sysrq.txt 10 KB

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  1. Linux Magic System Request Key Hacks
  2. Documentation for sysrq.c
  3. Last update: 2007-JAN-06
  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 (by a user with admin privileges).
  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. e.g.:
  52. echo t > /proc/sysrq-trigger
  53. * What are the 'command' keys?
  54. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  55. 'b' - Will immediately reboot the system without syncing or unmounting
  56. your disks.
  57. 'c' - Will perform a kexec reboot in order to take a crashdump.
  58. 'd' - Shows all locks that are held.
  59. 'e' - Send a SIGTERM to all processes, except for init.
  60. 'f' - Will call oom_kill to kill a memory hog process.
  61. 'g' - Used by kgdb on ppc platforms.
  62. 'h' - Will display help (actually any other key than those listed
  63. above will display help. but 'h' is easy to remember :-)
  64. 'i' - Send a SIGKILL to all processes, except for init.
  65. 'k' - Secure Access Key (SAK) Kills all programs on the current virtual
  66. console. NOTE: See important comments below in SAK section.
  67. 'm' - Will dump current memory info to your console.
  68. 'n' - Used to make RT tasks nice-able
  69. 'o' - Will shut your system off (if configured and supported).
  70. 'p' - Will dump the current registers and flags to your console.
  71. 'q' - Will dump a list of all running timers.
  72. 'r' - Turns off keyboard raw mode and sets it to XLATE.
  73. 's' - Will attempt to sync all mounted filesystems.
  74. 't' - Will dump a list of current tasks and their information to your
  75. console.
  76. 'u' - Will attempt to remount all mounted filesystems read-only.
  77. 'v' - Dumps Voyager SMP processor info to your console.
  78. 'w' - Dumps tasks that are in uninterruptable (blocked) state.
  79. 'x' - Used by xmon interface on ppc/powerpc platforms.
  80. '0'-'9' - Sets the console log level, controlling which kernel messages
  81. will be printed to your console. ('0', for example would make
  82. it so that only emergency messages like PANICs or OOPSes would
  83. make it to your console.)
  84. * Okay, so what can I use them for?
  85. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  86. Well, un'R'aw is very handy when your X server or a svgalib program crashes.
  87. sa'K' (Secure Access Key) is useful when you want to be sure there is no
  88. trojan program running at console which could grab your password
  89. when you would try to login. It will kill all programs on given console,
  90. thus letting you make sure that the login prompt you see is actually
  91. the one from init, not some trojan program.
  92. IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT
  93. IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT
  94. IMPORTANT: such. :IMPORTANT
  95. It seems others find it useful as (System Attention Key) which is
  96. useful when you want to exit a program that will not let you switch consoles.
  97. (For example, X or a svgalib program.)
  98. re'B'oot is good when you're unable to shut down. But you should also 'S'ync
  99. and 'U'mount first.
  100. 'C'rashdump can be used to manually trigger a crashdump when the system is hung.
  101. The kernel needs to have been built with CONFIG_KEXEC enabled.
  102. 'S'ync is great when your system is locked up, it allows you to sync your
  103. disks and will certainly lessen the chance of data loss and fscking. Note
  104. that the sync hasn't taken place until you see the "OK" and "Done" appear
  105. on the screen. (If the kernel is really in strife, you may not ever get the
  106. OK or Done message...)
  107. 'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync,
  108. 'U'mount, then re'B'oot when my system locks. It's saved me many a fsck.
  109. Again, the unmount (remount read-only) hasn't taken place until you see the
  110. "OK" and "Done" message appear on the screen.
  111. The loglevels '0'-'9' are useful when your console is being flooded with
  112. kernel messages you do not want to see. Selecting '0' will prevent all but
  113. the most urgent kernel messages from reaching your console. (They will
  114. still be logged if syslogd/klogd are alive, though.)
  115. t'E'rm and k'I'll are useful if you have some sort of runaway process you
  116. are unable to kill any other way, especially if it's spawning other
  117. processes.
  118. * Sometimes SysRq seems to get 'stuck' after using it, what can I do?
  119. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  120. That happens to me, also. I've found that tapping shift, alt, and control
  121. on both sides of the keyboard, and hitting an invalid sysrq sequence again
  122. will fix the problem. (i.e., something like alt-sysrq-z). Switching to another
  123. virtual console (ALT+Fn) and then back again should also help.
  124. * I hit SysRq, but nothing seems to happen, what's wrong?
  125. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  126. There are some keyboards that send different scancodes for SysRq than the
  127. pre-defined 0x54. So if SysRq doesn't work out of the box for a certain
  128. keyboard, run 'showkey -s' to find out the proper scancode sequence. Then
  129. use 'setkeycodes <sequence> 84' to define this sequence to the usual SysRq
  130. code (84 is decimal for 0x54). It's probably best to put this command in a
  131. boot script. Oh, and by the way, you exit 'showkey' by not typing anything
  132. for ten seconds.
  133. * I want to add SysRQ key events to a module, how does it work?
  134. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  135. In order to register a basic function with the table, you must first include
  136. the header 'include/linux/sysrq.h', this will define everything else you need.
  137. Next, you must create a sysrq_key_op struct, and populate it with A) the key
  138. handler function you will use, B) a help_msg string, that will print when SysRQ
  139. prints help, and C) an action_msg string, that will print right before your
  140. handler is called. Your handler must conform to the prototype in 'sysrq.h'.
  141. After the sysrq_key_op is created, you can call the kernel function
  142. register_sysrq_key(int key, struct sysrq_key_op *op_p); this will
  143. register the operation pointed to by 'op_p' at table key 'key',
  144. if that slot in the table is blank. At module unload time, you must call
  145. the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
  146. will remove the key op pointed to by 'op_p' from the key 'key', if and only if
  147. it is currently registered in that slot. This is in case the slot has been
  148. overwritten since you registered it.
  149. The Magic SysRQ system works by registering key operations against a key op
  150. lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has
  151. a number of operations registered into it at compile time, but is mutable,
  152. and 2 functions are exported for interface to it:
  153. register_sysrq_key and unregister_sysrq_key.
  154. Of course, never ever leave an invalid pointer in the table. I.e., when
  155. your module that called register_sysrq_key() exits, it must call
  156. unregister_sysrq_key() to clean up the sysrq key table entry that it used.
  157. Null pointers in the table are always safe. :)
  158. If for some reason you feel the need to call the handle_sysrq function from
  159. within a function called by handle_sysrq, you must be aware that you are in
  160. a lock (you are also in an interrupt handler, which means don't sleep!), so
  161. you must call __handle_sysrq_nolock instead.
  162. * I have more questions, who can I ask?
  163. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  164. And I'll answer any questions about the registration system you got, also
  165. responding as soon as possible.
  166. -Crutcher
  167. * Credits
  168. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  169. Written by Mydraal <vulpyne@vulpyne.net>
  170. Updated by Adam Sulmicki <adam@cfar.umd.edu>
  171. Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
  172. Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>