xref: /linux/Documentation/admin-guide/sysrq.rst (revision d99ff463ecf651437e9e4abe68f331dfb6b5bd9d)
1Linux Magic System Request Key Hacks
2====================================
3
4Documentation for sysrq.c
5
6What is the magic SysRq key?
7~~~~~~~~~~~~~~~~~~~~~~~~~~~~
8
9It is a 'magical' key combo you can hit which the kernel will respond to
10regardless of whatever else it is doing, unless it is completely locked up.
11
12How do I enable the magic SysRq key?
13~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
14
15You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when
16configuring the kernel. When running a kernel with SysRq compiled in,
17/proc/sys/kernel/sysrq controls the functions allowed to be invoked via
18the SysRq key. The default value in this file is set by the
19CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE config symbol, which itself defaults
20to 1. Here is the list of possible values in /proc/sys/kernel/sysrq:
21
22   -  0 - disable sysrq completely
23   -  1 - enable all functions of sysrq
24   - >1 - bitmask of allowed sysrq functions (see below for detailed function
25     description)::
26
27          2 =   0x2 - enable control of console logging level
28          4 =   0x4 - enable control of keyboard (SAK, unraw)
29          8 =   0x8 - enable debugging dumps of processes etc.
30         16 =  0x10 - enable sync command
31         32 =  0x20 - enable remount read-only
32         64 =  0x40 - enable signalling of processes (term, kill, oom-kill)
33        128 =  0x80 - allow reboot/poweroff
34        256 = 0x100 - allow nicing of all RT tasks
35
36You can set the value in the file by the following command::
37
38    echo "number" >/proc/sys/kernel/sysrq
39
40The number may be written here either as decimal or as hexadecimal
41with the 0x prefix. CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE must always be
42written in hexadecimal.
43
44Note that the value of ``/proc/sys/kernel/sysrq`` influences only the invocation
45via a keyboard. Invocation of any operation via ``/proc/sysrq-trigger`` is
46always allowed (by a user with admin privileges).
47
48How do I use the magic SysRq key?
49~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
50
51On x86
52	You press the key combo :kbd:`ALT-SysRq-<command key>`.
53
54	.. note::
55	   Some
56           keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is
57           also known as the 'Print Screen' key. Also some keyboards cannot
58	   handle so many keys being pressed at the same time, so you might
59	   have better luck with press :kbd:`Alt`, press :kbd:`SysRq`,
60	   release :kbd:`SysRq`, press :kbd:`<command key>`, release everything.
61
62On SPARC
63	You press :kbd:`ALT-STOP-<command key>`, I believe.
64
65On the serial console (PC style standard serial ports only)
66        You send a ``BREAK``, then within 5 seconds a command key. Sending
67        ``BREAK`` twice is interpreted as a normal BREAK.
68
69On PowerPC
70	Press :kbd:`ALT - Print Screen` (or :kbd:`F13`) - :kbd:`<command key>`.
71        :kbd:`Print Screen` (or :kbd:`F13`) - :kbd:`<command key>` may suffice.
72
73On other
74	If you know of the key combos for other architectures, please
75	submit a patch to be included in this section.
76
77On all
78	Write a character to /proc/sysrq-trigger.  e.g.::
79
80		echo t > /proc/sysrq-trigger
81
82The :kbd:`<command key>` is case sensitive.
83
84What are the 'command' keys?
85~~~~~~~~~~~~~~~~~~~~~~~~~~~~
86
87=========== ===================================================================
88Command	    Function
89=========== ===================================================================
90``b``	    Will immediately reboot the system without syncing or unmounting
91            your disks.
92
93``c``	    Will perform a system crash and a crashdump will be taken
94            if configured.
95
96``d``	    Shows all locks that are held.
97
98``e``	    Send a SIGTERM to all processes, except for init.
99
100``f``	    Will call the oom killer to kill a memory hog process, but do not
101	    panic if nothing can be killed.
