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Based on kernel version 2.6.34. Page generated on 2010-05-31 16:03 EST.

1	Linux Magic System Request Key Hacks
2	Documentation for sysrq.c
3	
4	*  What is the magic SysRq key?
5	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
6	It is a 'magical' key combo you can hit which the kernel will respond to
7	regardless of whatever else it is doing, unless it is completely locked up.
8	
9	*  How do I enable the magic SysRq key?
10	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
11	You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when
12	configuring the kernel. When running a kernel with SysRq compiled in,
13	/proc/sys/kernel/sysrq controls the functions allowed to be invoked via
14	the SysRq key. By default the file contains 1 which means that every
15	possible SysRq request is allowed (in older versions SysRq was disabled
16	by default, and you were required to specifically enable it at run-time
17	but this is not the case any more). Here is the list of possible values
18	in /proc/sys/kernel/sysrq:
19	   0 - disable sysrq completely
20	   1 - enable all functions of sysrq
21	  >1 - bitmask of allowed sysrq functions (see below for detailed function
22	       description):
23	          2 - enable control of console logging level
24	          4 - enable control of keyboard (SAK, unraw)
25	          8 - enable debugging dumps of processes etc.
26	         16 - enable sync command
27	         32 - enable remount read-only
28	         64 - enable signalling of processes (term, kill, oom-kill)
29	        128 - allow reboot/poweroff
30	        256 - allow nicing of all RT tasks
31	
32	You can set the value in the file by the following command:
33	    echo "number" >/proc/sys/kernel/sysrq
34	
35	Note that the value of /proc/sys/kernel/sysrq influences only the invocation
36	via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always
37	allowed (by a user with admin privileges).
38	
39	*  How do I use the magic SysRq key?
40	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
41	On x86   - You press the key combo 'ALT-SysRq-<command key>'. Note - Some
42	           keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is
43	           also known as the 'Print Screen' key. Also some keyboards cannot
44		   handle so many keys being pressed at the same time, so you might
45		   have better luck with "press Alt", "press SysRq", "release SysRq",
46		   "press <command key>", release everything.
47	
48	On SPARC - You press 'ALT-STOP-<command key>', I believe.
49	
50	On the serial console (PC style standard serial ports only) -
51	           You send a BREAK, then within 5 seconds a command key. Sending
52	           BREAK twice is interpreted as a normal BREAK.
53	
54	On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>,  
55	             Print Screen (or F13) - <command key> may suffice.
56	
57	On other - If you know of the key combos for other architectures, please
58	           let me know so I can add them to this section.
59	
60	On all -  write a character to /proc/sysrq-trigger.  e.g.:
61	
62			echo t > /proc/sysrq-trigger
63	
64	*  What are the 'command' keys?
65	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
66	'b'     - Will immediately reboot the system without syncing or unmounting
67	          your disks.
68	
69	'c'	- Will perform a system crash by a NULL pointer dereference.
70	          A crashdump will be taken if configured.
71	
72	'd'	- Shows all locks that are held.
73	
74	'e'     - Send a SIGTERM to all processes, except for init.
75	
76	'f'	- Will call oom_kill to kill a memory hog process.
77	
78	'g'	- Used by kgdb on ppc and sh platforms.
79	
80	'h'     - Will display help (actually any other key than those listed
81	          here will display help. but 'h' is easy to remember :-)
82	
83	'i'     - Send a SIGKILL to all processes, except for init.
84	
85	'j'     - Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.
86	
87	'k'     - Secure Access Key (SAK) Kills all programs on the current virtual
88	          console. NOTE: See important comments below in SAK section.
89	
90	'l'     - Shows a stack backtrace for all active CPUs.
91	
92	'm'     - Will dump current memory info to your console.
93	
94	'n'	- Used to make RT tasks nice-able
95	
96	'o'     - Will shut your system off (if configured and supported).
97	
98	'p'     - Will dump the current registers and flags to your console.
99	
100	'q'     - Will dump per CPU lists of all armed hrtimers (but NOT regular
101	          timer_list timers) and detailed information about all
102	          clockevent devices.
103	
104	'r'     - Turns off keyboard raw mode and sets it to XLATE.
105	
106	's'     - Will attempt to sync all mounted filesystems.
107	
108	't'     - Will dump a list of current tasks and their information to your
109	          console.
110	
111	'u'     - Will attempt to remount all mounted filesystems read-only.
112	
113	'v'	- Dumps Voyager SMP processor info to your console.
114	
115	'w'	- Dumps tasks that are in uninterruptable (blocked) state.
116	
117	'x'	- Used by xmon interface on ppc/powerpc platforms.
118	
119	'z'	- Dump the ftrace buffer
120	
121	'0'-'9' - Sets the console log level, controlling which kernel messages
122	          will be printed to your console. ('0', for example would make
123	          it so that only emergency messages like PANICs or OOPSes would
124	          make it to your console.)
125	
126	*  Okay, so what can I use them for?
127	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
128	Well, un'R'aw is very handy when your X server or a svgalib program crashes.
129	
130	sa'K' (Secure Access Key) is useful when you want to be sure there is no
131	trojan program running at console which could grab your password
132	when you would try to login. It will kill all programs on given console,
133	thus letting you make sure that the login prompt you see is actually
134	the one from init, not some trojan program.
135	IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT
136	IMPORTANT: c2 compliant system, and it should not be mistaken as   :IMPORTANT
137	IMPORTANT: such.                                                   :IMPORTANT
138	       It seems others find it useful as (System Attention Key) which is
139	useful when you want to exit a program that will not let you switch consoles.
