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Linux Magic System Request Key Hacks

Documentation for sysrq.c version 1.15

Last update: $Date: 2004-04-15 02:30:48 $

*  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 on a kernel with SysRq compiled in, it

may be DISABLED at run-time using following command:

        echo "0" > /proc/sys/kernel/sysrq

Note that previous versions disabled sysrq by default, and you were required

to specifically enable it at run-time. That is not the case any longer.

*  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.

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.  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.

'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.)

'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  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 interupt 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 

Updated by Adam Sulmicki 

Updated by Jeremy M. Dolan  2001/01/28 10:15:59

Added to by Crutcher Dunnavant