102
103``g``	    Used by kgdb (kernel debugger)
104
105``h``	    Will display help (actually any other key than those listed
106            here will display help. but ``h`` is easy to remember :-)
107
108``i``	    Send a SIGKILL to all processes, except for init.
109
110``j``	    Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.
111
112``k``	    Secure Access Key (SAK) Kills all programs on the current virtual
113            console. NOTE: See important comments below in SAK section.
114
115``l``	    Shows a stack backtrace for all active CPUs.
116
117``m``	    Will dump current memory info to your console.
118
119``n``	    Used to make RT tasks nice-able
120
121``o``	    Will shut your system off (if configured and supported).
122
123``p``	    Will dump the current registers and flags to your console.
124
125``q``	    Will dump per CPU lists of all armed hrtimers (but NOT regular
126            timer_list timers) and detailed information about all
127            clockevent devices.
128
129``r``	    Turns off keyboard raw mode and sets it to XLATE.
130
131``s``	    Will attempt to sync all mounted filesystems.
132
133``t``	    Will dump a list of current tasks and their information to your
134            console.
135
136``u``	    Will attempt to remount all mounted filesystems read-only.
137
138``v``	    Forcefully restores framebuffer console
139``v``	    Causes ETM buffer dump [ARM-specific]
140
141``w``	    Dumps tasks that are in uninterruptible (blocked) state.
142
143``x``	    Used by xmon interface on ppc/powerpc platforms.
144            Show global PMU Registers on sparc64.
145            Dump all TLB entries on MIPS.
146
147``y``	    Show global CPU Registers [SPARC-64 specific]
148
149``z``	    Dump the ftrace buffer
150
151``0``-``9`` Sets the console log level, controlling which kernel messages
152            will be printed to your console. (``0``, for example would make
153            it so that only emergency messages like PANICs or OOPSes would
154            make it to your console.)
155=========== ===================================================================
156
157Okay, so what can I use them for?
158~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
159
160Well, unraw(r) is very handy when your X server or a svgalib program crashes.
161
162sak(k) (Secure Access Key) is useful when you want to be sure there is no
163trojan program running at console which could grab your password
164when you would try to login. It will kill all programs on given console,
165thus letting you make sure that the login prompt you see is actually
166the one from init, not some trojan program.
167
168.. important::
169
170   In its true form it is not a true SAK like the one in a
171   c2 compliant system, and it should not be mistaken as
172   such.
173
174It seems others find it useful as (System Attention Key) which is
175useful when you want to exit a program that will not let you switch consoles.
176(For example, X or a svgalib program.)
177
178``reboot(b)`` is good when you're unable to shut down, it is an equivalent
179of pressing the "reset" button.
180
181``crash(c)`` can be used to manually trigger a crashdump when the system is hung.
182Note that this just triggers a crash if there is no dump mechanism available.
183
184``sync(s)`` is handy before yanking removable medium or after using a rescue
185shell that provides no graceful shutdown -- it will ensure your data is
186safely written to the disk. Note that the sync hasn't taken place until you see
187the "OK" and "Done" appear on the screen.
188
189``umount(u)`` can be used to mark filesystems as properly unmounted. From the
190running system's point of view, they will be remounted read-only. The remount
191isn't complete until you see the "OK" and "Done" message appear on the screen.
192
193The loglevels ``0``-``9`` are useful when your console is being flooded with
194kernel messages you do not want to see. Selecting ``0`` will prevent all but
195the most urgent kernel messages from reaching your console. (They will
196still be logged if syslogd/klogd are alive, though.)
197
198``term(e)`` and ``kill(i)`` are useful if you have some sort of runaway process
199you are unable to kill any other way, especially if it's spawning other
200processes.
201
202"just thaw ``it(j)``" is useful if your system becomes unresponsive due to a
203frozen (probably root) filesystem via the FIFREEZE ioctl.
204
205Sometimes SysRq seems to get 'stuck' after using it, what can I do?
