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TOMOYO Linux Cross Reference
Linux/Documentation/sysrq.txt

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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>

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