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author | Mauro Carvalho Chehab <mchehab@s-opensource.com> | 2016-09-21 09:51:11 -0300 |
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committer | Mauro Carvalho Chehab <mchehab@s-opensource.com> | 2016-10-24 08:12:35 -0200 |
commit | 9d85025b0418163fae079c9ba8f8445212de8568 (patch) | |
tree | 4629e2dedf4a9ed45a6855c129101f9b52138372 /Documentation/sysrq.txt | |
parent | 186128f75392f8478ad1b32a675627d738881ca4 (diff) | |
download | linux-9d85025b0418163fae079c9ba8f8445212de8568.tar.gz linux-9d85025b0418163fae079c9ba8f8445212de8568.tar.bz2 linux-9d85025b0418163fae079c9ba8f8445212de8568.zip |
docs-rst: create an user's manual book
Place README, REPORTING-BUGS, SecurityBugs and kernel-parameters
on an user's manual book.
As we'll be numbering the user's manual, remove the manual
numbering from SecurityBugs.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Diffstat (limited to 'Documentation/sysrq.txt')
-rw-r--r-- | Documentation/sysrq.txt | 289 |
1 files changed, 0 insertions, 289 deletions
diff --git a/Documentation/sysrq.txt b/Documentation/sysrq.txt deleted file mode 100644 index d1712ea2d314..000000000000 --- a/Documentation/sysrq.txt +++ /dev/null @@ -1,289 +0,0 @@ -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/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/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> |