206~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
207
208When this happens, try tapping shift, alt and control on both sides of the
209keyboard, and hitting an invalid sysrq sequence again. (i.e., something like
210:kbd:`alt-sysrq-z`).
211
212Switching to another virtual console (:kbd:`ALT+Fn`) and then back again
213should also help.
214
215I hit SysRq, but nothing seems to happen, what's wrong?
216~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
217
218There are some keyboards that produce a different keycode for SysRq than the
219pre-defined value of 99
220(see ``KEY_SYSRQ`` in ``include/uapi/linux/input-event-codes.h``), or
221which don't have a SysRq key at all. In these cases, run ``showkey -s`` to find
222an appropriate scancode sequence, and use ``setkeycodes <sequence> 99`` to map
223this sequence to the usual SysRq code (e.g., ``setkeycodes e05b 99``). It's
224probably best to put this command in a boot script. Oh, and by the way, you
225exit ``showkey`` by not typing anything for ten seconds.
226
227I want to add SysRQ key events to a module, how does it work?
228~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
229
230In order to register a basic function with the table, you must first include
231the header ``include/linux/sysrq.h``, this will define everything else you need.
232Next, you must create a ``sysrq_key_op`` struct, and populate it with A) the key
233handler function you will use, B) a help_msg string, that will print when SysRQ
234prints help, and C) an action_msg string, that will print right before your
235handler is called. Your handler must conform to the prototype in 'sysrq.h'.
236
237After the ``sysrq_key_op`` is created, you can call the kernel function
238``register_sysrq_key(int key, const struct sysrq_key_op *op_p);`` this will
239register the operation pointed to by ``op_p`` at table key 'key',
240if that slot in the table is blank. At module unload time, you must call
241the function ``unregister_sysrq_key(int key, const struct sysrq_key_op *op_p)``,
242which will remove the key op pointed to by 'op_p' from the key 'key', if and
243only if it is currently registered in that slot. This is in case the slot has
244been overwritten since you registered it.
245
246The Magic SysRQ system works by registering key operations against a key op
247lookup table, which is defined in 'drivers/tty/sysrq.c'. This key table has
248a number of operations registered into it at compile time, but is mutable,
249and 2 functions are exported for interface to it::
250
251	register_sysrq_key and unregister_sysrq_key.
252
253Of course, never ever leave an invalid pointer in the table. I.e., when
254your module that called register_sysrq_key() exits, it must call
255unregister_sysrq_key() to clean up the sysrq key table entry that it used.
256Null pointers in the table are always safe. :)
257
258If for some reason you feel the need to call the handle_sysrq function from
259within a function called by handle_sysrq, you must be aware that you are in
260a lock (you are also in an interrupt handler, which means don't sleep!), so
261you must call ``__handle_sysrq_nolock`` instead.
262
263When I hit a SysRq key combination only the header appears on the console?
264~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
265
266Sysrq output is subject to the same console loglevel control as all
267other console output.  This means that if the kernel was booted 'quiet'
268as is common on distro kernels the output may not appear on the actual
269console, even though it will appear in the dmesg buffer, and be accessible
270via the dmesg command and to the consumers of ``/proc/kmsg``.  As a specific
271exception the header line from the sysrq command is passed to all console
272consumers as if the current loglevel was maximum.  If only the header
273is emitted it is almost certain that the kernel loglevel is too low.
274Should you require the output on the console channel then you will need
275to temporarily up the console loglevel using :kbd:`alt-sysrq-8` or::
276
277    echo 8 > /proc/sysrq-trigger
278
279Remember to return the loglevel to normal after triggering the sysrq
280command you are interested in.
281
282I have more questions, who can I ask?
283~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
284
285Just ask them on the linux-kernel mailing list:
286	linux-kernel@vger.kernel.org
287
288Credits
289~~~~~~~
290
291- Written by Mydraal <vulpyne@vulpyne.net>
292- Updated by Adam Sulmicki <adam@cfar.umd.edu>
293- Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
294- Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>
295