The Design and Implementation of the FreeBSD Operating System, Second Edition
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FreeBSD/Linux Kernel Cross Reference
sys/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 
    5 *  What is the magic SysRq key?
    6 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    7 It is a 'magical' key combo you can hit which the kernel will respond to
    8 regardless of whatever else it is doing, unless it is completely locked up.
    9 
   10 *  How do I enable the magic SysRq key?
   11 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   12 You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when
   13 configuring the kernel. When running on a kernel with SysRq compiled in, it
   14 may be DISABLED at run-time using following command:
   15 
   16         echo "0" > /proc/sys/kernel/sysrq
   17 
   18 Note that previous versions disabled sysrq by default, and you were required
   19 to specifically enable it at run-time. That is not the case any longer.
   20 
   21 *  How do I use the magic SysRq key?
   22 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   23 On x86   - You press the key combo 'ALT-SysRq-<command key>'. Note - Some
   24            keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is
   25            also known as the 'Print Screen' key.
   26 
   27 On SPARC - You press 'ALT-STOP-<command key>', I believe.
   28 
   29 On the serial console (PC style standard serial ports only) -
   30            You send a BREAK, then within 5 seconds a command key. Sending
   31            BREAK twice is interpreted as a normal BREAK.
   32 
   33 On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>,  
   34              Print Screen (or F13) - <command key> may suffice.
   35 
   36 On other - If you know of the key combos for other architectures, please
   37            let me know so I can add them to this section.
   38 
   39 On all -  write a character to /proc/sysrq-trigger.  eg:
   40 
   41                 echo t > /proc/sysrq-trigger
   42 
   43 *  What are the 'command' keys?
   44 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   45 'r'     - Turns off keyboard raw mode and sets it to XLATE.
   46 
   47 'k'     - Secure Access Key (SAK) Kills all programs on the current virtual
   48           console. NOTE: See important comments below in SAK section.
   49 
   50 'b'     - Will immediately reboot the system without syncing or unmounting
   51           your disks.
   52 
   53 'o'     - Will shut your system off (if configured and supported).
   54 
   55 's'     - Will attempt to sync all mounted filesystems.
   56 
   57 'u'     - Will attempt to remount all mounted filesystems read-only.
   58 
   59 'p'     - Will dump the current registers and flags to your console.
   60 
   61 't'     - Will dump a list of current tasks and their information to your
   62           console.
   63 
   64 'm'     - Will dump current memory info to your console.
   65 
   66 '0'-'9' - Sets the console log level, controlling which kernel messages
   67           will be printed to your console. ('0', for example would make
   68           it so that only emergency messages like PANICs or OOPSes would
   69           make it to your console.)
   70 
   71 'e'     - Send a SIGTERM to all processes, except for init.
   72 
   73 'i'     - Send a SIGKILL to all processes, except for init.
   74 
   75 'l'     - Send a SIGKILL to all processes, INCLUDING init. (Your system
   76           will be non-functional after this.)
   77 
   78 'h'     - Will display help ( actually any other key than those listed
   79           above will display help. but 'h' is easy to remember :-)
   80 
   81 *  Okay, so what can I use them for?
   82 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   83 Well, un'R'aw is very handy when your X server or a svgalib program crashes.
   84 
   85 sa'K' (Secure Access Key) is useful when you want to be sure there are no
   86 trojan program is running at console and which could grab your password
   87 when you would try to login. It will kill all programs on given console
   88 and thus letting you make sure that the login prompt you see is actually
   89 the one from init, not some trojan program.
   90 IMPORTANT:In its true form it is not a true SAK like the one in   :IMPORTANT
   91 IMPORTANT:c2 compliant systems, and it should be mistook as such. :IMPORTANT
   92        It seems other find it useful as (System Attention Key) which is
   93 useful when you want to exit a program that will not let you switch consoles.
   94 (For example, X or a svgalib program.)
   95 
   96 re'B'oot is good when you're unable to shut down. But you should also 'S'ync
   97 and 'U'mount first.
   98 
   99 'S'ync is great when your system is locked up, it allows you to sync your
  100 disks and will certainly lessen the chance of data loss and fscking. Note
  101 that the sync hasn't taken place until you see the "OK" and "Done" appear
  102 on the screen. (If the kernel is really in strife, you may not ever get the
  103 OK or Done message...)
  104 
  105 'U'mount is basically useful in the same ways as 'S'ync. I generally 'S'ync,
  106 'U'mount, then re'B'oot when my system locks. It's saved me many a fsck.
