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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.

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