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Linux Magic System Request Key Hacks

Documentation for sysrq.c

Last update: 2007-AUG-04

*  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:

   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-'. 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 ", release everything.

On SPARC - You press 'ALT-STOP-', 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) - ,

             Print Screen (or F13) -  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 kexec reboot in order to take a crashdump.

'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 on ppc and sh platforms.

'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.

'k'     - Secure Access Key (SAK) Kills all programs on the current virtual

          console. NOTE: See important comments below in SAK section.

'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 a list of all running timers.

'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'     - Dumps Voyager SMP processor info to your console.

'w'     - Dumps tasks that are in uninterruptable (blocked) state.

'x'     - Used by xmon interface on ppc/powerpc platforms.

'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'rashdump can be used to manually trigger a crashdump when the system is hung.

The kernel needs to have been built with CONFIG_KEXEC enabled.

'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.

*  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 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  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 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.

*  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  2001/01/28 10:15:59

Added to by Crutcher Dunnavant