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293 lines
12 KiB
ReStructuredText
Linux Magic System Request Key Hacks
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====================================
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Documentation for sysrq.c
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What is the magic SysRq key?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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It is a 'magical' key combo you can hit which the kernel will respond to
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regardless of whatever else it is doing, unless it is completely locked up.
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How do I enable the magic SysRq key?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when
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configuring the kernel. When running a kernel with SysRq compiled in,
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/proc/sys/kernel/sysrq controls the functions allowed to be invoked via
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the SysRq key. The default value in this file is set by the
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CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE config symbol, which itself defaults
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to 1. Here is the list of possible values in /proc/sys/kernel/sysrq:
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- 0 - disable sysrq completely
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- 1 - enable all functions of sysrq
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- >1 - bitmask of allowed sysrq functions (see below for detailed function
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description)::
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2 = 0x2 - enable control of console logging level
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4 = 0x4 - enable control of keyboard (SAK, unraw)
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8 = 0x8 - enable debugging dumps of processes etc.
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16 = 0x10 - enable sync command
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32 = 0x20 - enable remount read-only
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64 = 0x40 - enable signalling of processes (term, kill, oom-kill)
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128 = 0x80 - allow reboot/poweroff
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256 = 0x100 - allow nicing of all RT tasks
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You can set the value in the file by the following command::
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echo "number" >/proc/sys/kernel/sysrq
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The number may be written here either as decimal or as hexadecimal
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with the 0x prefix. CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE must always be
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written in hexadecimal.
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Note that the value of ``/proc/sys/kernel/sysrq`` influences only the invocation
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via a keyboard. Invocation of any operation via ``/proc/sysrq-trigger`` is
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always allowed (by a user with admin privileges).
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How do I use the magic SysRq key?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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On x86
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You press the key combo :kbd:`ALT-SysRq-<command key>`.
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.. note::
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Some
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keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is
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also known as the 'Print Screen' key. Also some keyboards cannot
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handle so many keys being pressed at the same time, so you might
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have better luck with press :kbd:`Alt`, press :kbd:`SysRq`,
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release :kbd:`SysRq`, press :kbd:`<command key>`, release everything.
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On SPARC
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You press :kbd:`ALT-STOP-<command key>`, I believe.
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On the serial console (PC style standard serial ports only)
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You send a ``BREAK``, then within 5 seconds a command key. Sending
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``BREAK`` twice is interpreted as a normal BREAK.
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On PowerPC
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Press :kbd:`ALT - Print Screen` (or :kbd:`F13`) - :kbd:`<command key>`.
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:kbd:`Print Screen` (or :kbd:`F13`) - :kbd:`<command key>` may suffice.
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On other
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If you know of the key combos for other architectures, please
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let me know so I can add them to this section.
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On all
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Write a character to /proc/sysrq-trigger. e.g.::
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echo t > /proc/sysrq-trigger
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The :kbd:`<command key>` is case sensitive.
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What are the 'command' keys?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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=========== ===================================================================
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Command Function
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=========== ===================================================================
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``b`` Will immediately reboot the system without syncing or unmounting
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your disks.
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``c`` Will perform a system crash by a NULL pointer dereference.
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A crashdump will be taken if configured.
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``d`` Shows all locks that are held.
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``e`` Send a SIGTERM to all processes, except for init.
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``f`` Will call the oom killer to kill a memory hog process, but do not
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panic if nothing can be killed.
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``g`` Used by kgdb (kernel debugger)
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``h`` Will display help (actually any other key than those listed
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here will display help. but ``h`` is easy to remember :-)
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``i`` Send a SIGKILL to all processes, except for init.
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``j`` Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.
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``k`` Secure Access Key (SAK) Kills all programs on the current virtual
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console. NOTE: See important comments below in SAK section.
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``l`` Shows a stack backtrace for all active CPUs.
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``m`` Will dump current memory info to your console.
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``n`` Used to make RT tasks nice-able
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``o`` Will shut your system off (if configured and supported).
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``p`` Will dump the current registers and flags to your console.
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``q`` Will dump per CPU lists of all armed hrtimers (but NOT regular
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timer_list timers) and detailed information about all
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clockevent devices.
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``r`` Turns off keyboard raw mode and sets it to XLATE.
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``s`` Will attempt to sync all mounted filesystems.
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``t`` Will dump a list of current tasks and their information to your
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console.
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``u`` Will attempt to remount all mounted filesystems read-only.
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``v`` Forcefully restores framebuffer console
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``v`` Causes ETM buffer dump [ARM-specific]
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``w`` Dumps tasks that are in uninterruptable (blocked) state.
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``x`` Used by xmon interface on ppc/powerpc platforms.
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Show global PMU Registers on sparc64.
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Dump all TLB entries on MIPS.
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``y`` Show global CPU Registers [SPARC-64 specific]
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``z`` Dump the ftrace buffer
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``0``-``9`` Sets the console log level, controlling which kernel messages
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will be printed to your console. (``0``, for example would make
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it so that only emergency messages like PANICs or OOPSes would
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make it to your console.)
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=========== ===================================================================
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Okay, so what can I use them for?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Well, unraw(r) is very handy when your X server or a svgalib program crashes.
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sak(k) (Secure Access Key) is useful when you want to be sure there is no
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trojan program running at console which could grab your password
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when you would try to login. It will kill all programs on given console,
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thus letting you make sure that the login prompt you see is actually
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the one from init, not some trojan program.
