Based on kernel version 4.3. Page generated on 2015-11-02 12:51 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. The default value in this file is set by the 15 CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE config symbol, which itself defaults 16 to 1. Here is the list of possible values in /proc/sys/kernel/sysrq: 17 0 - disable sysrq completely 18 1 - enable all functions of sysrq 19 >1 - bitmask of allowed sysrq functions (see below for detailed function 20 description): 21 2 = 0x2 - enable control of console logging level 22 4 = 0x4 - enable control of keyboard (SAK, unraw) 23 8 = 0x8 - enable debugging dumps of processes etc. 24 16 = 0x10 - enable sync command 25 32 = 0x20 - enable remount read-only 26 64 = 0x40 - enable signalling of processes (term, kill, oom-kill) 27 128 = 0x80 - allow reboot/poweroff 28 256 = 0x100 - allow nicing of all RT tasks 29 30 You can set the value in the file by the following command: 31 echo "number" >/proc/sys/kernel/sysrq 32 33 The number may be written here either as decimal or as hexadecimal 34 with the 0x prefix. CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE must always be 35 written in hexadecimal. 36 37 Note that the value of /proc/sys/kernel/sysrq influences only the invocation 38 via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always 39 allowed (by a user with admin privileges). 40 41 * How do I use the magic SysRq key? 42 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 43 On x86 - You press the key combo 'ALT-SysRq-<command key>'. Note - Some 44 keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is 45 also known as the 'Print Screen' key. Also some keyboards cannot 46 handle so many keys being pressed at the same time, so you might 47 have better luck with "press Alt", "press SysRq", "release SysRq", 48 "press <command key>", release everything. 49 50 On SPARC - You press 'ALT-STOP-<command key>', I believe. 51 52 On the serial console (PC style standard serial ports only) - 53 You send a BREAK, then within 5 seconds a command key. Sending 54 BREAK twice is interpreted as a normal BREAK. 55 56 On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>, 57 Print Screen (or F13) - <command key> may suffice. 58 59 On other - If you know of the key combos for other architectures, please 60 let me know so I can add them to this section. 61 62 On all - write a character to /proc/sysrq-trigger. e.g.: 63 64 echo t > /proc/sysrq-trigger 65 66 * What are the 'command' keys? 67 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 68 'b' - Will immediately reboot the system without syncing or unmounting 69 your disks. 70 71 'c' - Will perform a system crash by a NULL pointer dereference. 72 A crashdump will be taken if configured. 73 74 'd' - Shows all locks that are held. 75 76 'e' - Send a SIGTERM to all processes, except for init. 77 78 'f' - Will call the oom killer to kill a memory hog process, but do not 79 panic if nothing can be killed. 80 81 'g' - Used by kgdb (kernel debugger) 82 83 'h' - Will display help (actually any other key than those listed 84 here will display help. but 'h' is easy to remember :-) 85 86 'i' - Send a SIGKILL to all processes, except for init. 87 88 'j' - Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl. 89 90 'k' - Secure Access Key (SAK) Kills all programs on the current virtual 91 console. NOTE: See important comments below in SAK section. 92 93 'l' - Shows a stack backtrace for all active CPUs. 94 95 'm' - Will dump current memory info to your console. 96 97 'n' - Used to make RT tasks nice-able 98 99 'o' - Will shut your system off (if configured and supported). 100 101 'p' - Will dump the current registers and flags to your console. 102 103 'q' - Will dump per CPU lists of all armed hrtimers (but NOT regular 104 timer_list timers) and detailed information about all 105 clockevent devices. 106 107 'r' - Turns off keyboard raw mode and sets it to XLATE. 108 109 's' - Will attempt to sync all mounted filesystems. 110 111 't' - Will dump a list of current tasks and their information to your 112 console. 113 114 'u' - Will attempt to remount all mounted filesystems read-only. 115 116 'v' - Forcefully restores framebuffer console 117 'v' - Causes ETM buffer dump [ARM-specific] 118 119 'w' - Dumps tasks that are in uninterruptable (blocked) state. 120 121 'x' - Used by xmon interface on ppc/powerpc platforms. 122 Show global PMU Registers on sparc64. 123 Dump all TLB entries on MIPS. 124 125 'y' - Show global CPU Registers [SPARC-64 specific] 126 127 'z' - Dump the ftrace buffer 128 129 '0'-'9' - Sets the console log level, controlling which kernel messages 130 will be printed to your console. ('0', for example would make 131 it so that only emergency messages like PANICs or OOPSes would 132 make it to your console.) 133 134 * Okay, so what can I use them for? 135 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 136 Well, unraw(r) is very handy when your X server or a svgalib program crashes. 137 138 sak(k) (Secure Access Key) is useful when you want to be sure there is no 139 trojan program running at console which could grab your password 140 when you would try to login. It will kill all programs on given console, 141 thus letting you make sure that the login prompt you see is actually 142 the one from init, not some trojan program. 143 IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT 144 IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT 145 IMPORTANT: such. :IMPORTANT 146 It seems others find it useful as (System Attention Key) which is 147 useful when you want to exit a program that will not let you switch consoles. 148 (For example, X or a svgalib program.) 149 150 reboot(b) is good when you're unable to shut down. But you should also 151 sync(s) and umount(u) first. 152 153 crash(c) can be used to manually trigger a crashdump when the system is hung. 154 Note that this just triggers a crash if there is no dump mechanism available. 