| 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 :kbd:`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 :kbd:`Alt`, press :kbd:`SysRq`, | |
| release :kbd:`SysRq`, press :kbd:`<command key>`, release everything. | |
| On SPARC - You press :kbd:`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 :kbd:`ALT - Print Screen` (or :kbd:`F13`) - :kbd:`<command key>`, | |
| :kbd:`Print Screen` (or :kbd:`F13`) - :kbd:`<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 | |
| What are the 'command' keys? | |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
| =========== =================================================================== | |
| Command Function | |
| =========== =================================================================== | |
| ``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 the oom killer to kill a memory hog process, but do not | |
| panic if nothing can be killed. | |
| ``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. | |
| Dump all TLB entries on MIPS. | |
| ``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 | |
| c2 compliant system, and it should not be mistaken as | |
| such. | |
| 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 :kbd:`alt-sysrq-z`). Switching to | |
| another virtual console (:kbd:`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/uapi/linux/input-event-codes.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/tty/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 :kbd:`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> |