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1da177e4 | 1 | Linux Magic System Request Key Hacks |
d346cce3 | 2 | Documentation for sysrq.c |
1da177e4 LT |
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 | |
d346cce3 | 37 | allowed (by a user with admin privileges). |
1da177e4 LT |
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 | |
dfb0042d | 45 | have better luck with "press Alt", "press SysRq", "release SysRq", |
1da177e4 LT |
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 | ||
d346cce3 | 60 | On all - write a character to /proc/sysrq-trigger. e.g.: |
1da177e4 LT |
61 | |
62 | echo t > /proc/sysrq-trigger | |
63 | ||
64 | * What are the 'command' keys? | |
65 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
1da177e4 LT |
66 | 'b' - Will immediately reboot the system without syncing or unmounting |
67 | your disks. | |
68 | ||
86b1ae38 HN |
69 | 'c' - Will perform a kexec reboot in order to take a crashdump. |
70 | ||
d346cce3 RD |
71 | 'd' - Shows all locks that are held. |
72 | ||
78831ba6 | 73 | 'e' - Send a SIGTERM to all processes, except for init. |
1da177e4 | 74 | |
78831ba6 | 75 | 'f' - Will call oom_kill to kill a memory hog process. |
1da177e4 | 76 | |
d29c91c7 | 77 | 'g' - Used by kgdb on ppc and sh platforms. |
1da177e4 | 78 | |
78831ba6 | 79 | 'h' - Will display help (actually any other key than those listed |
09736bd3 | 80 | here will display help. but 'h' is easy to remember :-) |
1da177e4 | 81 | |
78831ba6 RD |
82 | 'i' - Send a SIGKILL to all processes, except for init. |
83 | ||
84 | 'k' - Secure Access Key (SAK) Kills all programs on the current virtual | |
85 | console. NOTE: See important comments below in SAK section. | |
1da177e4 | 86 | |
5045bcae RR |
87 | 'l' - Shows a stack backtrace for all active CPUs. |
88 | ||
1da177e4 LT |
89 | 'm' - Will dump current memory info to your console. |
90 | ||
d346cce3 RD |
91 | 'n' - Used to make RT tasks nice-able |
92 | ||
78831ba6 RD |
93 | 'o' - Will shut your system off (if configured and supported). |
94 | ||
95 | 'p' - Will dump the current registers and flags to your console. | |
96 | ||
99ebcf82 LT |
97 | 'q' - Will dump per CPU lists of all armed hrtimers (but NOT regular |
98 | timer_list timers) and detailed information about all | |
99 | clockevent devices. | |
acf11fae | 100 | |
78831ba6 RD |
101 | 'r' - Turns off keyboard raw mode and sets it to XLATE. |
102 | ||
103 | 's' - Will attempt to sync all mounted filesystems. | |
104 | ||
105 | 't' - Will dump a list of current tasks and their information to your | |
106 | console. | |
107 | ||
108 | 'u' - Will attempt to remount all mounted filesystems read-only. | |
109 | ||
1da177e4 LT |
110 | 'v' - Dumps Voyager SMP processor info to your console. |
111 | ||
d346cce3 RD |
112 | 'w' - Dumps tasks that are in uninterruptable (blocked) state. |
113 | ||
114 | 'x' - Used by xmon interface on ppc/powerpc platforms. | |
115 | ||
1da177e4 LT |
116 | '0'-'9' - Sets the console log level, controlling which kernel messages |
117 | will be printed to your console. ('0', for example would make | |
118 | it so that only emergency messages like PANICs or OOPSes would | |
119 | make it to your console.) | |
120 | ||
1da177e4 LT |
121 | * Okay, so what can I use them for? |
122 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
123 | Well, un'R'aw is very handy when your X server or a svgalib program crashes. | |
124 | ||
d346cce3 RD |
125 | sa'K' (Secure Access Key) is useful when you want to be sure there is no |
126 | trojan program running at console which could grab your password | |
127 | when you would try to login. It will kill all programs on given console, | |
128 | thus letting you make sure that the login prompt you see is actually | |
1da177e4 | 129 | the one from init, not some trojan program. |
3eecd1dc JJ |
130 | IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT |
131 | IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT | |
132 | IMPORTANT: such. :IMPORTANT | |
d346cce3 | 133 | It seems others find it useful as (System Attention Key) which is |
1da177e4 LT |
134 | useful when you want to exit a program that will not let you switch consoles. |
135 | (For example, X or a svgalib program.) | |
136 | ||
137 | re'B'oot is good when you're unable to shut down. But you should also 'S'ync | |
138 | and 'U'mount first. | |
139 | ||
86b1ae38 HN |
140 | 'C'rashdump can be used to manually trigger a crashdump when the system is hung. |
141 | The kernel needs to have been built with CONFIG_KEXEC enabled. | |
142 | ||
1da177e4 LT |
143 | 'S'ync is great when your system is locked up, it allows you to sync your |
144 | disks and will certainly lessen the chance of data loss and fscking. Note | |
145 | that the sync hasn't taken place until you see the "OK" and "Done" appear | |
146 | on the screen. (If the kernel is really in strife, you may not ever get the | |
147 | OK or Done message...) | |
148 | ||
149 | 'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync, | |
150 | 'U'mount, then re'B'oot when my system locks. It's saved me many a fsck. | |
151 | Again, the unmount (remount read-only) hasn't taken place until you see the | |
152 | "OK" and "Done" message appear on the screen. | |
153 | ||
d346cce3 RD |
154 | The loglevels '0'-'9' are useful when your console is being flooded with |
155 | kernel messages you do not want to see. Selecting '0' will prevent all but | |
1da177e4 LT |
156 | the most urgent kernel messages from reaching your console. (They will |