  107 Again, the unmount (remount read-only) hasn't taken place until you see the
  108 "OK" and "Done" message appear on the screen.
  109 
  110 The loglevel'0'-'9' is useful when your console is being flooded with
  111 kernel messages you do not want to see. Setting '0' will prevent all but
  112 the most urgent kernel messages from reaching your console. (They will
  113 still be logged if syslogd/klogd are alive, though.)
  114 
  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 
  119 *  Sometimes SysRq seems to get 'stuck' after using it, what can I do?
  120 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  121 That happens to me, also. I've found that tapping shift, alt, and control
  122 on both sides of the keyboard, and hitting an invalid sysrq sequence again
  123 will fix the problem. (ie, something like alt-sysrq-z). Switching to another
  124 virtual console (ALT+Fn) and then back again should also help.
  125 
  126 *  I hit SysRq, but nothing seems to happen, what's wrong?
  127 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  128 There are some keyboards that send different scancodes for SysRq than the
  129 pre-defined 0x54. So if SysRq doesn't work out of the box for a certain
  130 keyboard, run 'showkey -s' to find out the proper scancode sequence. Then
  131 use 'setkeycodes <sequence> 84' to define this sequence to the usual SysRq
  132 code (84 is decimal for 0x54). It's probably best to put this command in a
  133 boot script. Oh, and by the way, you exit 'showkey' by not typing anything
  134 for ten seconds.
  135 
  136 *  I want to add SysRQ key events to a module, how does it work?
  137 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  138 In order to register a basic function with the table, you must first include
  139 the header 'include/linux/sysrq.h', this will define everything else you need.
  140 Next, you must create a sysrq_key_op struct, and populate it with A) the key
  141 handler function you will use, B) a help_msg string, that will print when SysRQ
  142 prints help, and C) an action_msg string, that will print right before your
  143 handler is called. Your handler must conform to the protoype in 'sysrq.h'.
  144 
  145 After the sysrq_key_op is created, you can call the macro 
  146 register_sysrq_key(int key, struct sysrq_key_op *op_p) that is defined in
  147 sysrq.h, this will register the operation pointed to by 'op_p' at table
  148 key 'key', if that slot in the table is blank. At module unload time, you must
  149 call the macro unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
  150 will remove the key op pointed to by 'op_p' from the key 'key', if and only if
  151 it is currently registered in that slot. This is in case the slot has been
  152 overwritten since you registered it.
  153 
  154 The Magic SysRQ system works by registering key operations against a key op
  155 lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has
  156 a number of operations registered into it at compile time, but is mutable,
  157 and 4 functions are exported for interface to it: __sysrq_lock_table,
  158 __sysrq_unlock_table, __sysrq_get_key_op, and __sysrq_put_key_op. The
  159 functions __sysrq_swap_key_ops and __sysrq_swap_key_ops_nolock are defined
  160 in the header itself, and the REGISTER and UNREGISTER macros are built from
  161 these. More complex (and dangerous!) manipulations of the table are possible
  162 using these functions, but you must be careful to always lock the table before
  163 you read or write from it, and to unlock it again when you are done. (And of
  164 course, to never ever leave an invalid pointer in the table). Null pointers in
  165 the table are always safe :)
  166 
  167 If for some reason you feel the need to call the handle_sysrq function from
  168 within a function called by handle_sysrq, you must be aware that you are in
  169 a lock (you are also in an interupt handler, which means don't sleep!), so
  170 you must call __handle_sysrq_nolock instead.
  171 
  172 *  I have more questions, who can I ask?
  173 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  174 You may feel free to send email to myrdraal@deathsdoor.com, and I will
  175 respond as soon as possible.
  176  -Myrdraal
  177 
  178 And I'll answer any questions about the registration system you got, also
  179 responding as soon as possible.
  180  -Crutcher
  181 
  182 *  Credits
  183 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  184 Written by Mydraal <myrdraal@deathsdoor.com>
  185 Updated by Adam Sulmicki <adam@cfar.umd.edu>
  186 Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
  187 Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>

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