/Documentation/sysrq.txt

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