--- /dev/null
+Linux Magic System Request Key Hacks
+Documentation for sysrq.c
+
+* What is the magic SysRq key?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+It is a 'magical' key combo you can hit which the kernel will respond to
+regardless of whatever else it is doing, unless it is completely locked up.
+
+* How do I enable the magic SysRq key?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when
+configuring the kernel. When running a kernel with SysRq compiled in,
+/proc/sys/kernel/sysrq controls the functions allowed to be invoked via
+the SysRq key. The default value in this file is set by the
+CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE config symbol, which itself defaults
+to 1. Here is the list of possible values in /proc/sys/kernel/sysrq:
+ 0 - disable sysrq completely
+ 1 - enable all functions of sysrq
+ >1 - bitmask of allowed sysrq functions (see below for detailed function
+ description):
+ 2 = 0x2 - enable control of console logging level
+ 4 = 0x4 - enable control of keyboard (SAK, unraw)
+ 8 = 0x8 - enable debugging dumps of processes etc.
+ 16 = 0x10 - enable sync command
+ 32 = 0x20 - enable remount read-only
+ 64 = 0x40 - enable signalling of processes (term, kill, oom-kill)
+ 128 = 0x80 - allow reboot/poweroff
+ 256 = 0x100 - allow nicing of all RT tasks
+
+You can set the value in the file by the following command:
+ echo "number" >/proc/sys/kernel/sysrq
+
+The number may be written here either as decimal or as hexadecimal
+with the 0x prefix. CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE must always be
+written in hexadecimal.
+
+Note that the value of /proc/sys/kernel/sysrq influences only the invocation
+via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always
+allowed (by a user with admin privileges).
+
+* How do I use the magic SysRq key?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+On x86 - You press the key combo 'ALT-SysRq-<command key>'. Note - Some
+ keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is
+ also known as the 'Print Screen' key. Also some keyboards cannot
+ handle so many keys being pressed at the same time, so you might
+ have better luck with "press Alt", "press SysRq", "release SysRq",
+ "press <command key>", release everything.
+
+On SPARC - You press 'ALT-STOP-<command key>', I believe.
+
+On the serial console (PC style standard serial ports only) -
+ You send a BREAK, then within 5 seconds a command key. Sending
+ BREAK twice is interpreted as a normal BREAK.
+
+On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>,
+ Print Screen (or F13) - <command key> may suffice.
+
+On other - If you know of the key combos for other architectures, please
+ let me know so I can add them to this section.
+
+On all - write a character to /proc/sysrq-trigger, e.g.:
+ echo t > /proc/sysrq-trigger
+
+On all - Enable network SysRq by writing a cookie to icmp_echo_sysrq, e.g.
+ echo 0x01020304 >/proc/sys/net/ipv4/icmp_echo_sysrq
+ Send an ICMP echo request with this pattern plus the particular
+ SysRq command key. Example:
+ # ping -c1 -s57 -p0102030468
+ will trigger the SysRq-H (help) command.
+
+
+* What are the 'command' keys?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+'b' - Will immediately reboot the system without syncing or unmounting
+ your disks.
+
+'c' - Will perform a system crash by a NULL pointer dereference.
+ A crashdump will be taken if configured.
+
+'d' - Shows all locks that are held.
+
+'e' - Send a SIGTERM to all processes, except for init.
+
+'f' - Will call oom_kill to kill a memory hog process.
+
+'g' - Used by kgdb (kernel debugger)
+
+'h' - Will display help (actually any other key than those listed
+ here will display help. but 'h' is easy to remember :-)
+
+'i' - Send a SIGKILL to all processes, except for init.
+
+'j' - Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.
+
+'k' - Secure Access Key (SAK) Kills all programs on the current virtual
+ console. NOTE: See important comments below in SAK section.
+
+'l' - Shows a stack backtrace for all active CPUs.
+
+'m' - Will dump current memory info to your console.
+
+'n' - Used to make RT tasks nice-able
+
+'o' - Will shut your system off (if configured and supported).
+
+'p' - Will dump the current registers and flags to your console.
+
+'q' - Will dump per CPU lists of all armed hrtimers (but NOT regular
+ timer_list timers) and detailed information about all
+ clockevent devices.
+
+'r' - Turns off keyboard raw mode and sets it to XLATE.
+
+'s' - Will attempt to sync all mounted filesystems.
+
+'t' - Will dump a list of current tasks and their information to your
+ console.
+
+'u' - Will attempt to remount all mounted filesystems read-only.
+
+'v' - Forcefully restores framebuffer console
+'v' - Causes ETM buffer dump [ARM-specific]
+
+'w' - Dumps tasks that are in uninterruptable (blocked) state.
+
+'x' - Used by xmon interface on ppc/powerpc platforms.
+ Show global PMU Registers on sparc64.
+
+'y' - Show global CPU Registers [SPARC-64 specific]
+
+'z' - Dump the ftrace buffer
+
+'0'-'9' - Sets the console log level, controlling which kernel messages
+ will be printed to your console. ('0', for example would make
+ it so that only emergency messages like PANICs or OOPSes would
+ make it to your console.)
+
+* Okay, so what can I use them for?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Well, unraw(r) is very handy when your X server or a svgalib program crashes.
+
+sak(k) (Secure Access Key) is useful when you want to be sure there is no
+trojan program running at console which could grab your password
+when you would try to login. It will kill all programs on given console,
+thus letting you make sure that the login prompt you see is actually
+the one from init, not some trojan program.
+IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT
+IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT
+IMPORTANT: such. :IMPORTANT
+ It seems others find it useful as (System Attention Key) which is
+useful when you want to exit a program that will not let you switch consoles.
+(For example, X or a svgalib program.)
+
+reboot(b) is good when you're unable to shut down. But you should also
+sync(s) and umount(u) first.
+
+crash(c) can be used to manually trigger a crashdump when the system is hung.
+Note that this just triggers a crash if there is no dump mechanism available.
+
+sync(s) is great when your system is locked up, it allows you to sync your
+disks and will certainly lessen the chance of data loss and fscking. Note
+that the sync hasn't taken place until you see the "OK" and "Done" appear
+on the screen. (If the kernel is really in strife, you may not ever get the
+OK or Done message...)
+
+umount(u) is basically useful in the same ways as sync(s). I generally sync(s),
+umount(u), then reboot(b) when my system locks. It's saved me many a fsck.
+Again, the unmount (remount read-only) hasn't taken place until you see the
+"OK" and "Done" message appear on the screen.
+
+The loglevels '0'-'9' are useful when your console is being flooded with
+kernel messages you do not want to see. Selecting '0' will prevent all but
+the most urgent kernel messages from reaching your console. (They will
+still be logged if syslogd/klogd are alive, though.)
+
+term(e) and kill(i) are useful if you have some sort of runaway process you
+are unable to kill any other way, especially if it's spawning other
+processes.
+
+"just thaw it(j)" is useful if your system becomes unresponsive due to a frozen
+(probably root) filesystem via the FIFREEZE ioctl.
+
+* Sometimes SysRq seems to get 'stuck' after using it, what can I do?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+That happens to me, also. I've found that tapping shift, alt, and control
+on both sides of the keyboard, and hitting an invalid sysrq sequence again
+will fix the problem. (i.e., something like alt-sysrq-z). Switching to another
+virtual console (ALT+Fn) and then back again should also help.
+
+* I hit SysRq, but nothing seems to happen, what's wrong?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+There are some keyboards that produce a different keycode for SysRq than the
+pre-defined value of 99 (see KEY_SYSRQ in include/linux/input.h), or which
+don't have a SysRq key at all. In these cases, run 'showkey -s' to find an
+appropriate scancode sequence, and use 'setkeycodes <sequence> 99' to map
+this sequence to the usual SysRq code (e.g., 'setkeycodes e05b 99'). It's
+probably best to put this command in a boot script. Oh, and by the way, you
+exit 'showkey' by not typing anything for ten seconds.
+
+* I want to add SysRQ key events to a module, how does it work?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+In order to register a basic function with the table, you must first include
+the header 'include/linux/sysrq.h', this will define everything else you need.
+Next, you must create a sysrq_key_op struct, and populate it with A) the key
+handler function you will use, B) a help_msg string, that will print when SysRQ
+prints help, and C) an action_msg string, that will print right before your
+handler is called. Your handler must conform to the prototype in 'sysrq.h'.
+
+After the sysrq_key_op is created, you can call the kernel function
+register_sysrq_key(int key, struct sysrq_key_op *op_p); this will
+register the operation pointed to by 'op_p' at table key 'key',
+if that slot in the table is blank. At module unload time, you must call
+the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
+will remove the key op pointed to by 'op_p' from the key 'key', if and only if
+it is currently registered in that slot. This is in case the slot has been
+overwritten since you registered it.
+
+The Magic SysRQ system works by registering key operations against a key op
+lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has
+a number of operations registered into it at compile time, but is mutable,
+and 2 functions are exported for interface to it:
+ register_sysrq_key and unregister_sysrq_key.
+Of course, never ever leave an invalid pointer in the table. I.e., when
+your module that called register_sysrq_key() exits, it must call
+unregister_sysrq_key() to clean up the sysrq key table entry that it used.
+Null pointers in the table are always safe. :)
+
+If for some reason you feel the need to call the handle_sysrq function from
+within a function called by handle_sysrq, you must be aware that you are in
+a lock (you are also in an interrupt handler, which means don't sleep!), so
+you must call __handle_sysrq_nolock instead.
+
+* When I hit a SysRq key combination only the header appears on the console?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Sysrq output is subject to the same console loglevel control as all
+other console output. This means that if the kernel was booted 'quiet'
+as is common on distro kernels the output may not appear on the actual
+console, even though it will appear in the dmesg buffer, and be accessible
+via the dmesg command and to the consumers of /proc/kmsg. As a specific
+exception the header line from the sysrq command is passed to all console
+consumers as if the current loglevel was maximum. If only the header
+is emitted it is almost certain that the kernel loglevel is too low.
+Should you require the output on the console channel then you will need
+to temporarily up the console loglevel using alt-sysrq-8 or:
+
+ echo 8 > /proc/sysrq-trigger
+
+Remember to return the loglevel to normal after triggering the sysrq
+command you are interested in.
+
+* I have more questions, who can I ask?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Just ask them on the linux-kernel mailing list:
+ linux-kernel@vger.kernel.org
+
+* Credits
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Written by Mydraal <vulpyne@vulpyne.net>
+Updated by Adam Sulmicki <adam@cfar.umd.edu>
+Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
+Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>
Code Review / kvmfornfv.git / blobdiff