diff options
Diffstat (limited to 'Documentation/admin-guide/sysrq.rst')
| -rw-r--r-- | Documentation/admin-guide/sysrq.rst | 290 |
1 files changed, 290 insertions, 0 deletions
diff --git a/Documentation/admin-guide/sysrq.rst b/Documentation/admin-guide/sysrq.rst new file mode 100644 index 000000000000..7b9035c01a2e --- /dev/null +++ b/Documentation/admin-guide/sysrq.rst @@ -0,0 +1,290 @@ +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> |