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.. important::
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In its true form it is not a true SAK like the one in a
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c2 compliant system, and it should not be mistaken as
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such.
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It seems others find it useful as (System Attention Key) which is
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useful when you want to exit a program that will not let you switch consoles.
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(For example, X or a svgalib program.)
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``reboot(b)`` is good when you're unable to shut down, it is an equivalent
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of pressing the "reset" button.
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``crash(c)`` can be used to manually trigger a crashdump when the system is hung.
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Note that this just triggers a crash if there is no dump mechanism available.
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``sync(s)`` is handy before yanking removable medium or after using a rescue
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shell that provides no graceful shutdown -- it will ensure your data is
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safely written to the disk. Note that the sync hasn't taken place until you see
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the "OK" and "Done" appear on the screen.
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``umount(u)`` can be used to mark filesystems as properly unmounted. From the
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running system's point of view, they will be remounted read-only. The remount
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isn't complete until you see the "OK" and "Done" message appear on the screen.
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The loglevels ``0``-``9`` are useful when your console is being flooded with
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kernel messages you do not want to see. Selecting ``0`` will prevent all but
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the most urgent kernel messages from reaching your console. (They will
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still be logged if syslogd/klogd are alive, though.)
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``term(e)`` and ``kill(i)`` are useful if you have some sort of runaway process
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you are unable to kill any other way, especially if it's spawning other
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processes.
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"just thaw ``it(j)``" is useful if your system becomes unresponsive due to a
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frozen (probably root) filesystem via the FIFREEZE ioctl.
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Sometimes SysRq seems to get 'stuck' after using it, what can I do?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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That happens to me, also. I've found that tapping shift, alt, and control
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on both sides of the keyboard, and hitting an invalid sysrq sequence again
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will fix the problem. (i.e., something like :kbd:`alt-sysrq-z`). Switching to
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another virtual console (:kbd:`ALT+Fn`) and then back again should also help.
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I hit SysRq, but nothing seems to happen, what's wrong?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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There are some keyboards that produce a different keycode for SysRq than the
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pre-defined value of 99
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(see ``KEY_SYSRQ`` in ``include/uapi/linux/input-event-codes.h``), or
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which don't have a SysRq key at all. In these cases, run ``showkey -s`` to find
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an appropriate scancode sequence, and use ``setkeycodes <sequence> 99`` to map
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this sequence to the usual SysRq code (e.g., ``setkeycodes e05b 99``). It's
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probably best to put this command in a boot script. Oh, and by the way, you
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exit ``showkey`` by not typing anything for ten seconds.
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I want to add SysRQ key events to a module, how does it work?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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In order to register a basic function with the table, you must first include
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the header ``include/linux/sysrq.h``, this will define everything else you need.
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Next, you must create a ``sysrq_key_op`` struct, and populate it with A) the key
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handler function you will use, B) a help_msg string, that will print when SysRQ
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prints help, and C) an action_msg string, that will print right before your
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handler is called. Your handler must conform to the prototype in 'sysrq.h'.
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After the ``sysrq_key_op`` is created, you can call the kernel function
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``register_sysrq_key(int key, const struct sysrq_key_op *op_p);`` this will
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register the operation pointed to by ``op_p`` at table key 'key',
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if that slot in the table is blank. At module unload time, you must call
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the function ``unregister_sysrq_key(int key, const struct sysrq_key_op *op_p)``,
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which will remove the key op pointed to by 'op_p' from the key 'key', if and
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only if it is currently registered in that slot. This is in case the slot has
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been overwritten since you registered it.
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The Magic SysRQ system works by registering key operations against a key op
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lookup table, which is defined in 'drivers/tty/sysrq.c'. This key table has
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a number of operations registered into it at compile time, but is mutable,
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and 2 functions are exported for interface to it::
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register_sysrq_key and unregister_sysrq_key.
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Of course, never ever leave an invalid pointer in the table. I.e., when
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your module that called register_sysrq_key() exits, it must call
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unregister_sysrq_key() to clean up the sysrq key table entry that it used.
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Null pointers in the table are always safe. :)
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If for some reason you feel the need to call the handle_sysrq function from
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within a function called by handle_sysrq, you must be aware that you are in
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a lock (you are also in an interrupt handler, which means don't sleep!), so
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you must call ``__handle_sysrq_nolock`` instead.
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When I hit a SysRq key combination only the header appears on the console?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Sysrq output is subject to the same console loglevel control as all
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other console output. This means that if the kernel was booted 'quiet'
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as is common on distro kernels the output may not appear on the actual
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console, even though it will appear in the dmesg buffer, and be accessible
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via the dmesg command and to the consumers of ``/proc/kmsg``. As a specific
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exception the header line from the sysrq command is passed to all console
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consumers as if the current loglevel was maximum. If only the header
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is emitted it is almost certain that the kernel loglevel is too low.
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Should you require the output on the console channel then you will need
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to temporarily up the console loglevel using :kbd:`alt-sysrq-8` or::
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echo 8 > /proc/sysrq-trigger
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Remember to return the loglevel to normal after triggering the sysrq
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command you are interested in.
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I have more questions, who can I ask?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Just ask them on the linux-kernel mailing list:
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linux-kernel@vger.kernel.org
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Credits
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~~~~~~~
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- Written by Mydraal <vulpyne@vulpyne.net>
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- Updated by Adam Sulmicki <adam@cfar.umd.edu>
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- Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
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- Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>
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