140	(For example, X or a svgalib program.)
141	
142	re'B'oot is good when you're unable to shut down. But you should also 'S'ync
143	and 'U'mount first.
144	
145	'C'rash can be used to manually trigger a crashdump when the system is hung.
146	Note that this just triggers a crash if there is no dump mechanism available.
147	
148	'S'ync is great when your system is locked up, it allows you to sync your
149	disks and will certainly lessen the chance of data loss and fscking. Note
150	that the sync hasn't taken place until you see the "OK" and "Done" appear
151	on the screen. (If the kernel is really in strife, you may not ever get the
152	OK or Done message...)
153	
154	'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync,
155	'U'mount, then re'B'oot when my system locks. It's saved me many a fsck.
156	Again, the unmount (remount read-only) hasn't taken place until you see the
157	"OK" and "Done" message appear on the screen.
158	
159	The loglevels '0'-'9' are useful when your console is being flooded with
160	kernel messages you do not want to see. Selecting '0' will prevent all but
161	the most urgent kernel messages from reaching your console. (They will
162	still be logged if syslogd/klogd are alive, though.)
163	
164	t'E'rm and k'I'll are useful if you have some sort of runaway process you
165	are unable to kill any other way, especially if it's spawning other
166	processes.
167	
168	"'J'ust thaw it" is useful if your system becomes unresponsive due to a frozen
169	(probably root) filesystem via the FIFREEZE ioctl.
170	
171	*  Sometimes SysRq seems to get 'stuck' after using it, what can I do?
172	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
173	That happens to me, also. I've found that tapping shift, alt, and control
174	on both sides of the keyboard, and hitting an invalid sysrq sequence again
175	will fix the problem. (i.e., something like alt-sysrq-z). Switching to another
176	virtual console (ALT+Fn) and then back again should also help.
177	
178	*  I hit SysRq, but nothing seems to happen, what's wrong?
179	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
180	There are some keyboards that send different scancodes for SysRq than the
181	pre-defined 0x54. So if SysRq doesn't work out of the box for a certain
182	keyboard, run 'showkey -s' to find out the proper scancode sequence. Then
183	use 'setkeycodes <sequence> 84' to define this sequence to the usual SysRq
184	code (84 is decimal for 0x54). It's probably best to put this command in a
185	boot script. Oh, and by the way, you exit 'showkey' by not typing anything
186	for ten seconds.
187	
188	*  I want to add SysRQ key events to a module, how does it work?
189	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
190	In order to register a basic function with the table, you must first include
191	the header 'include/linux/sysrq.h', this will define everything else you need.
192	Next, you must create a sysrq_key_op struct, and populate it with A) the key
193	handler function you will use, B) a help_msg string, that will print when SysRQ
194	prints help, and C) an action_msg string, that will print right before your
195	handler is called. Your handler must conform to the prototype in 'sysrq.h'.
196	
197	After the sysrq_key_op is created, you can call the kernel function
198	register_sysrq_key(int key, struct sysrq_key_op *op_p); this will
199	register the operation pointed to by 'op_p' at table key 'key',
200	if that slot in the table is blank. At module unload time, you must call
201	the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
202	will remove the key op pointed to by 'op_p' from the key 'key', if and only if
203	it is currently registered in that slot. This is in case the slot has been
204	overwritten since you registered it.
205	
206	The Magic SysRQ system works by registering key operations against a key op
207	lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has
208	a number of operations registered into it at compile time, but is mutable,
209	and 2 functions are exported for interface to it:
210		register_sysrq_key and unregister_sysrq_key.
211	Of course, never ever leave an invalid pointer in the table. I.e., when
212	your module that called register_sysrq_key() exits, it must call
213	unregister_sysrq_key() to clean up the sysrq key table entry that it used.
214	Null pointers in the table are always safe. :)
215	
216	If for some reason you feel the need to call the handle_sysrq function from
217	within a function called by handle_sysrq, you must be aware that you are in
218	a lock (you are also in an interrupt handler, which means don't sleep!), so
219	you must call __handle_sysrq_nolock instead.
220	
221	*  When I hit a SysRq key combination only the header appears on the console?
222	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
223	Sysrq output is subject to the same console loglevel control as all
224	other console output.  This means that if the kernel was booted 'quiet'
225	as is common on distro kernels the output may not appear on the actual
226	console, even though it will appear in the dmesg buffer, and be accessible
227	via the dmesg command and to the consumers of /proc/kmsg.  As a specific
228	exception the header line from the sysrq command is passed to all console
229	consumers as if the current loglevel was maximum.  If only the header
230	is emitted it is almost certain that the kernel loglevel is too low.
231	Should you require the output on the console channel then you will need
232	to temporarily up the console loglevel using alt-sysrq-8 or:
233	
234	    echo 8 > /proc/sysrq-trigger
235	
236	Remember to return the loglevel to normal after triggering the sysrq
237	command you are interested in.
238	
239	*  I have more questions, who can I ask?
240	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
241	And I'll answer any questions about the registration system you got, also
242	responding as soon as possible.
243	 -Crutcher
244	
245	*  Credits
246	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
247	Written by Mydraal <vulpyne[AT]vulpyne[DOT]net>
248	Updated by Adam Sulmicki <adam[AT]cfar.umd[DOT]edu>
249	Updated by Jeremy M. Dolan <jmd[AT]turbogeek[DOT]org> 2001/01/28 10:15:59
250	Added to by Crutcher Dunnavant <crutcher+kernel[AT]datastacks[DOT]com>
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