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diff --git a/Documentation/sysrq.txt b/Documentation/sysrq.txt new file mode 100644 index 000000000000..f98c2e31c143 --- /dev/null +++ b/Documentation/sysrq.txt @@ -0,0 +1,213 @@ +Linux Magic System Request Key Hacks +Documentation for sysrq.c version 1.15 +Last update: $Date: 2001/01/28 10:15:59 $ + +* What is the magic SysRq key? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +It is a 'magical' key combo you can hit which the kernel will respond to +regardless of whatever else it is doing, unless it is completely locked up. + +* How do I enable the magic SysRq key? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when +configuring the kernel. When running a kernel with SysRq compiled in, +/proc/sys/kernel/sysrq controls the functions allowed to be invoked via +the SysRq key. By default the file contains 1 which means that every +possible SysRq request is allowed (in older versions SysRq was disabled +by default, and you were required to specifically enable it at run-time +but this is not the case any more). Here is the list of possible values +in /proc/sys/kernel/sysrq: + 0 - disable sysrq completely + 1 - enable all functions of sysrq + >1 - bitmask of allowed sysrq functions (see below for detailed function + description): + 2 - enable control of console logging level + 4 - enable control of keyboard (SAK, unraw) + 8 - enable debugging dumps of processes etc. + 16 - enable sync command + 32 - enable remount read-only + 64 - enable signalling of processes (term, kill, oom-kill) + 128 - allow reboot/poweroff + 256 - allow nicing of all RT tasks + +You can set the value in the file by the following command: + echo "number" >/proc/sys/kernel/sysrq + +Note that the value of /proc/sys/kernel/sysrq influences only the invocation +via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always +allowed. + +* How do I use the magic SysRq key? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +On x86 - You press the key combo 'ALT-SysRq-<command key>'. Note - Some + keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is + also known as the 'Print Screen' key. Also some keyboards cannot + handle so many keys being pressed at the same time, so you might + have better luck with "press Alt", "press SysRq", "release Alt", + "press <command key>", release everything. + +On SPARC - You press 'ALT-STOP-<command key>', I believe. + +On the serial console (PC style standard serial ports only) - + You send a BREAK, then within 5 seconds a command key. Sending + BREAK twice is interpreted as a normal BREAK. + +On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>, + Print Screen (or F13) - <command key> may suffice. + +On other - If you know of the key combos for other architectures, please + let me know so I can add them to this section. + +On all - write a character to /proc/sysrq-trigger. eg: + + echo t > /proc/sysrq-trigger + +* What are the 'command' keys? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +'r' - Turns off keyboard raw mode and sets it to XLATE. + +'k' - Secure Access Key (SAK) Kills all programs on the current virtual + console. NOTE: See important comments below in SAK section. + +'b' - Will immediately reboot the system without syncing or unmounting + your disks. + +'o' - Will shut your system off (if configured and supported). + +'s' - Will attempt to sync all mounted filesystems. + +'u' - Will attempt to remount all mounted filesystems read-only. + +'p' - Will dump the current registers and flags to your console. + +'t' - Will dump a list of current tasks and their information to your + console. + +'m' - Will dump current memory info to your console. + +'v' - Dumps Voyager SMP processor info to your console. + +'0'-'9' - Sets the console log level, controlling which kernel messages + will be printed to your console. ('0', for example would make + it so that only emergency messages like PANICs or OOPSes would + make it to your console.) + +'f' - Will call oom_kill to kill a memory hog process + +'e' - Send a SIGTERM to all processes, except for init. + +'i' - Send a SIGKILL to all processes, except for init. + +'l' - Send a SIGKILL to all processes, INCLUDING init. (Your system + will be non-functional after this.) + +'h' - Will display help ( actually any other key than those listed + above will display help. but 'h' is easy to remember :-) + +* Okay, so what can I use them for? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Well, un'R'aw is very handy when your X server or a svgalib program crashes. + +sa'K' (Secure Access Key) is useful when you want to be sure there are no +trojan program is running at console and which could grab your password +when you would try to login. It will kill all programs on given console +and thus letting you make sure that the login prompt you see is actually +the one from init, not some trojan program. +IMPORTANT:In its true form it is not a true SAK like the one in :IMPORTANT +IMPORTANT:c2 compliant systems, and it should be mistook as such. :IMPORTANT + It seems other find it useful as (System Attention Key) which is +useful when you want to exit a program that will not let you switch consoles. +(For example, X or a svgalib program.) + +re'B'oot is good when you're unable to shut down. But you should also 'S'ync +and 'U'mount first. + +'S'ync is great when your system is locked up, it allows you to sync your +disks and will certainly lessen the chance of data loss and fscking. Note +that the sync hasn't taken place until you see the "OK" and "Done" appear +on the screen. (If the kernel is really in strife, you may not ever get the +OK or Done message...) + +'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync, +'U'mount, then re'B'oot when my system locks. It's saved me many a fsck. +Again, the unmount (remount read-only) hasn't taken place until you see the +"OK" and "Done" message appear on the screen. + +The loglevel'0'-'9' is useful when your console is being flooded with +kernel messages you do not want to see. Setting '0' will prevent all but +the most urgent kernel messages from reaching your console. (They will +still be logged if syslogd/klogd are alive, though.) + +t'E'rm and k'I'll are useful if you have some sort of runaway process you +are unable to kill any other way, especially if it's spawning other +processes. + +* Sometimes SysRq seems to get 'stuck' after using it, what can I do? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +That happens to me, also. I've found that tapping shift, alt, and control +on both sides of the keyboard, and hitting an invalid sysrq sequence again +will fix the problem. (ie, something like alt-sysrq-z). Switching to another +virtual console (ALT+Fn) and then back again should also help. + +* I hit SysRq, but nothing seems to happen, what's wrong? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +There are some keyboards that send different scancodes for SysRq than the +pre-defined 0x54. So if SysRq doesn't work out of the box for a certain +keyboard, run 'showkey -s' to find out the proper scancode sequence. Then +use 'setkeycodes <sequence> 84' to define this sequence to the usual SysRq +code (84 is decimal for 0x54). It's probably best to put this command in a +boot script. Oh, and by the way, you exit 'showkey' by not typing anything +for ten seconds. + +* I want to add SysRQ key events to a module, how does it work? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +In order to register a basic function with the table, you must first include +the header 'include/linux/sysrq.h', this will define everything else you need. +Next, you must create a sysrq_key_op struct, and populate it with A) the key +handler function you will use, B) a help_msg string, that will print when SysRQ +prints help, and C) an action_msg string, that will print right before your +handler is called. Your handler must conform to the protoype in 'sysrq.h'. + +After the sysrq_key_op is created, you can call the macro +register_sysrq_key(int key, struct sysrq_key_op *op_p) that is defined in +sysrq.h, this will register the operation pointed to by 'op_p' at table +key 'key', if that slot in the table is blank. At module unload time, you must +call the macro unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which +will remove the key op pointed to by 'op_p' from the key 'key', if and only if +it is currently registered in that slot. This is in case the slot has been +overwritten since you registered it. + +The Magic SysRQ system works by registering key operations against a key op +lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has +a number of operations registered into it at compile time, but is mutable, +and 4 functions are exported for interface to it: __sysrq_lock_table, +__sysrq_unlock_table, __sysrq_get_key_op, and __sysrq_put_key_op. The +functions __sysrq_swap_key_ops and __sysrq_swap_key_ops_nolock are defined +in the header itself, and the REGISTER and UNREGISTER macros are built from +these. More complex (and dangerous!) manipulations of the table are possible +using these functions, but you must be careful to always lock the table before +you read or write from it, and to unlock it again when you are done. (And of +course, to never ever leave an invalid pointer in the table). Null pointers in +the table are always safe :) + +If for some reason you feel the need to call the handle_sysrq function from +within a function called by handle_sysrq, you must be aware that you are in +a lock (you are also in an interrupt handler, which means don't sleep!), so +you must call __handle_sysrq_nolock instead. + +* I have more questions, who can I ask? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +You may feel free to send email to myrdraal@deathsdoor.com, and I will +respond as soon as possible. + -Myrdraal + +And I'll answer any questions about the registration system you got, also +responding as soon as possible. + -Crutcher + +* Credits +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Written by Mydraal <myrdraal@deathsdoor.com> +Updated by Adam Sulmicki <adam@cfar.umd.edu> +Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59 +Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com> |