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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. 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 (kernel debugger) |
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' - Forcefully restores framebuffer console |
114 | 'v' - Causes ETM buffer dump [ARM-specific] |
115 | |
116 | 'w' - Dumps tasks that are in uninterruptable (blocked) state. |
117 | |
118 | 'x' - Used by xmon interface on ppc/powerpc platforms. |
119 | |
120 | 'y' - Show global CPU Registers [SPARC-64 specific] |
121 | |
122 | 'z' - Dump the ftrace buffer |
123 | |
124 | '0'-'9' - Sets the console log level, controlling which kernel messages |
125 | will be printed to your console. ('0', for example would make |
126 | it so that only emergency messages like PANICs or OOPSes would |
127 | make it to your console.) |
128 | |
129 | * Okay, so what can I use them for? |
130 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
131 | Well, un'R'aw is very handy when your X server or a svgalib program crashes. |
132 | |
133 | sa'K' (Secure Access Key) is useful when you want to be sure there is no |
134 | trojan program running at console which could grab your password |
135 | when you would try to login. It will kill all programs on given console, |
136 | thus letting you make sure that the login prompt you see is actually |
137 | the one from init, not some trojan program. |
138 | IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT |
139 | IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT |
140 | IMPORTANT: such. :IMPORTANT |
141 | It seems others find it useful as (System Attention Key) which is |
142 | useful when you want to exit a program that will not let you switch consoles. |
143 | (For example, X or a svgalib program.) |
144 | |
145 | re'B'oot is good when you're unable to shut down. But you should also 'S'ync |
146 | and 'U'mount first. |
147 | |
148 | 'C'rash can be used to manually trigger a crashdump when the system is hung. |
149 | Note that this just triggers a crash if there is no dump mechanism available. |
150 | |
151 | 'S'ync is great when your system is locked up, it allows you to sync your |
152 | disks and will certainly lessen the chance of data loss and fscking. Note |
153 | that the sync hasn't taken place until you see the "OK" and "Done" appear |
154 | on the screen. (If the kernel is really in strife, you may not ever get the |
155 | OK or Done message...) |
156 | |
157 | 'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync, |
158 | 'U'mount, then re'B'oot when my system locks. It's saved me many a fsck. |
159 | Again, the unmount (remount read-only) hasn't taken place until you see the |
160 | "OK" and "Done" message appear on the screen. |
161 | |
162 | The loglevels '0'-'9' are useful when your console is being flooded with |
163 | kernel messages you do not want to see. Selecting '0' will prevent all but |
164 | the most urgent kernel messages from reaching your console. (They will |
165 | still be logged if syslogd/klogd are alive, though.) |
166 | |
167 | t'E'rm and k'I'll are useful if you have some sort of runaway process you |
168 | are unable to kill any other way, especially if it's spawning other |
169 | processes. |
170 | |
171 | "'J'ust thaw it" is useful if your system becomes unresponsive due to a frozen |
172 | (probably root) filesystem via the FIFREEZE ioctl. |
173 | |
174 | * Sometimes SysRq seems to get 'stuck' after using it, what can I do? |
175 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
176 | That happens to me, also. I've found that tapping shift, alt, and control |
177 | on both sides of the keyboard, and hitting an invalid sysrq sequence again |
178 | will fix the problem. (i.e., something like alt-sysrq-z). Switching to another |
179 | virtual console (ALT+Fn) and then back again should also help. |
180 | |
181 | * I hit SysRq, but nothing seems to happen, what's wrong? |
182 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
183 | There are some keyboards that produce a different keycode for SysRq than the |
184 | pre-defined value of 99 (see KEY_SYSRQ in include/linux/input.h), or which |
185 | don't have a SysRq key at all. In these cases, run 'showkey -s' to find an |
186 | appropriate scancode sequence, and use 'setkeycodes <sequence> 99' to map |
187 | this sequence to the usual SysRq code (e.g., 'setkeycodes e05b 99'). It's |
188 | probably best to put this command in a boot script. Oh, and by the way, you |
189 | exit 'showkey' by not typing anything for ten seconds. |
190 | |
191 | * I want to add SysRQ key events to a module, how does it work? |
192 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
193 | In order to register a basic function with the table, you must first include |
194 | the header 'include/linux/sysrq.h', this will define everything else you need. |
195 | Next, you must create a sysrq_key_op struct, and populate it with A) the key |
196 | handler function you will use, B) a help_msg string, that will print when SysRQ |
197 | prints help, and C) an action_msg string, that will print right before your |
198 | handler is called. Your handler must conform to the prototype in 'sysrq.h'. |
199 | |
200 | After the sysrq_key_op is created, you can call the kernel function |
201 | register_sysrq_key(int key, struct sysrq_key_op *op_p); this will |
202 | register the operation pointed to by 'op_p' at table key 'key', |
203 | if that slot in the table is blank. At module unload time, you must call |
204 | the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which |
205 | will remove the key op pointed to by 'op_p' from the key 'key', if and only if |
206 | it is currently registered in that slot. This is in case the slot has been |
207 | overwritten since you registered it. |
208 | |
209 | The Magic SysRQ system works by registering key operations against a key op |
210 | lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has |
211 | a number of operations registered into it at compile time, but is mutable, |
212 | and 2 functions are exported for interface to it: |
213 | register_sysrq_key and unregister_sysrq_key. |
214 | Of course, never ever leave an invalid pointer in the table. I.e., when |
215 | your module that called register_sysrq_key() exits, it must call |
216 | unregister_sysrq_key() to clean up the sysrq key table entry that it used. |
217 | Null pointers in the table are always safe. :) |
218 | |
219 | If for some reason you feel the need to call the handle_sysrq function from |
220 | within a function called by handle_sysrq, you must be aware that you are in |
221 | a lock (you are also in an interrupt handler, which means don't sleep!), so |
222 | you must call __handle_sysrq_nolock instead. |
223 | |
224 | * When I hit a SysRq key combination only the header appears on the console? |
225 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
226 | Sysrq output is subject to the same console loglevel control as all |
227 | other console output. This means that if the kernel was booted 'quiet' |
228 | as is common on distro kernels the output may not appear on the actual |
229 | console, even though it will appear in the dmesg buffer, and be accessible |
230 | via the dmesg command and to the consumers of /proc/kmsg. As a specific |
231 | exception the header line from the sysrq command is passed to all console |
232 | consumers as if the current loglevel was maximum. If only the header |
233 | is emitted it is almost certain that the kernel loglevel is too low. |
234 | Should you require the output on the console channel then you will need |
235 | to temporarily up the console loglevel using alt-sysrq-8 or: |
236 | |
237 | echo 8 > /proc/sysrq-trigger |
238 | |
239 | Remember to return the loglevel to normal after triggering the sysrq |
240 | command you are interested in. |
241 | |
242 | * I have more questions, who can I ask? |
243 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
244 | Just ask them on the linux-kernel mailing list: |
245 | linux-kernel@vger.kernel.org |
246 | |
247 | * Credits |
248 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
249 | Written by Mydraal <vulpyne@vulpyne.net> |
250 | Updated by Adam Sulmicki <adam@cfar.umd.edu> |
251 | Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59 |
252 | Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com> |
253 |
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