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Linus Torvalds1da177e2005-04-16 15:20:36 -07001Linux Magic System Request Key Hacks
2Documentation for sysrq.c version 1.15
3Last update: $Date: 2001/01/28 10:15:59 $
4
5* What is the magic SysRq key?
6~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
7It is a 'magical' key combo you can hit which the kernel will respond to
8regardless of whatever else it is doing, unless it is completely locked up.
9
10* How do I enable the magic SysRq key?
11~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
12You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when
13configuring the kernel. When running a kernel with SysRq compiled in,
14/proc/sys/kernel/sysrq controls the functions allowed to be invoked via
15the SysRq key. By default the file contains 1 which means that every
16possible SysRq request is allowed (in older versions SysRq was disabled
17by default, and you were required to specifically enable it at run-time
18but this is not the case any more). Here is the list of possible values
19in /proc/sys/kernel/sysrq:
20 0 - disable sysrq completely
21 1 - enable all functions of sysrq
22 >1 - bitmask of allowed sysrq functions (see below for detailed function
23 description):
24 2 - enable control of console logging level
25 4 - enable control of keyboard (SAK, unraw)
26 8 - enable debugging dumps of processes etc.
27 16 - enable sync command
28 32 - enable remount read-only
29 64 - enable signalling of processes (term, kill, oom-kill)
30 128 - allow reboot/poweroff
31 256 - allow nicing of all RT tasks
32
33You can set the value in the file by the following command:
34 echo "number" >/proc/sys/kernel/sysrq
35
36Note that the value of /proc/sys/kernel/sysrq influences only the invocation
37via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always
38allowed.
39
40* How do I use the magic SysRq key?
41~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
42On x86 - You press the key combo 'ALT-SysRq-<command key>'. Note - Some
43 keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is
44 also known as the 'Print Screen' key. Also some keyboards cannot
45 handle so many keys being pressed at the same time, so you might
46 have better luck with "press Alt", "press SysRq", "release Alt",
47 "press <command key>", release everything.
48
49On SPARC - You press 'ALT-STOP-<command key>', I believe.
50
51On the serial console (PC style standard serial ports only) -
52 You send a BREAK, then within 5 seconds a command key. Sending
53 BREAK twice is interpreted as a normal BREAK.
54
55On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>,
56 Print Screen (or F13) - <command key> may suffice.
57
58On other - If you know of the key combos for other architectures, please
59 let me know so I can add them to this section.
60
61On all - write a character to /proc/sysrq-trigger. eg:
62
63 echo t > /proc/sysrq-trigger
64
65* What are the 'command' keys?
66~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
67'r' - Turns off keyboard raw mode and sets it to XLATE.
68
69'k' - Secure Access Key (SAK) Kills all programs on the current virtual
70 console. NOTE: See important comments below in SAK section.
71
72'b' - Will immediately reboot the system without syncing or unmounting
73 your disks.
74
Hariprasad Nellitheertha86b1ae32005-06-25 14:58:25 -070075'c' - Will perform a kexec reboot in order to take a crashdump.
76
Linus Torvalds1da177e2005-04-16 15:20:36 -070077'o' - Will shut your system off (if configured and supported).
78
79's' - Will attempt to sync all mounted filesystems.
80
81'u' - Will attempt to remount all mounted filesystems read-only.
82
83'p' - Will dump the current registers and flags to your console.
84
85't' - Will dump a list of current tasks and their information to your
86 console.
87
88'm' - Will dump current memory info to your console.
89
90'v' - Dumps Voyager SMP processor info to your console.
91
92'0'-'9' - Sets the console log level, controlling which kernel messages
93 will be printed to your console. ('0', for example would make
94 it so that only emergency messages like PANICs or OOPSes would
95 make it to your console.)
96
97'f' - Will call oom_kill to kill a memory hog process
98
99'e' - Send a SIGTERM to all processes, except for init.
100
101'i' - Send a SIGKILL to all processes, except for init.
102
103'l' - Send a SIGKILL to all processes, INCLUDING init. (Your system
104 will be non-functional after this.)
105
106'h' - Will display help ( actually any other key than those listed
107 above will display help. but 'h' is easy to remember :-)
108
109* Okay, so what can I use them for?
110~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
111Well, un'R'aw is very handy when your X server or a svgalib program crashes.
112
113sa'K' (Secure Access Key) is useful when you want to be sure there are no
114trojan program is running at console and which could grab your password
115when you would try to login. It will kill all programs on given console
116and thus letting you make sure that the login prompt you see is actually
117the one from init, not some trojan program.
118IMPORTANT:In its true form it is not a true SAK like the one in :IMPORTANT
119IMPORTANT:c2 compliant systems, and it should be mistook as such. :IMPORTANT
120 It seems other find it useful as (System Attention Key) which is
121useful when you want to exit a program that will not let you switch consoles.