157 | still be logged if syslogd/klogd are alive, though.) | |
158 | ||
159 | t'E'rm and k'I'll are useful if you have some sort of runaway process you | |
160 | are unable to kill any other way, especially if it's spawning other | |
161 | processes. | |
162 | ||
163 | * Sometimes SysRq seems to get 'stuck' after using it, what can I do? | |
164 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
165 | That happens to me, also. I've found that tapping shift, alt, and control | |
166 | on both sides of the keyboard, and hitting an invalid sysrq sequence again | |
d346cce3 | 167 | will fix the problem. (i.e., something like alt-sysrq-z). Switching to another |
1da177e4 LT |
168 | virtual console (ALT+Fn) and then back again should also help. |
169 | ||
170 | * I hit SysRq, but nothing seems to happen, what's wrong? | |
171 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
172 | There are some keyboards that send different scancodes for SysRq than the | |
173 | pre-defined 0x54. So if SysRq doesn't work out of the box for a certain | |
174 | keyboard, run 'showkey -s' to find out the proper scancode sequence. Then | |
175 | use 'setkeycodes <sequence> 84' to define this sequence to the usual SysRq | |
176 | code (84 is decimal for 0x54). It's probably best to put this command in a | |
177 | boot script. Oh, and by the way, you exit 'showkey' by not typing anything | |
178 | for ten seconds. | |
179 | ||
180 | * I want to add SysRQ key events to a module, how does it work? | |
181 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
182 | In order to register a basic function with the table, you must first include | |
183 | the header 'include/linux/sysrq.h', this will define everything else you need. | |
184 | Next, you must create a sysrq_key_op struct, and populate it with A) the key | |
185 | handler function you will use, B) a help_msg string, that will print when SysRQ | |
186 | prints help, and C) an action_msg string, that will print right before your | |
338cec32 | 187 | handler is called. Your handler must conform to the prototype in 'sysrq.h'. |
1da177e4 | 188 | |
d346cce3 RD |
189 | After the sysrq_key_op is created, you can call the kernel function |
190 | register_sysrq_key(int key, struct sysrq_key_op *op_p); this will | |
191 | register the operation pointed to by 'op_p' at table key 'key', | |
192 | if that slot in the table is blank. At module unload time, you must call | |
193 | the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which | |
1da177e4 LT |
194 | will remove the key op pointed to by 'op_p' from the key 'key', if and only if |
195 | it is currently registered in that slot. This is in case the slot has been | |
196 | overwritten since you registered it. | |
197 | ||
198 | The Magic SysRQ system works by registering key operations against a key op | |
199 | lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has | |
200 | a number of operations registered into it at compile time, but is mutable, | |
d346cce3 RD |
201 | and 2 functions are exported for interface to it: |
202 | register_sysrq_key and unregister_sysrq_key. | |
203 | Of course, never ever leave an invalid pointer in the table. I.e., when | |
204 | your module that called register_sysrq_key() exits, it must call | |
205 | unregister_sysrq_key() to clean up the sysrq key table entry that it used. | |
206 | Null pointers in the table are always safe. :) | |
1da177e4 LT |
207 | |
208 | If for some reason you feel the need to call the handle_sysrq function from | |
209 | within a function called by handle_sysrq, you must be aware that you are in | |
210 | a lock (you are also in an interrupt handler, which means don't sleep!), so | |
211 | you must call __handle_sysrq_nolock instead. | |
212 | ||
47c33d9c AW |
213 | * When I hit a SysRq key combination only the header appears on the console? |
214 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
215 | Sysrq output is subject to the same console loglevel control as all | |
216 | other console output. This means that if the kernel was booted 'quiet' | |
217 | as is common on distro kernels the output may not appear on the actual | |
218 | console, even though it will appear in the dmesg buffer, and be accessible | |
219 | via the dmesg command and to the consumers of /proc/kmsg. As a specific | |
220 | exception the header line from the sysrq command is passed to all console | |
221 | consumers as if the current loglevel was maximum. If only the header | |
222 | is emitted it is almost certain that the kernel loglevel is too low. | |
223 | Should you require the output on the console channel then you will need | |
224 | to temporarily up the console loglevel using alt-sysrq-8 or: | |
225 | ||
226 | echo 8 > /proc/sysrq-trigger | |
227 | ||
228 | Remember to return the loglevel to normal after triggering the sysrq | |
229 | command you are interested in. | |
230 | ||
1da177e4 LT |
231 | * I have more questions, who can I ask? |
232 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
1da177e4 LT |
233 | And I'll answer any questions about the registration system you got, also |
234 | responding as soon as possible. | |
235 | -Crutcher | |
236 | ||
237 | * Credits | |
238 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
5e03e2c4 | 239 | Written by Mydraal <vulpyne@vulpyne.net> |
1da177e4 LT |
240 | Updated by Adam Sulmicki <adam@cfar.umd.edu> |
241 | Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59 | |
242 | Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com> |