]>
Commit | Line | Data |
---|---|---|
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 | And I'll answer any questions about the registration system you got, also | |
245 | responding as soon as possible. | |
246 | -Crutcher | |
247 | ||
248 | * Credits | |
249 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
250 | Written by Mydraal <vulpyne@vulpyne.net> | |
251 | Updated by Adam Sulmicki <adam@cfar.umd.edu> | |
252 | Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59 | |
253 | Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com> |