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Linux Magic System Request Key Hacks

Linux Magic System Request Key Hacks
Documentation for sysrq.c
Last update: 2007-AUG-04

* 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 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.

* What are the ‘command’ keys?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
‘b’ – Will immediately reboot the system without syncing or unmounting
your disks.

‘c’ – Will perform a kexec reboot in order to take a crashdump.

‘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 on ppc and sh platforms.

‘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.

‘k’ – Secure Access Key (SAK) Kills all programs on the current virtual
console. NOTE: See important comments below in SAK section.

‘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 a list of all running timers.

‘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’ – Dumps Voyager SMP processor info to your console.

‘w’ – Dumps tasks that are in uninterruptable (blocked) state.

‘x’ – Used by xmon interface on ppc/powerpc platforms.

‘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, un’R’aw is very handy when your X server or a svgalib program crashes.

sa’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.)

re’B’oot is good when you’re unable to shut down. But you should also ‘S’ync
and ‘U’mount first.

‘C’rashdump can be used to manually trigger a crashdump when the system is hung.
The kernel needs to have been built with CONFIG_KEXEC enabled.

‘S’ync 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…)

‘U’mount is basically useful in the same ways as ‘S’ync. I generally ‘S’ync,
‘U’mount, then re’B’oot 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.

* 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>

#########################################################

ประมาณว่าถ้าลีนุกซ์ค้าง จนไม่สามารถทำอะไรได้

1. กดแป้น alt + sys rq (บางเครื่องอาจต้องกด คีย์ fn หรือ F Lock ก่อน 1 ครั้ง)

2. ปล่อยแป้น sys rq

3. กด R E I S U B

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