. (They will 165 still be logged if syslogd/klogd are alive, though.) 166 167 t'E'rm and k'I'll are useful if you have some sort of runaway process you 168 are unable to kill any other way, especially if it's spawning other 169 processes. 170 171 "'J'ust thaw it" is useful if your system becomes unresponsive due to a frozen 172 (probably root) filesystem via the FIFREEZE ioctl. 173 174 * Sometimes SysRq seems to get 'stuck' after using it, what can I do? 175 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 176 That happens to me, also. I've found that tapping shift, alt, and control 177 on both sides of the keyboard, and hitting an invalid sysrq sequence again 178 will fix the problem. (i.e., something like alt-sysrq-z). Switching to another 179 virtual console (ALT+Fn) and then back again should also help. 180 181 * I hit SysRq, but nothing seems to happen, what's wrong? 182 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 183 There are some keyboards that produce a different keycode for SysRq than the 184 pre-defined value of 99 (see KEY_SYSRQ in include/linux/input.h), or which 185 don't have a SysRq key at all. In these cases, run 'showkey -s' to find an 186 appropriate scancode sequence, and use 'setkeycodes <sequence> 99' to map 187 this sequence to the usual SysRq code (e.g., 'setkeycodes e05b 99'). It's 188 probably best to put this command in a boot script. Oh, and by the way, you 189 exit 'showkey' by not typing anything for ten seconds. 190 191 * I want to add SysRQ key events to a module, how does it work? 192 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 193 In order to register a basic function with the table, you must first include 194 the header 'include/linux/sysrq.h', this will define everything else you need. 195 Next, you must create a sysrq_key_op struct, and populate it with A) the key 196 handler function you will use, B) a help_msg string, that will print when SysRQ 197 prints help, and C) an action_msg string, that will print right before your 198 handler is called. Your handler must conform to the prototype in 'sysrq.h'. 199 200 After the sysrq_key_op is created, you can call the kernel function 201 register_sysrq_key(int key, struct sysrq_key_op *op_p); this will 202 register the operation pointed to by 'op_p' at table key 'key', 203 if that slot in the table is blank. At module unload time, you must call 204 the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which 205 will remove the key op pointed to by 'op_p' from the key 'key', if and only if 206 it is currently registered in that slot. This is in case the slot has been 207 overwritten since you registered it. 208 209 The Magic SysRQ system works by registering key operations against a key op 210 lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has 211 a number of operations registered into it at compile time, but is mutable, 212 and 2 functions are exported for interface to it: 213 register_sysrq_key and unregister_sysrq_key. 214 Of course, never ever leave an invalid pointer in the table. I.e., when 215 your module that called register_sysrq_key() exits, it must call 216 unregister_sysrq_key() to clean up the sysrq key table entry that it used. 217 Null pointers in the table are always safe. :) 218 219 If for some reason you feel the need to call the handle_sysrq function from 220 within a function called by handle_sysrq, you must be aware that you are in 221 a lock (you are also in an interrupt handler, which means don't sleep!), so 222 you must call __handle_sysrq_nolock instead. 223 224 * When I hit a SysRq key combination only the header appears on the console? 225 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 226 Sysrq output is subject to the same console loglevel control as all 227 other console output. This means that if the kernel was booted 'quiet' 228 as is common on distro kernels the output may not appear on the actual 229 console, even though it will appear in the dmesg buffer, and be accessible 230 via the dmesg command and to the consumers of /proc/kmsg. As a specific 231 exception the header line from the sysrq command is passed to all console 232 consumers as if the current loglevel was maximum. If only the header 233 is emitted it is almost certain that the kernel loglevel is too low. 234 Should you require the output on the console channel then you will need 235 to temporarily up the console loglevel using alt-sysrq-8 or: 236 237 echo 8 > /proc/sysrq-trigger 238 239 Remember to return the loglevel to normal after triggering the sysrq 240 command you are interested in. 241 242 * I have more questions, who can I ask? 243 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 244 Just ask them on the linux-kernel mailing list: 245 email@example.com 246 247 * Credits 248 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 249 Written by Mydraal <firstname.lastname@example.org> 250 Updated by Adam Sulmicki <email@example.com> 251 Updated by Jeremy M. Dolan <firstname.lastname@example.org> 2001/01/28 10:15:59 252 Added to by Crutcher Dunnavant <email@example.com>
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