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