1Linux Magic System Request Key Hacks 2Documentation for sysrq.c version 1.15 3Last update: $Date: 2001/01/28 10:15:59 $ 4 5* What is the magic SysRq key? 6~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 7It is a 'magical' key combo you can hit which the kernel will respond to 8regardless of whatever else it is doing, unless it is completely locked up. 9 10* How do I enable the magic SysRq key? 11~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 12You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when 13configuring the kernel. When running on a kernel with SysRq compiled in, it 14may be DISABLED at run-time using following command: 15 16 echo "0" > /proc/sys/kernel/sysrq 17 18Note that previous versions disabled sysrq by default, and you were required 19to specifically enable it at run-time. That is not the case any longer. 20 21* How do I use the magic SysRq key? 22~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 23On x86 - You press the key combo 'ALT-SysRq-<command key>'. Note - Some 24 keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is 25 also known as the 'Print Screen' key. 26 27On SPARC - You press 'ALT-STOP-<command key>', I believe. 28 29On the serial console (PC style standard serial ports only) - 30 You send a BREAK, then within 5 seconds a command key. Sending 31 BREAK twice is interpreted as a normal BREAK. 32 33On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>, 34 Print Screen (or F13) - <command key> may suffice. 35 36On other - If you know of the key combos for other architectures, please 37 let me know so I can add them to this section. 38 39On all - write a character to /proc/sysrq-trigger. eg: 40 41 echo t > /proc/sysrq-trigger 42 43* What are the 'command' keys? 44~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 45'r' - Turns off keyboard raw mode and sets it to XLATE. 46 47'k' - Secure Access Key (SAK) Kills all programs on the current virtual 48 console. NOTE: See important comments below in SAK section. 49 50'b' - Will immediately reboot the system without syncing or unmounting 51 your disks. 52 53'o' - Will shut your system off (if configured and supported). 54 55's' - Will attempt to sync all mounted filesystems. 56 57'u' - Will attempt to remount all mounted filesystems read-only. 58 59'p' - Will dump the current registers and flags to your console. 60 61't' - Will dump a list of current tasks and their information to your 62 console. 63 64'm' - Will dump current memory info to your console. 65 66'0'-'9' - Sets the console log level, controlling which kernel messages 67 will be printed to your console. ('0', for example would make 68 it so that only emergency messages like PANICs or OOPSes would 69 make it to your console.) 70 71'e' - Send a SIGTERM to all processes, except for init. 72 73'i' - Send a SIGKILL to all processes, except for init. 74 75'l' - Send a SIGKILL to all processes, INCLUDING init. (Your system 76 will be non-functional after this.) 77 78'h' - Will display help ( actually any other key than those listed 79 above will display help. but 'h' is easy to remember :-) 80 81* Okay, so what can I use them for? 82~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 83Well, un'R'aw is very handy when your X server or a svgalib program crashes. 84 85sa'K' (Secure Access Key) is useful when you want to be sure there are no 86trojan program is running at console and which could grab your password 87when you would try to login. It will kill all programs on given console 88and thus letting you make sure that the login prompt you see is actually 89the one from init, not some trojan program. 90IMPORTANT:In its true form it is not a true SAK like the one in :IMPORTANT 91IMPORTANT:c2 compliant systems, and it should be mistook as such. :IMPORTANT 92 It seems other find it useful as (System Attention Key) which is 93useful when you want to exit a program that will not let you switch consoles. 94(For example, X or a svgalib program.) 95 96re'B'oot is good when you're unable to shut down. But you should also 'S'ync 97and 'U'mount first. 98 99'S'ync is great when your system is locked up, it allows you to sync your 100disks and will certainly lessen the chance of data loss and fscking. Note 101that the sync hasn't taken place until you see the "OK" and "Done" appear 102on the screen. (If the kernel is really in strife, you may not ever get the 103OK or Done message...) 104 105'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync, 106'U'mount, then re'B'oot when my system locks. It's saved me many a fsck. 107Again, the unmount (remount read-only) hasn't taken place until you see the 108"OK" and "Done" message appear on the screen. 