155 156 sync(s) is great when your system is locked up, it allows you to sync your 157 disks and will certainly lessen the chance of data loss and fscking. Note 158 that the sync hasn't taken place until you see the "OK" and "Done" appear 159 on the screen. (If the kernel is really in strife, you may not ever get the 160 OK or Done message...) 161 162 umount(u) is basically useful in the same ways as sync(s). I generally sync(s), 163 umount(u), then reboot(b) when my system locks. It's saved me many a fsck. 164 Again, the unmount (remount read-only) hasn't taken place until you see the 165 "OK" and "Done" message appear on the screen. 166 167 The loglevels '0'-'9' are useful when your console is being flooded with 168 kernel messages you do not want to see. Selecting '0' will prevent all but 169 the most urgent kernel messages from reaching your console. (They will 170 still be logged if syslogd/klogd are alive, though.) 171 172 term(e) and kill(i) are useful if you have some sort of runaway process you 173 are unable to kill any other way, especially if it's spawning other 174 processes. 175 176 "just thaw it(j)" is useful if your system becomes unresponsive due to a frozen 177 (probably root) filesystem via the FIFREEZE ioctl. 178 179 * Sometimes SysRq seems to get 'stuck' after using it, what can I do? 180 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 181 That happens to me, also. I've found that tapping shift, alt, and control 182 on both sides of the keyboard, and hitting an invalid sysrq sequence again 183 will fix the problem. (i.e., something like alt-sysrq-z). Switching to another 184 virtual console (ALT+Fn) and then back again should also help. 185 186 * I hit SysRq, but nothing seems to happen, what's wrong? 187 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 188 There are some keyboards that produce a different keycode for SysRq than the 189 pre-defined value of 99 (see KEY_SYSRQ in include/linux/input.h), or which 190 don't have a SysRq key at all. In these cases, run 'showkey -s' to find an 191 appropriate scancode sequence, and use 'setkeycodes <sequence> 99' to map 192 this sequence to the usual SysRq code (e.g., 'setkeycodes e05b 99'). It's 193 probably best to put this command in a boot script. Oh, and by the way, you 194 exit 'showkey' by not typing anything for ten seconds. 195 196 * I want to add SysRQ key events to a module, how does it work? 197 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 198 In order to register a basic function with the table, you must first include 199 the header 'include/linux/sysrq.h', this will define everything else you need. 200 Next, you must create a sysrq_key_op struct, and populate it with A) the key 201 handler function you will use, B) a help_msg string, that will print when SysRQ 202 prints help, and C) an action_msg string, that will print right before your 203 handler is called. Your handler must conform to the prototype in 'sysrq.h'. 204 205 After the sysrq_key_op is created, you can call the kernel function 206 register_sysrq_key(int key, struct sysrq_key_op *op_p); this will 207 register the operation pointed to by 'op_p' at table key 'key', 208 if that slot in the table is blank. At module unload time, you must call 209 the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which 210 will remove the key op pointed to by 'op_p' from the key 'key', if and only if 211 it is currently registered in that slot. This is in case the slot has been 212 overwritten since you registered it. 213 214 The Magic SysRQ system works by registering key operations against a key op 215 lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has 216 a number of operations registered into it at compile time, but is mutable, 217 and 2 functions are exported for interface to it: 218 register_sysrq_key and unregister_sysrq_key. 219 Of course, never ever leave an invalid pointer in the table. I.e., when 220 your module that called register_sysrq_key() exits, it must call 221 unregister_sysrq_key() to clean up the sysrq key table entry that it used. 222 Null pointers in the table are always safe. :) 223 224 If for some reason you feel the need to call the handle_sysrq function from 225 within a function called by handle_sysrq, you must be aware that you are in 226 a lock (you are also in an interrupt handler, which means don't sleep!), so 227 you must call __handle_sysrq_nolock instead. 228 229 * When I hit a SysRq key combination only the header appears on the console? 230 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 231 Sysrq output is subject to the same console loglevel control as all 232 other console output. This means that if the kernel was booted 'quiet' 233 as is common on distro kernels the output may not appear on the actual 234 console, even though it will appear in the dmesg buffer, and be accessible 235 via the dmesg command and to the consumers of /proc/kmsg. As a specific 236 exception the header line from the sysrq command is passed to all console 237 consumers as if the current loglevel was maximum. If only the header 238 is emitted it is almost certain that the kernel loglevel is too low. 239 Should you require the output on the console channel then you will need 240 to temporarily up the console loglevel using alt-sysrq-8 or: 241 242 echo 8 > /proc/sysrq-trigger 243 244 Remember to return the loglevel to normal after triggering the sysrq 245 command you are interested in. 246 247 * I have more questions, who can I ask? 248 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 249 Just ask them on the linux-kernel mailing list: 250 email@example.com 251 252 * Credits 253 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 254 Written by Mydraal <firstname.lastname@example.org> 255 Updated by Adam Sulmicki <email@example.com> 256 Updated by Jeremy M. Dolan <firstname.lastname@example.org> 2001/01/28 10:15:59 257 Added to by Crutcher Dunnavant <email@example.com>