122(For example, X or a svgalib program.)
123
124re'B'oot is good when you're unable to shut down. But you should also 'S'ync
125and 'U'mount first.
126
Hariprasad Nellitheertha86b1ae32005-06-25 14:58:25 -0700127'C'rashdump can be used to manually trigger a crashdump when the system is hung.
128The kernel needs to have been built with CONFIG_KEXEC enabled.
129
Linus Torvalds1da177e2005-04-16 15:20:36 -0700130'S'ync is great when your system is locked up, it allows you to sync your
131disks and will certainly lessen the chance of data loss and fscking. Note
132that the sync hasn't taken place until you see the "OK" and "Done" appear
133on the screen. (If the kernel is really in strife, you may not ever get the
134OK or Done message...)
135
136'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync,
137'U'mount, then re'B'oot when my system locks. It's saved me many a fsck.
138Again, the unmount (remount read-only) hasn't taken place until you see the
139"OK" and "Done" message appear on the screen.
140
141The loglevel'0'-'9' is useful when your console is being flooded with
142kernel messages you do not want to see. Setting '0' will prevent all but
143the most urgent kernel messages from reaching your console. (They will
144still be logged if syslogd/klogd are alive, though.)
145
146t'E'rm and k'I'll are useful if you have some sort of runaway process you
147are unable to kill any other way, especially if it's spawning other
148processes.
149
150* Sometimes SysRq seems to get 'stuck' after using it, what can I do?
151~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
152That happens to me, also. I've found that tapping shift, alt, and control
153on both sides of the keyboard, and hitting an invalid sysrq sequence again
154will fix the problem. (ie, something like alt-sysrq-z). Switching to another
155virtual console (ALT+Fn) and then back again should also help.
156
157* I hit SysRq, but nothing seems to happen, what's wrong?
158~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
159There are some keyboards that send different scancodes for SysRq than the
160pre-defined 0x54. So if SysRq doesn't work out of the box for a certain
161keyboard, run 'showkey -s' to find out the proper scancode sequence. Then
162use 'setkeycodes <sequence> 84' to define this sequence to the usual SysRq
163code (84 is decimal for 0x54). It's probably best to put this command in a
164boot script. Oh, and by the way, you exit 'showkey' by not typing anything
165for ten seconds.
166
167* I want to add SysRQ key events to a module, how does it work?
168~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
169In order to register a basic function with the table, you must first include
170the header 'include/linux/sysrq.h', this will define everything else you need.
171Next, you must create a sysrq_key_op struct, and populate it with A) the key
172handler function you will use, B) a help_msg string, that will print when SysRQ
173prints help, and C) an action_msg string, that will print right before your
Adrian Bunk338cec32005-09-10 00:26:54 -0700174handler is called. Your handler must conform to the prototype in 'sysrq.h'.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700175
176After the sysrq_key_op is created, you can call the macro
177register_sysrq_key(int key, struct sysrq_key_op *op_p) that is defined in
178sysrq.h, this will register the operation pointed to by 'op_p' at table
179key 'key', if that slot in the table is blank. At module unload time, you must
180call the macro unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
181will remove the key op pointed to by 'op_p' from the key 'key', if and only if
182it is currently registered in that slot. This is in case the slot has been
183overwritten since you registered it.
184
185The Magic SysRQ system works by registering key operations against a key op
186lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has
187a number of operations registered into it at compile time, but is mutable,
188and 4 functions are exported for interface to it: __sysrq_lock_table,
189__sysrq_unlock_table, __sysrq_get_key_op, and __sysrq_put_key_op. The
190functions __sysrq_swap_key_ops and __sysrq_swap_key_ops_nolock are defined
191in the header itself, and the REGISTER and UNREGISTER macros are built from
192these. More complex (and dangerous!) manipulations of the table are possible
193using these functions, but you must be careful to always lock the table before
194you read or write from it, and to unlock it again when you are done. (And of
195course, to never ever leave an invalid pointer in the table). Null pointers in
196the table are always safe :)
197
198If for some reason you feel the need to call the handle_sysrq function from
199within a function called by handle_sysrq, you must be aware that you are in
200a lock (you are also in an interrupt handler, which means don't sleep!), so
201you must call __handle_sysrq_nolock instead.
202
203* I have more questions, who can I ask?
204~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
205You may feel free to send email to myrdraal@deathsdoor.com, and I will
206respond as soon as possible.
207 -Myrdraal
208
209And I'll answer any questions about the registration system you got, also
210responding as soon as possible.
211 -Crutcher
212
213* Credits
214~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
215Written by Mydraal <myrdraal@deathsdoor.com>
216Updated by Adam Sulmicki <adam@cfar.umd.edu>
217Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
218Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>