109 110The loglevel'0'-'9' is useful when your console is being flooded with 111kernel messages you do not want to see. Setting '0' will prevent all but 112the most urgent kernel messages from reaching your console. (They will 113still be logged if syslogd/klogd are alive, though.) 114 115t'E'rm and k'I'll are useful if you have some sort of runaway process you 116are unable to kill any other way, especially if it's spawning other 117processes. 118 119* Sometimes SysRq seems to get 'stuck' after using it, what can I do? 120~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 121That happens to me, also. I've found that tapping shift, alt, and control 122on both sides of the keyboard, and hitting an invalid sysrq sequence again 123will fix the problem. (ie, something like alt-sysrq-z). Switching to another 124virtual console (ALT+Fn) and then back again should also help. 125 126* I hit SysRq, but nothing seems to happen, what's wrong? 127~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 128There are some keyboards that send different scancodes for SysRq than the 129pre-defined 0x54. So if SysRq doesn't work out of the box for a certain 130keyboard, run 'showkey -s' to find out the proper scancode sequence. Then 131use 'setkeycodes <sequence> 84' to define this sequence to the usual SysRq 132code (84 is decimal for 0x54). It's probably best to put this command in a 133boot script. Oh, and by the way, you exit 'showkey' by not typing anything 134for ten seconds. 135 136* I want to add SysRQ key events to a module, how does it work? 137~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 138In order to register a basic function with the table, you must first include 139the header 'include/linux/sysrq.h', this will define everything else you need. 140Next, you must create a sysrq_key_op struct, and populate it with A) the key 141handler function you will use, B) a help_msg string, that will print when SysRQ 142prints help, and C) an action_msg string, that will print right before your 143handler is called. Your handler must conform to the protoype in 'sysrq.h'. 144 145After the sysrq_key_op is created, you can call the macro 146register_sysrq_key(int key, struct sysrq_key_op *op_p) that is defined in 147sysrq.h, this will register the operation pointed to by 'op_p' at table 148key 'key', if that slot in the table is blank. At module unload time, you must 149call the macro unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which 150will remove the key op pointed to by 'op_p' from the key 'key', if and only if 151it is currently registered in that slot. This is in case the slot has been 152overwritten since you registered it. 153 154The Magic SysRQ system works by registering key operations against a key op 155lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has 156a number of operations registered into it at compile time, but is mutable, 157and 4 functions are exported for interface to it: __sysrq_lock_table, 158__sysrq_unlock_table, __sysrq_get_key_op, and __sysrq_put_key_op. The 159functions __sysrq_swap_key_ops and __sysrq_swap_key_ops_nolock are defined 160in the header itself, and the REGISTER and UNREGISTER macros are built from 161these. More complex (and dangerous!) manipulations of the table are possible 162using these functions, but you must be careful to always lock the table before 163you read or write from it, and to unlock it again when you are done. (And of 164course, to never ever leave an invalid pointer in the table). Null pointers in 165the table are always safe :) 166 167If for some reason you feel the need to call the handle_sysrq function from 168within a function called by handle_sysrq, you must be aware that you are in 169a lock (you are also in an interupt handler, which means don't sleep!), so 170you must call __handle_sysrq_nolock instead. 171 172* I have more questions, who can I ask? 173~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 174You may feel free to send email to myrdraal@deathsdoor.com, and I will 175respond as soon as possible. 176 -Myrdraal 177 178And I'll answer any questions about the registration system you got, also 179responding as soon as possible. 180 -Crutcher 181 182* Credits 183~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 184Written by Mydraal <myrdraal@deathsdoor.com> 185Updated by Adam Sulmicki <adam@cfar.umd.edu> 186Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59 187Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com> 188