| Linux Magic System Request Key Hacks | |
| Documentation for sysrq.c | |
| * 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 (by a user with admin privileges). | |
| * 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 SysRq", | |
| "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. e.g.: | |
| echo t > /proc/sysrq-trigger | |
| * What are the 'command' keys? | |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
| 'b' - Will immediately reboot the system without syncing or unmounting | |
| your disks. | |
| 'c' - Will perform a system crash by a NULL pointer dereference. | |
| A crashdump will be taken if configured. | |
| 'd' - Shows all locks that are held. | |
| 'e' - Send a SIGTERM to all processes, except for init. | |
| 'f' - Will call oom_kill to kill a memory hog process. | |
| 'g' - Used by kgdb (kernel debugger) | |
| 'h' - Will display help (actually any other key than those listed | |
| here will display help. but 'h' is easy to remember :-) | |
| 'i' - Send a SIGKILL to all processes, except for init. | |
| 'j' - Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl. | |
| 'k' - Secure Access Key (SAK) Kills all programs on the current virtual | |
| console. NOTE: See important comments below in SAK section. | |
| 'l' - Shows a stack backtrace for all active CPUs. | |
| 'm' - Will dump current memory info to your console. | |
| 'n' - Used to make RT tasks nice-able | |
| 'o' - Will shut your system off (if configured and supported). | |
| 'p' - Will dump the current registers and flags to your console. | |
| 'q' - Will dump per CPU lists of all armed hrtimers (but NOT regular | |
| timer_list timers) and detailed information about all | |
| clockevent devices. | |
| 'r' - Turns off keyboard raw mode and sets it to XLATE. | |
| 's' - Will attempt to sync all mounted filesystems. | |
| 't' - Will dump a list of current tasks and their information to your | |
| console. | |
| 'u' - Will attempt to remount all mounted filesystems read-only. | |
| 'v' - Forcefully restores framebuffer console | |
| 'v' - Causes ETM buffer dump [ARM-specific] | |
| 'w' - Dumps tasks that are in uninterruptable (blocked) state. | |
| 'x' - Used by xmon interface on ppc/powerpc platforms. | |
| 'y' - Show global CPU Registers [SPARC-64 specific] | |
| 'z' - Dump the ftrace buffer | |
| '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.) | |
| * 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 is no | |
| trojan program running at console which could grab your password | |
| when you would try to login. It will kill all programs on given console, | |
| 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 a :IMPORTANT | |
| IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT | |
| IMPORTANT: such. :IMPORTANT | |
| It seems others 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. | |
| 'C'rash can be used to manually trigger a crashdump when the system is hung. | |
| Note that this just triggers a crash if there is no dump mechanism available. | |
| '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 loglevels '0'-'9' are useful when your console is being flooded with | |
| kernel messages you do not want to see. Selecting '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. | |
| "'J'ust thaw it" is useful if your system becomes unresponsive due to a frozen | |
| (probably root) filesystem via the FIFREEZE ioctl. | |
| * 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. (i.e., 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 produce a different keycode for SysRq than the | |
| pre-defined value of 99 (see KEY_SYSRQ in include/linux/input.h), or which | |
| don't have a SysRq key at all. In these cases, run 'showkey -s' to find an | |
| appropriate scancode sequence, and use 'setkeycodes <sequence> 99' to map | |
| this sequence to the usual SysRq code (e.g., 'setkeycodes e05b 99'). 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 prototype in 'sysrq.h'. | |
| After the sysrq_key_op is created, you can call the kernel function | |
| register_sysrq_key(int key, struct sysrq_key_op *op_p); 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 function 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 2 functions are exported for interface to it: | |
| register_sysrq_key and unregister_sysrq_key. | |
| Of course, never ever leave an invalid pointer in the table. I.e., when | |
| your module that called register_sysrq_key() exits, it must call | |
| unregister_sysrq_key() to clean up the sysrq key table entry that it used. | |
| 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. | |
| * When I hit a SysRq key combination only the header appears on the console? | |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
| Sysrq output is subject to the same console loglevel control as all | |
| other console output. This means that if the kernel was booted 'quiet' | |
| as is common on distro kernels the output may not appear on the actual | |
| console, even though it will appear in the dmesg buffer, and be accessible | |
| via the dmesg command and to the consumers of /proc/kmsg. As a specific | |
| exception the header line from the sysrq command is passed to all console | |
| consumers as if the current loglevel was maximum. If only the header | |
| is emitted it is almost certain that the kernel loglevel is too low. | |
| Should you require the output on the console channel then you will need | |
| to temporarily up the console loglevel using alt-sysrq-8 or: | |
| echo 8 > /proc/sysrq-trigger | |
| Remember to return the loglevel to normal after triggering the sysrq | |
| command you are interested in. | |
| * I have more questions, who can I ask? | |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
| And I'll answer any questions about the registration system you got, also | |
| responding as soon as possible. | |
| -Crutcher | |
| * Credits | |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
| Written by Mydraal < |
|
| Updated by Adam Sulmicki < |
|
| Updated by Jeremy M. Dolan < |
|
| Added to by Crutcher Dunnavant < |
|
http://kernel.org/doc/Documentation/sysrq.txt